Editing Cardiac output (section)

== Measurement ==
There are a number of clinical methods to measure cardiac output, ranging from direct intracardiac catheterization to non-invasive measurement of the arterial pulse. Each method has advantages and drawbacks. Relative comparison is limited by the absence of a widely accepted "gold standard" measurement. Cardiac output can also be affected significantly by the phase of respiration – intra-thoracic pressure changes influence diastolic filling and therefore cardiac output. This is especially important during mechanical ventilation, in which cardiac output can vary by up to 50% across a single respiratory cycle.{{Citation needed|date=October 2013}} Cardiac output should therefore be measured at evenly spaced points over a single cycle or averaged over several cycles.{{citation needed|date=October 2014}}

Invasive methods are well accepted, but there is increasing evidence that these methods are neither accurate nor effective in guiding therapy. Consequently, the focus on development of non-invasive methods is growing.<ref name="Stevenson">{{cite journal | vauthors = Binanay C, Califf RM, Hasselblad V, O'Connor CM, Shah MR, Sopko G, Stevenson LW, Francis GS, Leier CV, Miller LW | title = Evaluation study of congestive heart failure and pulmonary artery catheterization effectiveness: the ESCAPE trial | journal = JAMA | volume = 294 | issue = 13 | pages = 1625–33 | date = October 2005 | pmid = 16204662 | doi = 10.1001/jama.294.13.1625 | author11 = ESCAPE Investigators ESCAPE Study Coordinators | doi-access = free }}</ref><ref name="Shah">{{cite journal | vauthors = Pasche B, Knobloch TJ, Bian Y, Liu J, Phukan S, Rosman D, Kaklamani V, Baddi L, Siddiqui FS, Frankel W, Prior TW, Schuller DE, Agrawal A, Lang J, Dolan ME, Vokes EE, Lane WS, Huang CC, Caldes T, Di Cristofano A, Hampel H, Nilsson I, von Heijne G, Fodde R, Murty VV, de la Chapelle A, Weghorst CM | title = Somatic acquisition and signaling of TGFBR1*6A in cancer | journal = JAMA | volume = 294 | issue = 13 | pages = 1634–46 | date = October 2005 | pmid = 16204663 | doi = 10.1001/jama.294.13.1634 | s2cid = 25937172 | doi-access =  }}</ref><ref name="Hall">{{cite journal | vauthors = Hall JB | title = Searching for evidence to support pulmonary artery catheter use in critically ill patients | journal = JAMA | volume = 294 | issue = 13 | pages = 1693–94 | date = October 2005 | pmid = 16204671 | doi = 10.1001/jama.294.13.1693 }}</ref>

===Doppler ultrasound===
[[File:VTI LVOT.png|thumb|Doppler signal in the left ventricular outflow tract: Velocity Time Integral (VTI)]]

This method uses [[ultrasound]] and the [[Doppler effect]] to measure cardiac output. The blood velocity through the heart causes a Doppler shift in the frequency of the returning ultrasound waves. This shift can then be used to calculate flow velocity and volume, and effectively cardiac output, using the following equations:{{citation needed|date=March 2021}}
* <math>Q = SV \times HR</math>
* <math>SV = VTI \times CSA</math>
* <math>CSA =  \pi r^2</math>
where:
* CSA is the valve orifice cross sectional area,
* r is the valve radius, and,
* VTI is the velocity time integral of the trace of the Doppler flow profile.

Being non-invasive, accurate and inexpensive, Doppler ultrasound is a routine part of clinical ultrasound; it has high levels of reliability and reproducibility, and has been in clinical use since the 1960s.{{citation needed|date=March 2021}}

====Echocardiography====
[[Echocardiography]] is a non-invasive method of quantifying cardiac output using ultrasound. Two-dimensional (2D) ultrasound and Doppler measurements are used together to calculate cardiac output. 2D measurement of the diameter (d) of the aortic annulus allows calculation of the flow cross-sectional area (CSA), which is then multiplied by the VTI of the Doppler flow profile across the aortic valve to determine the flow volume per beat ([[Stroke Volume|stroke volume]], SV). The result is then multiplied by the heart rate (HR) to obtain cardiac output. Although used in clinical medicine, it has a wide test-retest variability.<ref>{{cite journal | vauthors = Finegold JA, Manisty CH, Cecaro F, Sutaria N, Mayet J, Francis DP | title = Choosing between velocity-time-integral ratio and peak velocity ratio for calculation of the dimensionless index (or aortic valve area) in serial follow-up of aortic stenosis | journal = International Journal of Cardiology | volume = 167 | issue = 4 | pages = 1524–31 | date = August 2013 | pmid = 22575631 | doi = 10.1016/j.ijcard.2012.04.105 }}</ref> It is said to require extensive training and skill, but the exact steps needed to achieve clinically adequate precision have never been disclosed. 2D measurement of the aortic valve diameter is one source of noise; others are beat-to-beat variation in stroke volume and subtle differences in probe position. An alternative that is not necessarily more reproducible is the measurement of the pulmonary valve to calculate right-sided CO. Although it is in wide general use, the technique is time-consuming and is limited by the reproducibility of its component elements. In the manner used in clinical practice, precision of SV and CO is of the order of ±20%.{{citation needed|date=October 2014}}

====Transcutaneous====
Ultrasonic Cardiac Output Monitor (USCOM) uses [[Continuous wave doppler|continuous wave Doppler]] to measure the Doppler flow profile VTI. It uses [[anthropometry]] to calculate aortic and pulmonary valve diameters and CSAs, allowing right-sided and left-sided ''Q'' measurements. In comparison to the echocardiographic method, USCOM significantly improves reproducibility and increases sensitivity of the detection of changes in flow. Real-time, automatic tracing of the Doppler flow profile allows beat-to-beat right-sided and left-sided ''Q'' measurements, simplifying operation and reducing the time of acquisition compared to conventional echocardiography. USCOM has been validated from 0.12 L/min to 18.7 L/min<ref name="Su">{{cite journal | vauthors = Su BC, Yu HP, Yang MW, Lin CC, Kao MC, Chang CH, Lee WC | title = Reliability of a new ultrasonic cardiac output monitor in recipients of living donor liver transplantation | journal = Liver Transplantation | volume = 14 | issue = 7 | pages = 1029–37 | date = July 2008 | pmid = 18581505 | doi = 10.1002/lt.21461 | s2cid = 37185399 | doi-access = free }}</ref> in new-born babies,<ref name="Phillips et. al. 3">{{cite journal | vauthors = Phillips R, Paradisis M, Evans N, Southwell D, Burstow D, West M | year = 2006 |title=Cardiac output measurement in preterm neonates: validation of USCOM against echocardiography |journal=Critical Care |volume=10 |issue=Suppl 1 |page=343 |doi=10.1186/cc4690|pmc=4092718 | doi-access = free }}</ref> children<ref name="Cattermole">{{cite journal | vauthors = Cattermole GN, Leung PY, Mak PS, Chan SS, Graham CA, Rainer TH | title = The normal ranges of cardiovascular parameters in children measured using the Ultrasonic Cardiac Output Monitor | journal = Critical Care Medicine | volume = 38 | issue = 9 | pages = 1875–81 | date = September 2010 | pmid = 20562697 | doi = 10.1097/CCM.0b013e3181e8adee | s2cid = 24949904 }}</ref> and adults.<ref name="Jain et. al.">{{cite journal | vauthors = Jain S, Allins A, Salim A, Vafa A, Wilson MT, Margulies DR | title = Noninvasive Doppler ultrasonography for assessing cardiac function: can it replace the Swan-Ganz catheter? | journal = American Journal of Surgery | volume = 196 | issue = 6 | pages = 961–67; discussion 967–68 | date = December 2008 | pmid = 19095116 | doi = 10.1016/j.amjsurg.2008.07.039 }}</ref> The method can be applied with equal accuracy to patients of all ages for the development of physiologically rational haemodynamic protocols. USCOM is the only method of cardiac output measurement to have achieved equivalent accuracy to the implantable flow probe.<ref name="Phillips et. al. 2"/> This accuracy has ensured high levels of clinical use in conditions including sepsis, heart failure and hypertension.<ref name="Horster">{{cite journal | vauthors = Horster S, Stemmler HJ, Strecker N, Brettner F, Hausmann A, Cnossen J, Parhofer KG, Nickel T, Geiger S | title = Cardiac Output Measurements in Septic Patients: Comparing the Accuracy of USCOM to PiCCO | journal = Critical Care Research and Practice | volume = 2012 | pages = 1–5 | year = 2012 | pmid = 22191019 | pmc = 3235433 | doi = 10.1155/2012/270631 | doi-access = free }}</ref><ref name="Phillips et. al. 5">{{cite journal | vauthors = Phillips R, Lichtenthal P, Sloniger J, Burstow D, West M, Copeland J | title = Noninvasive cardiac output measurement in heart failure subjects on circulatory support | journal = Anesthesia and Analgesia | volume = 108 | issue = 3 | pages = 881–86 | date = March 2009 | pmid = 19224797 | doi = 10.1213/ane.0b013e318193174b | s2cid = 35618846 }}</ref><ref name="Kager">{{cite journal | vauthors = Kager CC, Dekker GA, Stam MC | title = Measurement of cardiac output in normal pregnancy by a non-invasive two-dimensional independent Doppler device | journal = The Australian & New Zealand Journal of Obstetrics & Gynaecology | volume = 49 | issue = 2 | pages = 142–44 | date = April 2009 | pmid = 19441163 | doi = 10.1111/j.1479-828X.2009.00948.x | s2cid = 25371483 | doi-access = free }}</ref>

====Transoesophageal====
[[File:TEE-Sonde.png|alt=A Transesophageal echocardiogram (BrE: TOE, AmE: TEE) probe.|thumb|A transoesophageal echocardiogram probe.]]

The Transoesophageal Doppler includes two main technologies; [[Transesophageal echocardiogram|transoesophageal echocardiogram]]—which is primarily used for diagnostic purposes, and [[Esophogeal doppler|oesophageal Doppler]] monitoring—which is primarily used for the clinical monitoring of cardiac output. The latter uses continuous wave Doppler to measure blood velocity in the [[descending aorta|descending thoracic aorta]]. An ultrasound probe is inserted either orally or nasally into the oesophagus to mid-thoracic level, at which point the oesophagus lies alongside the descending [[thoracic aorta]]. Because the transducer is close to the blood flow, the signal is clear. The probe may require re-focussing to ensure an optimal signal. This method has good validation, is widely used for fluid management during surgery with evidence for improved patient outcome,<ref>{{cite journal | vauthors = Mythen MG, Webb AR | title = Perioperative plasma volume expansion reduces the incidence of gut mucosal hypoperfusion during cardiac surgery | journal = Archives of Surgery | volume = 130 | issue = 4 | pages = 423–29 | date = April 1995 | pmid = 7535996 | doi = 10.1001/archsurg.1995.01430040085019 }}</ref><ref>{{cite journal | vauthors = Sinclair S, James S, Singer M | title = Intraoperative intravascular volume optimisation and length of hospital stay after repair of proximal femoral fracture: randomised controlled trial | journal = BMJ | volume = 315 | issue = 7113 | pages = 909–12 | date = October 1997 | pmid = 9361539 | pmc = 2127619 | doi = 10.1136/bmj.315.7113.909 }}</ref><ref>{{cite journal | vauthors = Conway DH, Mayall R, Abdul-Latif MS, Gilligan S, Tackaberry C | title = Randomised controlled trial investigating the influence of intravenous fluid titration using oesophageal Doppler monitoring during bowel surgery | journal = Anaesthesia | volume = 57 | issue = 9 | pages = 845–49 | date = September 2002 | pmid = 12190747 | doi = 10.1046/j.1365-2044.2002.02708.x | s2cid = 43755776 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Gan TJ, Soppitt A, Maroof M, el-Moalem H, Robertson KM, Moretti E, Dwane P, Glass PS | title = Goal-directed intraoperative fluid administration reduces length of hospital stay after major surgery | journal = Anesthesiology | volume = 97 | issue = 4 | pages = 820–26 | date = October 2002 | pmid = 12357146 | doi = 10.1097/00000542-200210000-00012 | s2cid = 10471164 }}</ref><ref>{{cite journal | vauthors = Venn R, Steele A, Richardson P, Poloniecki J, Grounds M, Newman P | title = Randomized controlled trial to investigate influence of the fluid challenge on duration of hospital stay and perioperative morbidity in patients with hip fractures | journal = British Journal of Anaesthesia | volume = 88 | issue = 1 | pages = 65–71 | date = January 2002 | pmid = 11881887 | doi = 10.1093/bja/88.1.65 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Wakeling HG, McFall MR, Jenkins CS, Woods WG, Miles WF, Barclay GR, Fleming SC | title = Intraoperative oesophageal Doppler guided fluid management shortens postoperative hospital stay after major bowel surgery | journal = British Journal of Anaesthesia | volume = 95 | issue = 5 | pages = 634–42 | date = November 2005 | pmid = 16155038 | doi = 10.1093/bja/aei223 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Noblett SE, Snowden CP, Shenton BK, Horgan AF | title = Randomized clinical trial assessing the effect of Doppler-optimized fluid management on outcome after elective colorectal resection | journal = The British Journal of Surgery | volume = 93 | issue = 9 | pages = 1069–76 | date = September 2006 | pmid = 16888706 | doi = 10.1002/bjs.5454 | s2cid = 25469534 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Pillai P, McEleavy I, Gaughan M, Snowden C, Nesbitt I, Durkan G, Johnson M, Cosgrove J, Thorpe A | title = A double-blind randomized controlled clinical trial to assess the effect of Doppler optimized intraoperative fluid management on outcome following radical cystectomy | journal = The Journal of Urology | volume = 186 | issue = 6 | pages = 2201–06 | date = December 2011 | pmid = 22014804 | doi = 10.1016/j.juro.2011.07.093 }}</ref> and has been recommended by the UK's National Institute for Health and Clinical Excellence ([[NICE]]).<ref>{{Cite web|url=https://www.nice.org.uk/mtg3|title=CardioQ-ODM oesophageal doppler monitor &#124; Guidance &#124; NICE|date=25 March 2011 |access-date=23 February 2022|archive-date=23 February 2022|archive-url=https://web.archive.org/web/20220223063027/https://www.nice.org.uk/guidance/mtg3|url-status=live}}</ref> Oesophageal Doppler monitoring measures the velocity of blood and not true ''Q'', therefore relies on a nomogram<ref>{{cite web |first1=Graham D. |last1=Lowe |first2=Barry M. |last2=Chamberlain |first3=Eleanor J. |last3=Philpot |first4=Richard J. |last4=Willshire |name-list-style=vanc |year=2010 |title=Oesophageal Doppler Monitor (ODM) guided individualised goal directed fluid management (iGDFM) in surgery – a technical review |work=Deltex Medical Technical Review |url=https://www.deltexmedical.com/downloads/TechnicalReview.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150923213441/https://www.deltexmedical.com/downloads/TechnicalReview.pdf |archive-date=23 September 2015 |access-date=13 October 2014 }}</ref> based on patient age, height and weight to convert the measured velocity into stroke volume and cardiac output. This method generally requires patient sedation and is accepted for use in both adults and children.{{citation needed|date=March 2021}}

===Pulse pressure methods===
[[Pulse pressure]] (PP) methods measure the pressure in an artery over time to derive a waveform and use this information to calculate cardiac performance. However, any measure from the artery includes changes in pressure associated with changes in arterial function, for example compliance and impedance. Physiological or therapeutic changes in vessel diameter are assumed to reflect changes in ''Q''. PP methods measure the combined performance of the heart and the blood vessels, thus limiting their application for measurement of ''Q''. This can be partially compensated for by intermittent calibration of the waveform to another ''Q'' measurement method then monitoring the PP waveform. Ideally, the PP waveform should be calibrated on a beat-to-beat basis. There are invasive and non-invasive methods of measuring PP.{{citation needed|date=March 2021}}

==== Finapres methodology ====

In 1967, the Czech physiologist Jan Peňáz invented and patented the [[Continuous noninvasive arterial pressure|volume clamp method]] of measuring continuous blood pressure. The principle of the volume clamp method is to dynamically provide equal pressures, on either side of an artery wall. By clamping the artery to a certain volume, inside pressure—intra-arterial pressure—balances outside pressure—finger cuff pressure. Peñáz decided the finger was the optimal site to apply this volume clamp method. The use of finger cuffs excludes the device from application in patients without vasoconstriction, such as in sepsis or in patients on vasopressors.{{citation needed|date=June 2015}}

In 1978, scientists at BMI-TNO, the research unit of [[Netherlands Organisation for Applied Scientific Research]] at the [[University of Amsterdam]], invented and patented a series of additional key elements that make the volume clamp work in clinical practice. These methods include the use of modulated infrared light in the optical system inside the sensor, the lightweight, easy-to-wrap finger cuff with [[velcro]] fixation, a new pneumatic proportional control valve principle, and a set point strategy for the determining and tracking the correct volume at which to clamp the finger arteries—the Physiocal system. An acronym for physiological calibration of the finger arteries, this Physiocal tracker was found to be accurate, robust and reliable.{{citation needed|date=June 2015}}

The Finapres methodology was developed to use this information to calculate arterial pressure from finger cuff pressure data. A generalised algorithm to correct for the pressure level difference between the finger and brachial sites in patients was developed. This correction worked under all of the circumstances it was tested in—even when it was not designed for it—because it applied general physiological principles. This innovative brachial pressure waveform reconstruction method was first implemented in the Finometer, the successor of Finapres that BMI-TNO introduced to the market in 2000.{{Citation needed|date = June 2015}}

The availability of a continuous, high-fidelity, calibrated blood pressure waveform opened up the perspective of beat-to-beat computation of integrated haemodynamics, based on two notions: pressure and flow are inter-related at each site in the arterial system by their so-called characteristic impedance. At the proximal aortic site, the 3-element [[Windkessel effect|Windkessel]] model of this impedance can be modelled with sufficient accuracy in an individual patient with known age, gender, height and weight. According to comparisons of non-invasive peripheral vascular monitors, modest clinical utility is restricted to patients with normal and invariant circulation.<ref name="de Wilde">{{cite journal | vauthors = de Wilde RB, Schreuder JJ, van den Berg PC, Jansen JR | title = An evaluation of cardiac output by five arterial pulse contour techniques during cardiac surgery | journal = Anaesthesia | volume = 62 | issue = 8 | pages = 760–68 | date = August 2007 | pmid = 17635422 | doi = 10.1111/j.1365-2044.2007.05135.x | doi-access = free }}</ref>

====Invasive====
Invasive PP monitoring involves inserting a [[manometer]] pressure sensor into an artery—usually the [[radial artery|radial]] or [[femoral artery]]—and continuously measuring the PP waveform. This is generally done by connecting the catheter to a signal processing device with a display. The PP waveform can then be analysed to provide measurements of cardiovascular performance. Changes in vascular function, the position of the catheter tip or damping of the pressure waveform signal will affect the accuracy of the readings. Invasive PP measurements can be calibrated or uncalibrated.{{citation needed|date=June 2015}}

=====Calibrated PP – PiCCO, LiDCO=====
{{abbr|PiCCO|Pulse contour cardiac output}} ([[:de:PULSION Medical Systems|PULSION Medical Systems]] AG, Munich, Germany) and PulseCO (LiDCO Ltd, London, England) generate continuous ''Q'' by analysing the arterial PP waveform. In both cases, an independent technique is required to provide calibration of continuous ''Q'' analysis because arterial PP analysis cannot account for unmeasured variables such as the changing compliance of the vascular bed. Recalibration is recommended after changes in patient position, therapy or condition.{{citation needed|date=June 2015}}

In PiCCO, transpulmonary thermodilution, which uses the Stewart-Hamilton principle but measures temperatures changes from central venous line to a central arterial line, i.e., the femoral or axillary arterial line, is used as the calibrating technique. The ''Q'' value derived from cold-saline thermodilution is used to calibrate the arterial PP contour, which can then provide continuous ''Q'' monitoring. The PiCCO algorithm is dependent on blood pressure waveform morphology (mathematical analysis of the PP waveform), and it calculates continuous ''Q'' as described by Wesseling and colleagues.<ref name="Wesseling">{{cite journal | vauthors = Wesseling KH, Jansen JR, Settels JJ, Schreuder JJ | title = Computation of aortic flow from pressure in humans using a nonlinear, three-element model | journal = Journal of Applied Physiology | volume = 74 | issue = 5 | pages = 2566–73 | date = May 1993 | pmid = 8335593 | doi = 10.1152/jappl.1993.74.5.2566 }}</ref> Transpulmonary thermodilution spans right heart, pulmonary circulation and left heart, allowing further mathematical analysis of the thermodilution curve and giving measurements of cardiac filling volumes ([[End-diastolic volume|{{abbr|GEDV|Global end diastolic volume}}]]), intrathoracic blood volume and extravascular lung water. Transpulmonary thermodilution allows for less invasive ''Q'' calibration but is less accurate than PA thermodilution and requires a central venous and arterial line with the accompanied infection risks.{{citation needed|date=June 2015}}

In LiDCO, the independent calibration technique is [[lithium chloride]] dilution using the Stewart-Hamilton principle. Lithium chloride dilution uses a peripheral vein and a peripheral arterial line. Like PiCCO, frequent calibration is recommended when there is a change in Q.<ref name="Bein2">{{cite journal | vauthors = Bein B, Meybohm P, Cavus E, Renner J, Tonner PH, Steinfath M, Scholz J, Doerges V | title = The reliability of pulse contour-derived cardiac output during hemorrhage and after vasopressor administration | journal = Anesthesia and Analgesia | volume = 105 | issue = 1 | pages = 107–13 | date = July 2007 | pmid = 17578965 | doi = 10.1213/01.ane.0000268140.02147.ed | s2cid = 5549744 | doi-access = free }}</ref> Calibration events are limited in frequency because they involve the injection of lithium chloride and can be subject to errors in the presence of certain muscle relaxants. The PulseCO algorithm used by LiDCO is based on pulse power derivation and is not dependent on waveform morphology.{{citation needed|date=March 2021}}

=====Statistical analysis of arterial pressure – FloTrac/Vigileo=====

FloTrac/Vigileo ([[Edwards Lifesciences]]) is an uncalibrated, haemodynamic monitor based on pulse contour analysis. It estimates cardiac output (''Q'') using a standard arterial catheter with a manometer located in the femoral or radial artery. The device consists of a high-fidelity pressure transducer, which, when used with a supporting monitor (Vigileo or EV1000 monitor), derives left-sided cardiac output (''Q'') from a sample of arterial pulsations. The device uses an algorithm based on the [[Frank–Starling law of the heart]], which states pulse pressure (PP) is proportional to stroke volume (SV). The algorithm calculates the product of the standard deviation of the arterial pressure (AP) wave over a sampled period of 20 seconds and a vascular tone factor (Khi, or χ) to generate stroke volume. The equation in simplified form is: <math display="inline">SV = \mathrm{std}(AP) \cdot \chi</math>, or, <math display="inline">BP \cdot k \mathrm{\ (constant)}</math>. Khi is designed to reflect arterial resistance; compliance is a multivariate polynomial equation that continuously quantifies arterial compliance and vascular resistance. Khi does this by analyzing the morphological changes of arterial pressure waveforms on a bit-by-bit basis, based on the principle that changes in compliance or resistance affect the shape of the arterial pressure waveform. By analyzing the shape of said waveforms, the effect of vascular tone is assessed, allowing the calculation of SV. ''Q'' is then derived using equation ({{EquationNote|1}}). Only perfused beats that generate an arterial waveform are counted for in HR.{{citation needed|date=October 2014}}

This system estimates Q using an existing arterial catheter with variable accuracy. These arterial monitors do not require intracardiac catheterisation from a pulmonary artery catheter. They require an arterial line and are therefore invasive. As with other arterial waveform systems, the short set-up and data acquisition times are benefits of this technology. Disadvantages include its inability to provide data regarding right-sided heart pressures or mixed venous oxygen saturation.<ref>{{cite journal | vauthors = Singh S, Taylor MA | title = Con: the FloTrac device should not be used to follow cardiac output in cardiac surgical patients | journal = Journal of Cardiothoracic and Vascular Anesthesia | volume = 24 | issue = 4 | pages = 709–11 | date = August 2010 | pmid = 20673749 | doi = 10.1053/j.jvca.2010.04.023 }}</ref><ref>{{cite journal | vauthors = Manecke GR | title = Edwards FloTrac sensor and Vigileo monitor: easy, accurate, reliable cardiac output assessment using the arterial pulse wave | journal = Expert Review of Medical Devices | volume = 2 | issue = 5 | pages = 523–27 | date = September 2005 | pmid = 16293062 | doi = 10.1586/17434440.2.5.523 | s2cid = 31049402 }}</ref> The measurement of Stroke Volume Variation (SVV), which predicts volume responsiveness is intrinsic to all arterial waveform technologies. It is used for managing fluid optimisation in high-risk surgical or critically ill patients. A physiologic optimization program based on haemodynamic principles that incorporates the data pairs SV and SVV has been published.<ref>{{cite journal | vauthors = McGee WT | title = A simple physiologic algorithm for managing hemodynamics using stroke volume and stroke volume variation: physiologic optimization program | journal = Journal of Intensive Care Medicine | volume = 24 | issue = 6 | pages = 352–60 | year = 2009 | pmid = 19736180 | doi = 10.1177/0885066609344908 | s2cid = 12806349 }}</ref>

Arterial monitoring systems are unable to predict changes in vascular tone; they estimate changes in vascular compliance. The measurement of pressure in the artery to calculate the flow in the heart is physiologically irrational and of questionable accuracy,<ref name="Se 2">{{cite journal | vauthors = Su BC, Tsai YF, Chen CY, Yu HP, Yang MW, Lee WC, Lin CC | title = Cardiac output derived from arterial pressure waveform analysis in patients undergoing liver transplantation: validity of a third-generation device | journal = Transplantation Proceedings | volume = 44 | issue = 2 | pages = 424–28 | date = March 2012 | pmid = 22410034 | doi = 10.1016/j.transproceed.2011.12.036 }}</ref> and of unproven benefit.<ref>{{cite journal | vauthors = Takala J, Ruokonen E, Tenhunen JJ, Parviainen I, Jakob SM | title = Early non-invasive cardiac output monitoring in hemodynamically unstable intensive care patients: a multi-center randomized controlled trial | journal = Critical Care | volume = 15 | issue = 3 | pages = R148 | date = June 2011 | pmid = 21676229 | pmc = 3219022 | doi = 10.1186/cc10273 | doi-access = free }}</ref> Arterial pressure monitoring is limited in patients off-ventilation, in atrial fibrillation, in patients on vasopressors, and in those with a dynamic autonomic system such as those with sepsis.<ref name="Bein2" />

===== Uncalibrated, pre-estimated demographic data-free – PRAM =====
Pressure Recording Analytical Method (PRAM), estimates ''Q'' from the analysis of the pressure wave profile obtained from an arterial catheter—radial or femoral access. This PP waveform can then be used to determine ''Q''. As the waveform is sampled at 1000&nbsp;Hz, the detected pressure curve can be measured to calculate the actual beat-to-beat stroke volume. Unlike FloTrac, neither constant values of impedance from external calibration, nor form pre-estimated [[in vivo]] or [[in vitro]] data, are needed.{{cn|date=July 2024}}

PRAM has been validated against the considered gold standard methods in stable condition<ref>{{cite journal | vauthors = Romano SM, Pistolesi M | title = Assessment of cardiac output from systemic arterial pressure in humans | journal = Critical Care Medicine | volume = 30 | issue = 8 | pages = 1834–41 | date = August 2002 | pmid = 12163802 | doi = 10.1097/00003246-200208000-00027 | s2cid = 12100251 }}</ref> and in various haemodynamic states.<ref>{{cite journal | vauthors = Scolletta S, Romano SM, Biagioli B, Capannini G, Giomarelli P | title = Pressure recording analytical method (PRAM) for measurement of cardiac output during various haemodynamic states | journal = British Journal of Anaesthesia | volume = 95 | issue = 2 | pages = 159–65 | date = August 2005 | pmid = 15894561 | doi = 10.1093/bja/aei154 | doi-access = free }}</ref> It can be used to monitor pediatric and mechanically supported patients.<ref>{{cite journal | vauthors = Calamandrei M, Mirabile L, Muschetta S, Gensini GF, De Simone L, Romano SM | title = Assessment of cardiac output in children: a comparison between the pressure recording analytical method and Doppler echocardiography | journal = Pediatric Critical Care Medicine | volume = 9 | issue = 3 | pages = 310–12 | date = May 2008 | pmid = 18446106 | doi = 10.1097/PCC.0b013e31816c7151 | s2cid = 25815656 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Scolletta S, Gregoric ID, Muzzi L, Radovancevic B, Frazier OH | title = Pulse wave analysis to assess systemic blood flow during mechanical biventricular support | journal = Perfusion | volume = 22 | issue = 1 | pages = 63–66 | date = January 2007 | pmid = 17633137 | doi = 10.1177/0267659106074784 | s2cid = 32129645 }}</ref>

Generally monitored haemodynamic values, fluid responsiveness parameters and an exclusive reference are provided by PRAM: Cardiac Cycle Efficiency (CCE). It is expressed by a pure number ranging from 1 (best) to -1 (worst) and it indicates the overall heart-vascular response coupling. The ratio between heart performance and consumed energy, represented as CCE "stress index", can be of paramount importance in understanding the patient's present and future courses.<ref>{{cite web | vauthors = Scolletta S, Romano SM, Maglioni H |year=2005 |title=Left ventricular performance by PRAM during cardiac surgery |page=S157}} in {{cite journal |year=2005 |title=OP 564–605 |journal=Intensive Care Medicine |volume=31 |issue=Suppl 1 |pages=S148–58 |doi=10.1007/s00134-005-2781-3|s2cid=30752685 }}</ref>

===Impedance cardiography===

[[Impedance cardiography]] (often abbreviated as ICG, or Thoracic Electrical Bioimpedance (TEB)) measures changes in [[electrical impedance]] across the thoracic region over the cardiac cycle. Lower impedance indicates greater intrathoracic fluid volume and blood flow. By synchronizing fluid volume changes with the heartbeat, the change in impedance can be used to calculate stroke volume, cardiac output and systemic vascular resistance.<ref>{{cite journal |doi=10.5617/jeb.51 |url=https://www.journals.uio.no/index.php/bioimpedance/article/view/51 |first1=Donald P |last1=Bernstein |year=2010 |title=Impedance cardiography: Pulsatile blood flow and the biophysical and electrodynamic basis for the stroke volume equations |journal=Journal of Electrical Bioimpedance |volume=1 |pages=2–17 |url-status=live |archive-url=https://web.archive.org/web/20151017150340/https://www.journals.uio.no/index.php/bioimpedance/article/view/51 |archive-date=17 October 2015|doi-access=free }}</ref>

Both invasive and non-invasive approaches are used.<ref>{{cite journal | vauthors = Costa PD, Rodrigues PP, Reis AH, Costa-Pereira A | title = A review on remote monitoring technology applied to implantable electronic cardiovascular devices | journal = Telemedicine Journal and e-Health | volume = 16 | issue = 10 | pages = 1042–50 | date = December 2010 | pmid = 21070132 | doi = 10.1089/tmj.2010.0082 }}</ref> The reliability and validity of the non-invasive approach has gained some acceptance,<ref>{{cite journal | vauthors = Tang WH, Tong W | title = Measuring impedance in congestive heart failure: current options and clinical applications | journal = American Heart Journal | volume = 157 | issue = 3 | pages = 402–11 | date = March 2009 | pmid = 19249408 | pmc = 3058607 | doi = 10.1016/j.ahj.2008.10.016 }}</ref><ref>{{cite journal | vauthors = Ferrario CM, Flack JM, Strobeck JE, Smits G, Peters C | title = Individualizing hypertension treatment with impedance cardiography: a meta-analysis of published trials | journal = Therapeutic Advances in Cardiovascular Disease | volume = 4 | issue = 1 | pages = 5–16 | date = February 2010 | pmid = 20042450 | doi = 10.1177/1753944709348236 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Moshkovitz Y, Kaluski E, Milo O, Vered Z, Cotter G | title = Recent developments in cardiac output determination by bioimpedance: comparison with invasive cardiac output and potential cardiovascular applications | journal = Current Opinion in Cardiology | volume = 19 | issue = 3 | pages = 229–37 | date = May 2004 | pmid = 15096956 | doi = 10.1097/00001573-200405000-00008 | s2cid = 28996732 }}</ref><ref>{{cite journal | vauthors = Parry MJ, McFetridge-Durdle J | title = Ambulatory impedance cardiography: a systematic review | journal = Nursing Research | volume = 55 | issue = 4 | pages = 283–91 | year = 2006 | pmid = 16849981 | doi = 10.1097/00006199-200607000-00009 | s2cid = 28726590 }}</ref> although there is not complete agreement on this point.<ref>{{cite journal | vauthors = Wang DJ, Gottlieb SS | title = Impedance cardiography: more questions than answers | journal = Current Heart Failure Reports | volume = 3 | issue = 3 | pages = 107–13 | date = September 2006 | pmid = 16914102 | doi = 10.1007/s11897-006-0009-7 | s2cid = 31094943 }}</ref> The clinical use of this approach in the diagnosis, prognosis and therapy of a variety of diseases continues.<ref>{{cite journal | vauthors = Ventura HO, Taler SJ, Strobeck JE | title = Hypertension as a hemodynamic disease: the role of impedance cardiography in diagnostic, prognostic, and therapeutic decision making | journal = American Journal of Hypertension | volume = 18 | issue = 2 Pt 2 | pages = 26S–43S | date = February 2005 | pmid = 15752931 | doi = 10.1016/j.amjhyper.2004.11.002 | doi-access = free }}</ref>

Non-invasive ICG equipment includes the Bio-Z Dx,<ref>{{cite web |url=https://www.sonosite.com/products/bioz-dx |title=BioZ Dx Diagnostics System &#124; Sonosite Inc |access-date=2010-11-30 |url-status=dead |archive-url=https://web.archive.org/web/20101203072604/https://www.sonosite.com/products/bioz-dx |archive-date=3 December 2010}}{{Verify source|date=March 2011}}{{psc|date=October 2014}}</ref> the Niccomo,<ref>{{Cite web|title = Niccomo – Non-Invasive Continuous Cardiac Output Monitor|url = https://www.medis-de.com/index.php?option=com_content&task=view&id=24&Itemid=49|website = www.medis-de.com|access-date = 2015-06-01|publisher = medis. GmbH Ilmenau.|url-status = live|archive-url = https://web.archive.org/web/20151017150340/https://www.medis-de.com/index.php?option=com_content&task=view&id=24&Itemid=49|archive-date = 17 October 2015}}</ref> and TEBCO products by BoMed.<ref>{{cite web |url=https://bomed.us/tebco.html |title=OEM Module TEBCO |access-date=2015-05-22 |url-status=live |archive-url=https://archive.wikiwix.com/cache/20150524202336/https://bomed.us/tebco.html |archive-date=24 May 2015}} TEBCO OEM</ref><ref>bomed.us/ext-teb.html EXT-TEBCO</ref>

=== Ultrasound dilution ===
Ultrasound dilution (UD) uses body-temperature normal saline (NS) as an indicator introduced into an extracorporeal loop to create an atrioventricular (AV) circulation with an ultrasound sensor, which is used to measure the dilution then to calculate cardiac output using a proprietary algorithm. A number of other haemodynamic variables, such as total end-diastole volume (TEDV), central blood volume (CBV) and active circulation volume (ACVI) can be calculated using this method.{{citation needed|date=October 2014}}

The UD method was firstly introduced in 1995.<ref>{{cite journal | vauthors = Krivitski NM | title = Theory and validation of access flow measurement by dilution technique during hemodialysis | journal = Kidney International | volume = 48 | issue = 1 | pages = 244–50 | date = July 1995 | pmid = 7564085 | doi = 10.1038/ki.1995.290 | doi-access = free }}</ref> It was extensively used to measure flow and volumes with extracorporeal circuit conditions, such as [[ECMO]]<ref>{{cite journal | vauthors = Tanke RB, van Heijst AF, Klaessens JH, Daniels O, Festen C | title = Measurement of the ductal L-R shunt during extracorporeal membrane oxygenation in the lamb | journal = Journal of Pediatric Surgery | volume = 39 | issue = 1 | pages = 43–47 | date = January 2004 | pmid = 14694369 | doi = 10.1016/j.jpedsurg.2003.09.017 }}</ref><ref>{{cite journal | vauthors = Casas F, Reeves A, Dudzinski D, Weber S, Lorenz M, Akiyama M, Kamohara K, Kopcak M, Ootaki Y, Zahr F, Sinkewich M, Foster R, Fukamachi K, Smith WA | title = Performance and reliability of the CPB/ECMO Initiative Forward Lines Casualty Management System | journal = ASAIO Journal | volume = 51 | issue = 6 | pages = 681–85 | year = 2005 | pmid = 16340350 | doi = 10.1097/01.mat.0000182472.63808.b9 | s2cid = 1897392 | doi-access = free }}</ref> and [[Hemodialysis|Haemodialysis]],<ref>{{cite journal | vauthors = Tessitore N, Bedogna V, Poli A, Mantovani W, Lipari G, Baggio E, Mansueto G, Lupo A | title = Adding access blood flow surveillance to clinical monitoring reduces thrombosis rates and costs, and improves fistula patency in the short term: a controlled cohort study | journal = Nephrology, Dialysis, Transplantation | volume = 23 | issue = 11 | pages = 3578–84 | date = November 2008 | pmid = 18511608 | doi = 10.1093/ndt/gfn275 | doi-access =  }}</ref><ref>{{cite journal | vauthors = van Loon M, van der Mark W, Beukers N, de Bruin C, Blankestijn PJ, Huisman RM, Zijlstra JJ, van der Sande FM, Tordoir JH | title = Implementation of a vascular access quality programme improves vascular access care | journal = Nephrology, Dialysis, Transplantation | volume = 22 | issue = 6 | pages = 1628–32 | date = June 2007 | pmid = 17400567 | doi = 10.1093/ndt/gfm076 | doi-access = free }}</ref> leading more than 150 peer reviewed publications. UD has now been adapted to [[intensive care unit]]s (ICU) as the COstatus device.<ref>([https://www.transonic.com/products/critical-care/product/costatus/ COstatus] {{webarchive|url=https://web.archive.org/web/20150512224027/https://www.transonic.com/products/critical-care/product/costatus/ |date=12 May 2015 }}, [https://www.transonic.com/ Transonic System Inc.] {{webarchive|url=https://web.archive.org/web/20081029072511/https://transonic.com/ |date=29 October 2008 }} Ithaca, NY){{psc|date = October 2014}}
</ref>

The UD method is based on ultrasound indicator dilution.<ref>{{cite journal | vauthors = Krivitski NM, Kislukhin VV, Thuramalla NV | title = Theory and in vitro validation of a new extracorporeal arteriovenous loop approach for hemodynamic assessment in pediatric and neonatal intensive care unit patients | journal = Pediatric Critical Care Medicine | volume = 9 | issue = 4 | pages = 423–28 | date = July 2008 | pmid = 18496416 | pmc = 2574659 | doi = 10.1097/01.PCC.0b013e31816c71bc }}</ref> Blood ultrasound velocity (1560–1585&nbsp;m/s) is a function of total blood protein concentration—sums of proteins in plasma and in red blood red cells—and temperature. Injection of body-temperature normal saline (ultrasound velocity of saline is 1533&nbsp;m/s) into a unique AV loop decreases blood ultrasound velocity, and produces dilution curves.{{citation needed|date=October 2014}}

UD requires the establishment of an extracorporeal circulation through its unique AV loop with two pre-existing arterial and central venous lines in ICU patients. When the saline indicator is injected into the AV loop, it is detected by the venous clamp-on sensor on the loop before it enters the patient's heart's right atrium. After the indicator traverses the heart and lung, the concentration curve in the arterial line is recorded and displayed on the COstatus HCM101 Monitor. Cardiac output is calculated from the area of the concentration curve using the Stewart-Hamilton equation. UD is a non-invasive procedure, requiring only a connection to the AV loop and two lines from a patient. UD has been specialised for application in pediatric ICU patients and has been demonstrated to be relatively safe although invasive and reproducible.{{citation needed|date=October 2014}}

===Electrical cardiometry===
[[Electrical cardiometry]] is a non-invasive method similar to Impedance cardiography; both methods measure thoracic electrical bioimpedance (TEB). The underlying model differs between the two methods; Electrical cardiometry attributes the steep increase of TEB beat-to-beat to the change in orientation of red blood cells. Four standard ECG electrodes are required for measurement of cardiac output. Electrical Cardiometry is a method trademarked by Cardiotronic, Inc., and shows promising results in a wide range of patients. It is currently approved in the US for use in adults, children and babies. Electrical cardiometry monitors have shown promise in postoperative cardiac surgical patients, in both haemodynamically stable and unstable cases.<ref>{{cite journal | vauthors = Funk DJ, Moretti EW, Gan TJ | title = Minimally invasive cardiac output monitoring in the perioperative setting | journal = Anesthesia and Analgesia | volume = 108 | issue = 3 | pages = 887–97 | date = March 2009 | pmid = 19224798 | doi = 10.1213/ane.0b013e31818ffd99 | s2cid = 15891210 | doi-access = free }}</ref>

===Magnetic resonance imaging===
Velocity-encoded phase contrast Magnetic resonance imaging (MRI)<ref>{{cite book |last1=Arheden |first1=Håkan |last2=Ståhlberg |first2=Freddy |name-list-style=vanc |chapter=Blood flow measurements |chapter-url=https://lup.lub.lu.se/record/1136006 |editor1-last=de Roos |editor1-first=Albert |editor2-last=Higgins |editor2-first=Charles B |title=MRI and CT of the Cardiovascular System |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |year=2006 |pages=71–90 |isbn=978-0-7817-6271-7 |edition=2nd |access-date=23 February 2022 |archive-date=23 February 2022 |archive-url=https://web.archive.org/web/20220223063034/https://lup.lub.lu.se/search/publication/1136006 |url-status=live }}</ref> is the most accurate technique for measuring flow in large vessels in mammals. MRI flow measurements have been shown to be highly accurate compared to measurements made with a beaker and timer,<ref>{{cite journal | vauthors = Arheden H, Holmqvist C, Thilen U, Hanséus K, Björkhem G, Pahlm O, Laurin S, Ståhlberg F | title = Left-to-right cardiac shunts: comparison of measurements obtained with MR velocity mapping and with radionuclide angiography | journal = Radiology | volume = 211 | issue = 2 | pages = 453–58 | date = May 1999 | pmid = 10228528 | doi = 10.1148/radiology.211.2.r99ma43453 }}</ref> and less variable than the Fick principle<ref>{{cite journal | vauthors = Razavi R, Hill DL, Keevil SF, Miquel ME, Muthurangu V, Hegde S, Rhode K, Barnett M, van Vaals J, Hawkes DJ, Baker E | title = Cardiac catheterisation guided by MRI in children and adults with congenital heart disease | journal = Lancet | volume = 362 | issue = 9399 | pages = 1877–82 | date = December 2003 | pmid = 14667742 | doi = 10.1016/S0140-6736(03)14956-2 | s2cid = 25380774 }}</ref> and thermodilution.<ref>{{cite journal | vauthors = Kuehne T, Yilmaz S, Schulze-Neick I, Wellnhofer E, Ewert P, Nagel E, Lange P | title = Magnetic resonance imaging guided catheterisation for assessment of pulmonary vascular resistance: in vivo validation and clinical application in patients with pulmonary hypertension | journal = Heart | volume = 91 | issue = 8 | pages = 1064–69 | date = August 2005 | pmid = 16020598 | pmc = 1769055 | doi = 10.1136/hrt.2004.038265 }}</ref>

Velocity-encoded MRI is based on the detection of changes in the phase of proton [[precession]]. These changes are proportional to the velocity of the protons' movement through a magnetic field with a known gradient. When using velocity-encoded MRI, the result is two sets of images, one for each time point in the cardiac cycle. One is an anatomical image and the other is an image in which the signal intensity in each [[pixel]] is directly proportional to the through-plane velocity. The average velocity in a vessel, i.e., the [[aorta]] or the [[pulmonary artery]], is quantified by measuring the average signal intensity of the pixels in the cross-section of the vessel then multiplying by a known constant. The flow is calculated by multiplying the mean velocity by the cross-sectional area of the vessel. This flow data can be used in a flow-versus-time graph. The area under the flow-versus-time curve for one [[cardiac cycle]] is the stroke volume. The length of the cardiac cycle is known and determines heart rate; ''Q'' can be calculated using equation ({{EquationNote|1}}). MRI is typically used to quantify the flow over one cardiac cycle as the average of several heart beats. It is also possible to quantify the stroke volume in real-time on a beat-for-beat basis.<ref>{{cite journal | vauthors = Petzina R, Ugander M, Gustafsson L, Engblom H, Sjögren J, Hetzer R, Ingemansson R, Arheden H, Malmsjö M | title = Hemodynamic effects of vacuum-assisted closure therapy in cardiac surgery: assessment using magnetic resonance imaging | journal = The Journal of Thoracic and Cardiovascular Surgery | volume = 133 | issue = 5 | pages = 1154–62 | date = May 2007 | pmid = 17467423 | doi = 10.1016/j.jtcvs.2007.01.011 | doi-access = free }}</ref>

While MRI is an important research tool for accurately measuring ''Q'', it is currently not clinically used for haemodynamic monitoring in emergency or intensive care settings. {{As of|2015}}, cardiac output measurement by MRI is routinely used in clinical cardiac MRI examinations.<ref>{{cite journal | vauthors = Pennell DJ, Sechtem UP, Higgins CB, Manning WJ, Pohost GM, Rademakers FE, van Rossum AC, Shaw LJ, Yucel EK | title = Clinical indications for cardiovascular magnetic resonance (CMR): Consensus Panel report | journal = European Heart Journal | volume = 25 | issue = 21 | pages = 1940–65 | date = November 2004 | pmid = 15522474 | doi = 10.1016/j.ehj.2004.06.040 | author10 = Society for Cardiovascular Magnetic Resonance | author11 = Working Group on Cardiovascular Magnetic Resonance of the European Society of Cardiology | doi-access = free }}</ref>

=== Dye dilution method ===
The dye dilution method is done by rapidly injecting a dye, [[indocyanine green]], into the right atrium of the heart. The dye flows with the blood into the aorta. A probe is inserted into the aorta to measure the concentration of the dye leaving the heart at equal time intervals [0, ''T''] until the dye has cleared. Let ''c''(''t)'' be the concentration of the dye at time ''t''. By dividing the time intervals from [0, ''T''] into subintervals Δ''t'', the amount of dye that flows past the measuring point during the subinterval from <math>t=t_{i-1}</math> to <math>t=t_i</math> is:

<math>(concentration)(volume)=c(t_i)(F\Delta t)</math>

where <math>F</math> is the rate of flow that is being calculated. The total amount of dye is:

<math>\sum_{i=1}^nc(t_i)(F\Delta t)=F \sum_{i=1}^nc(t_i)(\Delta t)</math>

and, letting <math>n\rightarrow\infty</math>, the amount of dye is:

<math>A=F\int_{0}^{T} c(t)dt</math>

Thus, the cardiac output is given by:

<math>F=\frac{A}{\int_{0}^{T} c(t)dt}

</math>

where the amount of dye injected <math>A</math> is known, and the integral can be determined using the concentration readings.<ref>{{Cite book|title=Calculus: Early Transcententals|last=Stewart|first=James | name-list-style = vanc |publisher=Cengage Learning|year=2010|isbn=9780538497909|pages=565–66 }}</ref>

The dye dilution method is one of the most accurate methods of determining cardiac output during exercise. The error of a single calculation of cardiac output values at rest and during exercise is less than 5%. This method does not allow measurement of 'beat to beat' changes, and requires a cardiac output that is stable for approximately 10 s during exercise and 30 s at rest.{{citation needed|date=March 2018}}