An exact science is dominated by the idea of approximation.
A few years after its introduction, the pulmonary artery (PA) catheter was upgraded by incorporating a thermistor at the catheter tip to measure blood flow using the thermodilution principle. This single addition to the PA catheter increased its monitoring capacity from 2 parameters (i.e., central venous pressure and wedge pressure) to 10 parameters (see Tables 10.1 and 10.2). More recent refinements in thermodilution methodology have further increased the monitoring capacity of the PA catheter, adding the ability to measure the ejection fraction of the right ventricle and to monitor cardiac output continuously. This chapter describes each of the thermodilution methods for monitoring cardiac performance.
BASIC THERMODILUTION METHOD
Thermodilution is an indicator-dilution method of measuring blood flow. This method is based on the premise that when an indicator substance is added to circulating blood, the rate of blood flow is inversely proportional to the change in concentration of the indicator over time. The indicator substance can be a dye (dye-dilution method) or a fluid with a different temperature than blood (thermodilution method). The thermodilution method of measuring cardiac output with a PA catheter is shown in Figure 12.1 (1-3). A dextrose or saline solution that is colder than blood is injected through the proximal port of the catheter in the right atrium. The cold fluid mixes with blood in the right heart chambers, and the cooled blood is ejected into the pulmonary artery and flows past the thermistor on the distal end of the catheter. The thermistor records the change in blood temperature with time and sends this information to an electronic instrument that records and displays a temperature-time curve like the one shown in Figure 12.1. The area under this curve is inversely proportional to the rate of blood flow in the pulmonary artery. In the absence of intracardiac shunts, this flow rate is equivalent to the (average) cardiac output.
Examples of thermodilution curves are shown in Figure 12.2. The low cardiac output curve (upper panel) has a gradual rise and fall, whereas the high output curve (middle panel) has a rapid rise, an abbreviated peak, and a steep downslope. Note that the area under the low cardiac output curve is greater than the area under the high output curve; that is, the area under the curves is inversely related to the flow rate. Electronic cardiac monitors integrate the area under the temperature-time curves and provide a digital display of the calculated cardiac output. There is a tendency to rely on this digital display of the calculated cardiac output without examining the temperature-time curve, and this can lead to errors in interpretation.
Cardiac output can be 30% higher in the supine position than in the semierect position (4). Therefore, consecutive cardiac output determinations should be performed with each patient in a uniform position, or the position of the patient should be recorded with each cardiac output determination.
INJECTING THE INDICATOR
Bolus injection of normal saline (0.9% sodium chloride) or 5% dextrose-in-water produces the most satisfactory measurements (3). Other injectate solutions can produce variable results (because of their varying specific heats) and are not recommended.
Injectate Volume and Temperature
The indicator solution can be cooled in ice or injected at room temperature, and can be administered in a volume of 5 mL or 10 mL. In general, higher-volume, lower-temperature injectates produce the highest signal-to-noise ratios, and thus the most accurate measurements (1-3,5). However, room temperature injectates (which are less tedious to administer than iced injectates) produce reliable measurements in most critically ill patients (6-9). When the indicator fluid is injected at room temperature, the large (10 mL) injection volume produces the most reliable results. When using smaller injectate volumes, using iced injectates increases the reliability of the measurements. Using small volumes of room temperature injectates can yield inaccurate results in low cardiac output states (5), and is thus not recommended.
Optimal results are produced when the bolus injection is completed within 2 seconds (10), but satisfactory results are obtained with injection times up to 4 seconds. Longer injection times can produce falsely low measurements.
Cardiac output can vary significantly during the respiratory cycle, particularly during mechanical ventilation. Random thermodilution measurements obtained in different phases of the respiratory cycle can vary by more than 10%, whereas injections that are timed to the end of expiration can reduce the variability to 5% (11). This has led to the recommendation that thermodilution cardiac outputs always be recorded at end-expiration. However, it is very difficult to time injections so that the thermodilution curve is recorded at precisely the same time in the respiratory cycle. In fact, the injection time can be longer than the duration of the respiratory cycle in patients with rapid breathing; for example, at a respiratory rate above 15 breaths/minute, an injection time of 4 seconds is longer than the duration of each respiratory cycle (less than 4 seconds). It is best in such cases to begin injecting the indicator solution at the same part of the respiratory cycle for each cardiac output measurement.
Alternative Injection Ports
If the proximal (right-atrial) port of the PA catheter is obstructed, the injectate can be introduced through an alternative infusion port on the catheter (if one is available) (8) or through the side arm of the introducer catheter (see Fig. 10.1) (9).
ACCURACY AND RELIABILITY
NUMBER OF MEASUREMENTS
Serial measurements are recommended for each cardiac output determination. Three measurements are sufficient if they differ by 10% or less. Cardiac output is determined by averaging the serial measurements. The initial measurement is often falsely elevated (7), so for optimal accuracy, the initial measurement should be discarded. Serial measurements that differ by more than 10% should be considered unreliable (12).
Tricuspid regurgitation causes the cold indicator fluid to be recycled back and forth across the tricuspid valve. This produces a prolonged, low-amplitude thermodilution curve like the one shown in the lower panel of Figure 12.2. This type of curve is an exaggerated version of a low-output curve (with a large area under the curve), so tricuspid regurgitation produces a falsely low thermodilution cardiac output (13). Tricuspid regurgitation may be common in mechanically ventilated patients because of the high right-sided cardiac pressures created by positive-pressure lung inflations. Therefore, this condition may be a common source of error in thermodilution cardiac outputs in the ICU.
Low cardiac outputs produce low-amplitude temperature-time curves, and this can affect the accuracy of thermodilution cardiac outputs by decreasing the signal-to-noise ratio. Accuracy is most affected when room temperature indicator solutions are injected in low (5 mL) volumes. In this situation, thermodilution can underestimate cardiac output by as much as 30% (5). In low-output states (cardiac index below 2.5 L/minute/m2), an iced injectate gives the most accurate measurements if low injectate volumes are used.
Intracardiac shunts produce falsely high thermodilution cardiac output measurements. In right-to-left shunts, a portion of the cold indicator fluid passes through the shunt, thereby creating an abbreviated thermodilution curve (similar to the abbreviated high-output curve). In left-to-right shunts, the thermodilution curve is abbreviated because the shunted blood increases the blood volume in the right heart chambers, and this dilutes the indicator solution that is injected.
Thermodilution cardiac output can vary by as much as 10% without a change in the clinical condition of the patient (14). This means that a baseline cardiac output of 5 L/min can vary spontaneously from 4.5 to 5.5 L/min (or a baseline cardiac index of 3 L/min/m2 can vary from 2.7 to 3.3 L/min/m2) without representing a change in the clinical condition of the patient. Therefore, the thermodilution cardiac output (or cardiac index) must change by more than 10% for the change to be considered clinically significant.
THERMODILUTION EJECTION FRACTION
In the mid-1980s, a PA catheter was introduced with a fast-response thermistor capable of measuring the ejection fraction of the right ventricle (15). This added the ability to evaluate right-ventricular function at the bedside.
Rapid-response thermistors can record the temperature changes associated with each cardiac cycle. This produces a ramplike thermodilution curve like the one shown in Figure 12.3. The change in temperature between each plateau on the curve is caused by dilution of the cold indicator fluid by venous blood that fills the ventricle during diastole. Because the volume that fills the ventricles during diastole is equivalent to the stroke volume, the temperature difference T1 – T2 is the thermal equivalent of the stroke volume. Thus, the points at each end of the temperature change can be taken as the thermal equivalents of the end-diastolic volume (T1) and end-systolic volume (T2), respectively. Because the ejection fraction is the ratio of stroke volume (SV) to end-diastolic volume (EDV), the right-ventricular ejection fraction (RVEF) can be derived using the appropriate thermal equivalents:
RVEF = SV/EDV
RVEF = (T1 – T2)/T1
Normal thermodilution RVEF is 0.45 to 0.50. This is slightly lower than radionuclide RVEF (the gold standard), but the difference is less than 10% (16). Thermodilution RVEF can be measured reliably using a room temperature injectate (given as a 10-mL bolus) (17).
Because the thermodilution PA catheter can measure stroke volume, RVEF can be used to derive the right-ventricular end-diastolic volume (RVEDV):
RVEDV = SV/RVEF
This allows for a determination of ventricular preload (end-diastolic volume) at the bedside, and bypasses the shortcomings of end-diastolic pressure (e.g., the central venous pressure) as a reflection of preload (18).
CONTINUOUS CARDIAC OUTPUT
The most recent development in the thermodilution method has led to the introduction of a PA catheter that can monitor cardiac output continuously, without the need for intermittent bolus injections of indicator fluid (19). This catheter (Baxter Edwards Critical Care, Irvine, California) is equipped with a 10-cm thermal filament located 15 to 25 cm from the catheter tip. The filament generates low-energy heat pulses that are transmitted to the surrounding blood. The resulting change in blood temperature is then used to generate a thermodilution curve for determining cardiac output. Although the output is called continuous cardiac output, the measurement is an average cardiac output recorded over 3-minute time intervals and updated every 30 to 60 seconds.
Continuous thermodilution cardiac output monitoring has proven to be both safe and reliable (19,20). However, it has yet to gain widespread popularity, probably because of the added cost of the newer technology. Nevertheless, the advantages of a continuous measure of cardiac output (e.g., more on-line information regarding trends in cardiac output) seem to justify the additional cost of the technology in patients with unstable or severely compromised hemodynamic function.