CARDIAC OUTPUT VIA THERMODILUTION METHOD

 

Cardiac output is the volume of blood being pumped by the ventricle of the heart in a minute.  Many diseases of the cardiovascular system are associated with changes in the cardiac output such as hypertension.  Cardiac output changes can also be associated with non-cardiovascular disease processes such as sepsis.  In this case one would often see a significant rise in the cardiac output.  Decreased cardiac output states are seen in patients with cardiomyopathy and heart failure.  The ability to monitor cardiac output increases the accuracy in the diagnosis of a disease process and can be a useful guide in determining a course of treatment for that disease process.

Cardiac Output is the blood flow out of the heart in L/min.  Normal Cardiac Output is 4.0-8.0L/min.  ( CI=2.5-4.0 L/min/M2 ).  Cardiac Output =Heart Rate*SV.  Cardiac index standardizes the cardiac output for body size.  For this one needs to have the patients weight and height.

There are various methods of measuring the cardiac output.  The thermodilution method is a very popular method used in the measuring of cardiac output.  This method requires the insertion of a thermodilution pulmonary artery catheter; know as a Swann Ganz catheter.  Placement of the catheter plays an important role in the measurement of cardiac output.

The usual sites for insertion of the Swann Ganz catheter are either the subclavian vein or the internal jugular artery.  Other sites include the femoral vein and the anterior cubital vein.  The site of choice is the subclavian vein for reasons of infection control.  Occlusiveness of the dressing over the insertion site is much easier to achieve at this area.  The next best place for insertion is the internal jugular vein.  Use of the femoral vein is not an optimal area for the insertion site due to the possibility of urine or stool damaging the integrity of the dressing.  The femoral vein has a higher risk of infection than the subclavian vein or internal jugular vein sites.  An important role for the ICU nurse is to maintain occlusive, dry dressings over central line insertion sites.

The Swann Ganz catheter has a balloon at its tip which when inflated allows for easier placement of the catheter by the floatation technique.  This procedure can be done at the bedside or in the cath lab using fluoroscopy to visualize while inserting the catheter.  There is a balloon at the end of the Swann Ganz catheter that provides the means with which to measure the PCW pressure by the balloon forming a seal that isolates the tip from the pulmonary artery ( PA ) flow.

The terminal end of the catheter has a connector for the thermistor to obtain blood temperature for cardiac output measurement and to monitor the temperature of blood continuously.

The thermodilution technique is based upon the principle that a known volume of saline, at a known temperature, is added to an unknown volume of fluid, the volume of the fluid may be determined by measuring the temperature of the mixture.  The cardiac output computer can then calculate the cardiac output.

The method to obtain the cardiac output involves

                     drawing up 10ml of normal saline

                          injecting this fluid rapidly and smoothly into the right atrium via the proximal port of the catheter

            this ensures uniform mixing of the solution with the blood returning to the right heart

            temperature change is measured in the PA by the thermistor

  Accuracy of this method of obtaining the cardiac output involves the following:

 unnecessary and excessive handling of syringes which could change the injectate temperature

 careful measurement of the volume of the injectate

  speed and consistency of injection ( <4 seconds )

   timing the infections so that they occur during the same phase of respiration…the end of expiration is the preferred phase however in practice the timing of injections is difficult to achieve

   to ensure the most accurate value for the cardiac output it is recommended to use the average of 3 injections each of which should be within 10% of each other

The presence of valvular insufficiency or intracardiac shunt will generally make cardiac output measurement via the thermodilution method inaccurate and unreliable.

Advantages of the thermodilution method of measuring cardiac output:

         Only one catheter is required, which can be inserted at the bedside

         Blood withdrawal is not required

         Serial measurements can be performed rapidly

         The results are not affected by oxygen administration

 

Disadvantages of the thermodilution method of measuring cardiac output:

       Possible complications from having an indwelling PA catheter

         Fluid overload may be a problem if numerous and frequent measurements are required.  This is especially true in patients that are already in a fluid overload state

        Potential for injectate contamination

         Reports of transient bradycardia and atrial fibrillation occurring with the use of iced injectate

         Less accurate in low cardiac output states and is definitely not accurate in the presence of intracardiac shunts or valvular insufficiency. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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