What is the clinical presentation of cardiac output in pulmonary artery catheterization (PAC)?

Updated: Dec 22, 2017
  • Author: Bojan Paunovic, MD; Chief Editor: Karlheinz Peter, MD, PhD  more...
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Answer

Cardiac output

CO can be determined via the PAC by several methods. It can be determined by using the Fick principle, which is a variation of the law of conservation and states that consumption of a substance must equal the product of blood flow to the organ and the difference between the arterial and venous concentrations of the substance. In this circumstance, the substance is oxygen, and CO is determined by the following formula:

CO equals oxygen consumption per minute (VO2) divided by arterial oxygen content (CaO2) minus mixed venous oxygen content (CvO2)

CO is determined by using systemic arterial and PA blood samples, and by measuring or estimating VO2. The Fick method is most accurate when the CO is low and the arterial-venous oxygen difference is high. Unfortunately, in critically ill patients, establishing a steady-state and estimating or measuring VO2 is difficult; thus, the reliability of this technique is poor.

The indicator-dilution technique is more accurate and reproducible. A known amount of dye (indocyanine green) is injected into the PA. Arterial blood is withdrawn from the aorta as the dye circulates, and a concentration-versus-time curve is derived. The first-pass curve is used to determine CO, which is calculated by dividing the initial mass of the injectate by the average concentration. This value then is corrected (60 s/time of the curve) to obtain CO. This procedure requires considerable blood sampling and is time consuming, because recirculation of the dye complicates the calculation.

Many PACs also allow CO to be measured by using a variation of the indicator-dilution method known as the thermodilution method. This method is more efficient because the injectate does not recirculate to a significant degree and no blood sampling is necessary. A saline bolus of known volume (5-10 mL) and temperature (usually ≤25ºC) is injected through the proximal (RA) lumen. The thermistor at the end of the PAC monitors the change in blood temperature, and a temperature-versus-time curve is generated (see image below).

Principle of cardiac output measurement. Principle of cardiac output measurement.

The change in temperature as warm venous blood dilutes the injectate is inversely proportional to the derived CO. The Stewart-Hamilton formula shows this relationship: CO equals the volume of injectate multiplied by blood temperature minus injectate temperature multiplied by computation constants, and divided by change of blood temperature as a function of time (area under the curve).

Understanding this formula allows discernment of artifact errors that can lead to underestimation or overestimation. Loss of injectate or inadvertent administration of a volume lower than required results in a low-amplitude temperature-versus-time curve that produces a falsely elevated CO value. Causes of this are system leak, right-to-left intracardiac shunts, inappropriately rapid injection, and a poorly positioned PAC. Conversely, too much injectate or too slow an injection leads to a falsely low CO reading. Temperature errors can occur when continuous infusions are used. Thrombus or vessel wall impingement can alter the thermistor function.

Physiologic causes for CO measurement discrepancies include tricuspid and pulmonary regurgitation, which may produce recirculation peaks and thus increase the area under the curve, resulting in a falsely low CO estimate. Arrhythmias alter steadiness of PA flow and may cause difficulty in obtaining a consistent CO.

CO alterations occur during the respiratory cycle and are accentuated by respiratory distress and positive pressure ventilation. Proper timing of the injection to the same phase of respiration (preferably end-expiration) provides more consistent measurements. Averaging the values of 3 injections is recommended to minimize sampling errors.

While CO is one of the most important measurements that the PAC provides, the absolute value should be normalized for the size of the patient. To account for this, the cardiac index (CI), which equals CO divided by body surface area [BSA], is calculated. The physician should keep in mind that, as independent variables, CO and CI are of limited use for assessing tissue perfusion because these must be interpreted along with other clinical data.


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