Editing Cardiac output (section)

===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>