Thermodilution cardiac output measurement. Effects of the respiratory cycle on its reproducibility.

Thermodilution cardiac output measurements are commonly employed in the management of critically ill patients. Serial measurements often show significant variation, and poor reproducibility limits their clinical utility. There are no clinical studies revealing when to perform thermodilution cardiac output measurements in relation to the respiratory cycle. We prospectively studied 32 patients in a randomized scheme comparing three thermodilution cardiac output measurements at peak-inspiration, at end-exhalation, or randomly in spontaneously breathing and mechanically ventilated patients. Saline injections initiated at peak-inspiration or end-exhalation resulted in cardiac output measurements with much smaller standard deviations than those seen with random injections. Thermodilution cardiac output measurements performed at random times in the respiratory cycle should be avoided, and we recommend initiating these measurements at end-exhalation.

Reliability of the thermodilution method in the determination of cardiac output in clinical practice.

The thermodilution (TD) method of determining cardiac output (CO) is widely used in clinical practice, but its reliability in this setting is not well understood and may be poor. This is especially true when dealing with unstable patients and using commercial TD devices. Numerous published reports have shown markedly variable results when comparing simultaneously the TD method and the Fick or dye-dilution methods in catheterization laboratories, intensive care units, or operating rooms. In order to assist clinicians in interpreting the reliability of the TD method in measuring CO, we analyzed all available published data (14 reports). The differing results in these reports were evaluated by standard statistical methods and by an extension of the influence function method of structural analysis developed to differentiate reproducibility and accuracy errors of each technique. Comparing the accuracy of the TD method with that of the Fick or the dye-dilution methods reveals that the 3 methods are of equal merit and can be used as independent references. Our reproducibility data show that when using commercial TD devices there must be a minimal difference of 12 to 15% (average, 13%) between determinations of cardiac output (3 measurements per determination) to suggest clinical significance. Minimal differences of 20 to 26% (average, 22%) are required between determinations when using 1 measurement per determination. There was no difference in the quality of the TD method when comparing rapid injection of iced or room temperature thermal indicator.

Pulmonary insufficiency induced by oleic acid in the sheep: a model for investigation of extracorporeal oxygenation.

Most previous studies of the efficiency of bypass techniques for respiratory support have been conducted in hypoxic but otherwise normal animals. However, mechanisms of improved oxygenation by partial venoarterial bypass in the presence of acute respiratory insufficiency can be better studied with an appropriate pathophysiologic model; for this purpose, acute hemorrhagic pulmonary edema was induced in sheep by injection of oleic acid into the right atrium. The model presented a 3 hour period of elevated pulmonary shunting with stable hemodynamics. This preparation is being employed to the study of the mechanisms of extracorporeal oxygenation.

Retrograde aortic perfusion by partial cardiopulmonary bypass: effect of mixing on measured arterial oxygen tension.

Partial venoarterial bypass with return of oxygenated blood to the femoral artery can produce mixing in the aorta of the blood from the oxygenator and left ventricular sources at flow rates of less than 50% of the total body flow. This was observed in six anesthetized sheep with normal lungs maintained on controlled ventilation on partial cardiopulmonary (venoarterial) bypass. The evidence of mixing increases with the extracorporeal flow rate, reaching the aortic arch in a majority of cases at the higher rates. Management of bypass and the interpretation of its effects depend on definite knowledge of the presence or absence of mixing. Partial bypass will raise the arterial oxygen tension (Pao-2) of the blood continuing to flow through the lungs. This effect is independent of mixing (Pao-2) and may be related either to the addition of oxygen to the blood or to an enhanced pulmonary ventilation-perfusion relationship.