Theoretically speaking, the Stewart-Hamilton equation should only be used under conditions of constant flow. During IPPV, respiration-induced cyclical flow modulations can cause significant errors in the estimation of mean cardiac output. A flow-correction algorithm, which halves the magnitude of this type of error, has recently been published 1.
Ten mls of crystalloid injectate are usually used to determine an output, although a smaller volume may be used if fluid overload is a potential problem 2. A room temperature solution can be used, but if a colder injectate is used, a better 'signal to noise' ratio results and an improvement in accuracy can be expected. (Ice-cold injectate does, however, carry an increased risk of inducing bradycardia - see 'Miscellaneous Complications').
The catheter should be attached to a cardiac output computer (or monitoring system) that displays a curve of change in temperature against time and that calculates the cardiac output and derived indices automatically. If such a computer is not available, the derived indices can be calculated by using the calculator on the 'Calculators' page of the simulator.
Before accepting a calculated value of output the temperature curve should be inspected. If the curve is irregular, that particular output calculation should be rejected. The area under the curve is an inverse measure of the cardiac output (Figure 2). (The lower the output, the less the dilution of cold injectate with warm blood and therefore the larger in area the temperature-change curve).
Thermodilution has become the most commonly employed method for estimating cardiac output, but it remains a relatively imprecise technique. The accuracy of the technique can be improved if a set of basic rules are observed.
1. Estimates of cardiac output should be made in triplicate and the average of the three values taken as the output measurement for that particular time period. Obvious 'rogue results' should be excluded from the averaging process.
2. A standard injection technique should be adopted. In particular, the injection should be made at a constant, consistent rate and the duration of injection should be brief (< 3 seconds) 3.
3. Thought should be given to the timing of the injection in relation to the phase of respiration. Cardiac output estimates vary cyclically according to the timing of the injection moment during the respiratory cycle although the phase and amplitude of this variation is not predictable 4.
Because the pulmonary artery occlusion pressure is best measured at the end of expiration, it is probably reasonable to suggest that determination of output also be made during the expiratory pause. This will have the effect of reducing the variability of multiple cardiac output measurements, although the averaged value will not necessarily be the most representative value of cardiac output for the complete respiratory cycle.
4. The same volume of injectate at the same temperature should be used for all series of determinations. If injectate volume is not accurately measured, output determinations will be proportionately inaccurate - a 0.5ml error in injectate volume resulting in a 10% error in cardiac output measurement 5.
5. If the PAC incorporates a separate venous infusion port, fluid administration through this port should be suspended (if reasonable) at the time of CO determination. The simultaneous administration of fluid via this port will tend to lead to an underestimation of CO 6.
6. Finally, it should be remembered that the thermodilution technique may be inaccurate in the presence of structural abnormalities of the heart. For example, in the presence of tricuspid incompetence, the method probably underestimates forward flow 7.
Even if all these precautions are observed by experienced clinicians, measured outputs within a single series will still vary by 10-20%. In spite of these shortcomings, the thermodilution technique is probably the best method of cardiac output determination available today. - It is far more useable than either the original Fick (Figure 4) or dye dilution (Figure 1) methods and is more accurate and less observer-dependent than any echocardiographic technique.
Continuous cardiac output measurement catheters:
It is possible to measure cardiac output 'continuously' using a specially modified PAC 8, 9. These 'CCO' catheters work on the thermal dilution principle, using a randomly pulsed heating filament situated in the right ventricular portion of the catheter. A rapid-response thermistor analyses the thermal transients, and in conjunction with specialised hard- and soft-ware calculates the cardiac output. The measurements which are obtained appear to be clinically acceptable, but may show a significant bias and lack of precision when compared to the traditional, intermittent thermal dilution technique.
More recently designed CCO systems use different modalities of heat delivery and computational techniques and appear to be associated with an improvement in the accuracy and precision of the output measurement when compared to the earlier systems 10.
These catheters may also incorporate a fibreoptic oximetric channel to permit continuous mixed venous oxygen saturation measurement. The new generation of pulmonary artery catheters has recently been reviewed by Nelson 11.
Other indicator dilution techniques:
The use of lithium chloride as an alternative indicator to indocyanine green has recently been described 12, 13. The drug is injected via a central venous catheter and its dilution curve measured in the arterial blood using a lithium-selective electrode. In animals, the method compares favourably with both conventional thermodilution cardiac output measurement and with electromagnetic flow measurement 14. As electrode technology improves, it is possible that such techniques will become more widely used.
Other methods of cardiac output measurement:
In ventilated patients with normal lung function, Parkin et al have recently developed a simple technique for non-invasive measurement of cardiac output using a derivative Fick technique (Figure 4) and carbon dioxide as the exchanging gas 15. Some other methods of determining cardiac output are described in the section entitled "Alternatives to the PAC".
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10. Seguin P, Colcanap O, Le Rouzo A et al Evaluation of a new semi-continuous cardiac output system in the intensive care unit. Can J Anaesth (1998) 45: 578-583
11. Nelson LD The new pulmonary arterial catheters. Right ventricular ejection fraction and continuous cardiac output. Crit Care Clin, 12(4):795-818 1996 Oct
12. Linton R; Band D; O'Brien T; Jonas M; Leach R Lithium dilution cardiac output measurement: a comparison with thermodilution. Crit Care Med, 25:1796-800, 1997
13. Linton RA; Linton NW; Band DM A new method of analysing indicator dilution curves. Cardiovasc Res, 30:930-8, 1995
14. Kurita T; Morita K; Kato S; Kikura M; Horie M; Ikeda K Comparison of the accuracy of the lithium dilution technique with the thermodilution technique for measurement of cardiac output. Br J Anaesth, 79:770-5, 1997 Dec
15. Parkin WG, Turner BR, Boyd H Four minutely automated cardiac output using simple capnography. Anaesth Intensive Care 27:94, 1999.
Last edited on: 13/11/2000