Editing Cardiac output (section)

==Historical methods==

===Fick principle===
{{main|Fick principle}}
[[File:Spirometry NIH.jpg|alt=An illustration of how spirometry is done|thumb|An illustration of how spirometry is done]]

The Fick principle, first described by [[Adolf Eugen Fick]] in 1870, assumes the rate of oxygen consumption is a function of the rate of blood flow and the rate of oxygen picked up by the red blood cells. Application of the Fick principle involves calculating the oxygen consumed over time by measuring the oxygen concentration of venous blood and arterial blood. ''Q'' is calculated from these measurements as follows:
* ''V''<sub>O<sub>2</sub></sub> consumption per minute using a [[spirometer]] (with the subject re-breathing air) and a [[Carbon dioxide|CO<sub>2</sub>]] absorber
* the oxygen content of blood taken from the pulmonary artery (representing mixed venous blood)
* the oxygen content of blood from a cannula in a peripheral artery (representing arterial blood)

From these values, we know that:

:<math chem>V_\ce{O2} = (Q \times C_A) - (Q \times C_V)</math>

where
* ''C''<sub>A</sub> is the oxygen content of arterial blood, and,
* ''C''<sub>V</sub> is the oxygen content of venous blood.
This allows us to say

:<math chem> Q\ = \frac{{V}_\ce{O2}}{{C}_{A} - {C}_{V}} </math>

and therefore calculate ''Q''. (''C<sub>A</sub>'' – ''C<sub>V</sub>'') is also known as the [[arteriovenous oxygen difference]].{{cn|date=August 2022}}

While considered to be the most accurate method of measuring ''Q'', the Fick method is invasive and requires time for sample analysis, and accurate oxygen consumption samples are difficult to acquire. There have been modifications to the Fick method where respiratory oxygen content is measured as part of a closed system and the consumed oxygen is calculated using an assumed oxygen consumption index, which is then used to calculate ''Q''. Other variations use [[inert gas]]es as tracers and measure the change in inspired and expired gas concentrations to calculate ''Q'' (Innocor, Innovision A/S, Denmark).

The calculation of the arterial and venous oxygen content of the blood is a straightforward process. Almost all oxygen in the blood is bound to [[hemoglobin|hæmoglobin]] molecules in the red blood cells. Measuring the content of hæmoglobin in the blood and the percentage of saturation of hæmoglobin—the oxygen saturation of the blood—is a simple process and is readily available to physicians. Each [[gram]] of haemoglobin can carry 1.34 mL of [[oxygen|O<sub>2</sub>]]; the oxygen content of the blood—either arterial or venous—can be estimated using the following formula:

:<math chem>\begin{align}
\text{Oxygen content of blood} &= \left [\text{haemoglobin} \right] \left ( \text{g/dL} \right ) \ \times\ 1.34 \left ( \text{mL}\ \ce{O2} /\text{g of haemoglobin} \right ) \\
&\times\ \text{saturation of blood}\ \left ( \text{percent} \right )\ +\ 0.0032\ \times\ \text{partial pressure of oxygen} \left ( \text{torr} \right )
\end{align} </math>

===Pulmonary artery thermodilution (trans-right-heart thermodilution)===
[[File:Pulmonary artery catheter english.JPG|alt=Diagram of Pulmonary artery catheter (PAC)|thumb|Diagram of Pulmonary artery catheter (PAC)]]

The indicator method was further developed by replacing the indicator dye with heated or cooled fluid. Temperature changes rather than dye concentration are measured at sites in the circulation; this method is known as thermodilution. The [[pulmonary artery catheter]] (PAC) introduced to clinical practice in 1970, also known as the [[Swan-Ganz catheter]], provides direct access to the right heart for thermodilution measurements. Continuous, invasive, cardiac monitoring in intensive care units has been mostly phased out. The PAC remains useful in right-heart study done in cardiac catheterisation laboratories.{{citation needed|date=June 2015}}

The PAC is balloon tipped and is inflated, which helps "sail" the catheter balloon through the right ventricle to occlude a small branch of the pulmonary artery system. The balloon is then deflated. The PAC thermodilution method involves the injection of a small amount (10 mL) of cold glucose at a known temperature into the pulmonary artery and measuring the temperature a known distance away {{Convert|6–10|cm|inch|abbr=on}} using the same catheter with temperature sensors set apart at a known distance.{{citation needed|date=June 2015}}

The historically significant Swan-Ganz multi-lumen catheter allows reproducible calculation of cardiac output from a measured time-temperature curve, also known as the thermodilution curve. [[Thermistor]] technology enabled the observations that low CO registers temperature change slowly and high CO registers temperature change rapidly. The degree of temperature change is directly proportional to the cardiac output. In this unique method, three or four repeated measurements or passes are usually averaged to improve accuracy.<ref>{{cite journal | vauthors = Iberti TJ, Fischer EP, Leibowitz AB, Panacek EA, Silverstein JH, Albertson TE | title = A multicenter study of physicians' knowledge of the pulmonary artery catheter. Pulmonary Artery Catheter Study Group | journal = JAMA | volume = 264 | issue = 22 | pages = 2928–32 | date = December 1990 | pmid = 2232089 | doi = 10.1001/jama.264.22.2928 }}</ref><ref>{{cite journal | vauthors = Johnston IG, Jane R, Fraser JF, Kruger P, Hickling K | title = Survey of intensive care nurses' knowledge relating to the pulmonary artery catheter | journal = Anaesthesia and Intensive Care | volume = 32 | issue = 4 | pages = 564–68 | date = August 2004 | pmid = 15675218 | doi = 10.1177/0310057X0403200415 | doi-access = free }}</ref> Modern catheters are fitted with heating filaments that intermittently heat up and measure the thermodilution curve, providing serial ''Q'' measurements. These instruments average measurements over 2–9 minutes depending on the stability of the circulation, and thus do not provide continuous monitoring.

PAC use can be complicated by arrhythmias, infection, pulmonary artery rupture and damage to the right heart valve. Recent studies in patients with critical illnesses, sepsis, acute respiratory failure and heart failure suggest that use of the PAC does not improve patient outcomes.<ref name="Stevenson"/><ref name="Shah"/><ref name="Hall"/> This clinical ineffectiveness may relate to its poor accuracy and sensitivity, which have been demonstrated by comparison with flow probes across a sixfold range of ''Q'' values.<ref name="Phillips et. al. 2">{{cite journal | vauthors = Phillips RA, Hood SG, Jacobson BM, West MJ, Wan L, May CN | title = Pulmonary Artery Catheter (PAC) Accuracy and Efficacy Compared with Flow Probe and Transcutaneous Doppler (USCOM): An Ovine Cardiac Output Validation | journal = Critical Care Research and Practice | volume = 2012 | pages = 1–9 | year = 2012 | pmid = 22649718 | pmc = 3357512 | doi = 10.1155/2012/621496 | doi-access = free }}</ref> Use of PAC is in decline as clinicians move to less invasive and more accurate technologies for monitoring hæmodynamics.<ref>{{cite journal | vauthors = Alhashemi JA, Cecconi M, Hofer CK | title = Cardiac output monitoring: an integrative perspective | journal = Critical Care | volume = 15 | issue = 2 | pages = 214 | year = 2011 | pmid = 21457508 | pmc = 3219410 | doi = 10.1186/cc9996 | doi-access = free }}</ref>