Cardiac Output, Thermodilution
Norm of Cardiac Output, Thermodilution
4–8 L/minute.
Usage of Cardiac Output, Thermodilution
Evaluation of hemodynamic instability (heart failure, pulmonary hypertension) and shock states, determination of optimal myocardial function preoperatively by Starling curve, and evaluation of response to fluid administration and inotropic drugs. Cardiac output is performed to determine the amount of blood being propelled forward by the heart.
Description of Cardiac Output, Thermodilution
Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV). It is the volume of blood ejected from the heart over a period of 1 minute. The determinants of cardiac output are preload, afterload, and heart rate in beats per minute and stroke volume in milliliters per beat (CO = HR × SV). Stroke volume is the volume of blood ejected with each ventricular contraction and is the difference between the volume of the left ventricle at end diastole and the volume remaining in the ventricle at end systole. In an average-sized adult at rest, cardiac output is approximately 4–8 L/min. In diseased states, cardiac output is usually found to be less than normal and may be so low that an adequate blood supply to the body's tissues cannot be delivered. A low cardiac output may be the result of poor filling of the ventricle (reduced preload) or poor forward emptying of the ventricle (increased afterload). Some causes of low resting cardiac output are diminished myocardial function resulting from myocardial infarction, aortic stenosis, arterial hypertension, and cardiomyopathy. The thermodilution method of cardiac output determination measures the change in core temperature in the pulmonary artery before and after injection of a specific quantity of injectate of a known temperature. The change in temperature reflects the cardiac output in an inverse manner and is used to plot a cardiac output/thermodilution curve. A low cardiac output produces a greater change in temperature for a longer period of time than does a high cardiac output.
Professional Considerations of Cardiac Output, Thermodilution
Consent form IS required for insertion of pulmonary artery catheter.
Risks
Pulmonary embolus from dislodgement of clot on catheter. See Pulmonary artery catheterization for catheter-specific risks and contraindications.
Contraindications
None. Injections should be kept to the minimum volume needed for clients who are fluid overloaded.
Preparation
- Client must have a pulmonary artery catheter in place.
- Obtain cardiac output tubing, a 10-mL syringe, and injectate. Also obtain ice if injectate is less than 10 degrees C cooler than the client's core temperature. Iced injectate should also be used for hemodynamically unstable clients and hypothermic clients.
- Just before beginning the procedure, take a “time out” to verify the correct client, procedure, and site.
Procedure
- The client may be positioned up to 60 degrees of head-of-bed elevation but should be positioned similarly for each cardiac output measurement. Hemodynamically unstable clients should be positioned supine.
- Cardiac output is performed through a 2- or 3-lumen pulmonary artery catheter. A 3-lumen catheter contains two lumens that exit into the right atrium for measurements of central venous pressure, cardiac output injection, and fluid infusions and one lumen that exits the pulmonary artery, plus a thermistor at the distal catheter tip in the pulmonary artery for measurement of core blood temperature.
- A computation constant is selected for the specific injectate temperature and quantity, and the catheter in use is entered into the computer that will calculate cardiac output. The injectate used must be at least 10 degrees C cooler than the client's core temperature for the most accurate thermodilution curve.
- After the catheter placement has been verified, a bolus of 5 or 10 mL of iced or room-temperature intravenous fluid (D5W or NS) is injected into the external catheter port that exits into the right atrium. The injection should begin as the client begins exhalation and should be completed within 4 seconds.
- As the fluid exits into the right atrium, it cools the blood that is in the right atrium. This volume of cooled blood moves into the right ventricle and then into the pulmonary artery.
- In the pulmonary artery, the catheter thermistor senses the temperature change as the cooled blood passes over it. The thermistor will record a decrease in temperature followed by a gradual return to body temperature as the cold solution flows distally. The resulting temperature change is plotted on a temperature/time curve by the cardiac output computer.
- Generally, three cardiac output readings are obtained and averaged to calculate cardiac output. However, the procedure may be stopped and the cardiac output calculated if the second measurement is within 10% of the first measurement.
Postprocedure Care
- Resume slow flush infusion to maintain patency of the cardiac output lumen, if used before injection.
Client and Family Teaching
- The client will not feel injections.
Factors That Affect Results
- Too much or too little of injectate solution injected will produce erroneous values.
- Injection not completed within 4 seconds will produce a falsely high value.
- If the catheter is kinked, the cardiac output value will be falsely high.
- If the catheter is not inserted far enough for the cardiac output port to be distal to the tip of the introducer (sheath, Cordis), retrograde injection into the Cordis will produce a falsely high cardiac output.
- Changes in stroke volume resulting from dysrhythmias or changing heart rates can produce wide variations in serial cardiac output readings.
- An incorrect catheter computation constant entered into the cardiac output calculation will produce an erroneous value.
Other Data
- One single duration-controlled injection thermodilution measurement is as accurate as the mean of four phase-controlled measurements.
