Thermodilution Cardiac Output (CO)

Stroke Volume = the volume of blood ejected from either ventricle during one contraction (usually we pay attention to left ventricular stroke volume.)

Heart Rate = the number of ventricular contractions within one minute

Thermodilution cardiac output measurements are computations of cardiac output, but the measurements are made very quickly and calculated with a computer.

Normal Range of CO = 4 – 6 L/min

© Edwards Lifesciences, LLC, as reproduced by Albert, N.M. (2005). Cardiac output measurement techniques (invasive).  In D.J.L.Wiegand & K.K. Carlson (Eds)  AACN Procedure Manual for Critical Care (5th ed., p. 487).  St. Louis: Elsevier Saunders.

Thermodilution cardiac output measurements require a pulmonary artery (PA) catheter and are calculated by a computerized monitoring system based on temperature changes in the heart when a cooler solution is injected into the right atrium via the proximal port of a pulmonary artery catheter. 

1. A cooler solution–D5W or NS either chilled in an ice bath or at room temperature–is injected by a 10 ml bolus syringe rapidly through the proximal port of the PA catheter into the right atrium.  (Notice the temperature probe connected to the injection site to record the solution's temperature before entering the circulatory system.)  This solution is often referred to as the injectate.  Cardiac output measurements can be made with smaller boluses of 5 ml or even 3 ml of injectate.  [Injections are made with sterile technique.  A rule-of-thumb in clinical practice is not to hold onto the barrel of the syringe of the injectate; that will warm the injectate before it reaches the right atrium.]

2. A momentary fluctuation in temperature occurs in the blood circulating through the heart; this fluctuation in temperature of the blood is recorded by the thermistor port which is located near the distal end of the PA catheter in the pulmonary artery. 

3. The fluctuation in temperature (y-axis) produces a graphed curve over time (x-axis) on the computerized monitoring system.  The monitor computes a cardiac output value based on a calculus formula from the graphed curve.

4. Generally, the nurse will make 5 cardiac output measurements with 5 separate injections.

5. Then the nurse will average the three closest readings of cardiac output to determine the patient's cardiac output.

6. In measuring a patient's intake, it is important not to forget the 50 ml that are injected during each time cardiac output measurements are collected.

Thermodilution cardiac output measurements ARE NOT PRESSURE RECORDINGS.  Therefore, the leveling of the transducers does not affect the results.  However, when cardiac output measurements are  made, usually the nurse will also record all of the pressure readings for PA pressures, PAOP, CVP, and arterial blood pressures.  These pressure readings require the transducers to be leveled with the patient's phlebostatic axis.

Video of Bolus Thermodilution Cardiac Output Measurements

http://media.wiley.com/CurrentProtocols/PH/ph0556/ph0556-fig-0005-1-full.gif

• Cardiogenic shock
• Cardiac surgery in post-operative care
• Cardiac valvular diseases
• Conditions requiring detailed assessment of the cardiovascular system

Continuous Thermodilution Cardiac Output

More modern technologies for thermodilution cardiac output allow for continuous monitoring of CO through heating of blood in the right atrium (with a thermal filament) every 30 – 60 seconds.  The monitoring system shows an average cardiac output averaged from readings over the past 3 – 6  minutes.  Continuous cardiac output monitoring is most reliable when the patient's core body temperature is between 30 – 40^oC.

Unreliable readings of continuous thermodilution cardiac output can be caused by:

• Febrile patient (T > 40^oC)
• Rapid infusion of cool IV solutions through central line ports upstream from thermistor on PA catheter.

Animation Video of Continuous Cardiac Output monitoring

Lithium Dilution Cardiac Output (LiDCO)

Cardiac output can also be monitored via an arterial line instead of a pulmonary artery catheter.  This reduces risks associated with a pulmonary artery catheter such as sepsis or fluid overload from thermodilution injections.  The LiDCO system operates under an indicator dilution principle with lithium in subtherapeutic doses being administered intravenously and measured at an arterial sensor.  Continuous CO readings are based on arterial waveform analysis after being calibrated with lithium injection.   Recalibration is required every 4 hours to insure accuracy of cardiac output readings.

Contraindicated for patients receiving lithium therapy!

©Jonas, M., Hett, D., & Morgan, J. (2002). Real time, continuous monitoring of cardiac output and oxygen delivery.  International Journal of Intensive Care, 9(1), http://www.pulseco.com/intensive_care.html

(picture) http://www.pulseco.com/images/lidco-pulsco.gif

Transesophageal Echocardiography (TEE)

Transesophageal echocardiography is a less invasive method for monitoring cardiac output, but the equipment requires an advanced competency working with echocardiography technology.  An esophageal doppler is advanced down the esophagus to the location of approximately T5-T6 vertebrae. 

http://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Transesophageal_echocardiography_diagram.svg/524px-Transesophageal_echocardiography_diagram.svg.png

(Computed Parameter)

Cardiac Index (CI) = CO/BSA

CO = Cardiac Output

BSA = Body Surface Area

(Cardiac index is a more accurate reflection of myocardial strength or contractility because it places cardiac output in the context of the patient's size and weight.)