University of Virginia Health System

The Physiology:

 Pulmonary Artery Catheters

  As we’ve learned, pulmonary artery catheters (PACs) can assist physicians caring for patients undergoing an array of procedures.  However, in anesthesiology, one of the most important functions of a PAC is to provide accurate, precise, frequent information regarding the cardiac function and hemodynamic state of a patient undergoing a surgical procedure.  Two measurements obtained with a PAC which are most helpful, and which can enable the anesthesiologist to calculate other aspects of cardiac function are:

• Pulmonary capillary wedge pressure
• Cardiac output

Pulmonary capillary wedge pressure :

       Pulmonary capillary occlusion pressure (PCOP, also called pulmonary artery wedge pressure ) is measured when the balloon on the tip of the PAC is inflated within a pulmonary artery.  This enables the catheter to obtain an indirect measurement of left ventricular end diastolic pressure (normal range 6-12 mmHg)  How does it do this? 

     Analogy:  Imagine a small creek.  First standing on the edge of the creek and looking at the water, it is difficult to know how that water is flowing, and what may be occurring further downstream that you can’t see.  Imagine then that you could instantly place a temporary dam in the creek.  Now, by looking at what the water does on the downsteam side of the dam, you can infer what may be going on further down the creek.  If the water quickly drops, then you can surmise that the creek is flowing rapidly and nothing is obstructing its path.  However, if the water slowly drops, you could guess that something must be holding up the flow of the creek - perhaps the water is simply flowing slowly, or perhaps a beaver has made his home downstream! 

     When the balloon is inflated, the pressure monitor at the tip of the catheter is shielded from all right sided pressures, and “sees” only what is happening “downstream, “ namely the pressures from the left atrium and left ventricle. 

So, in cases where left and right cardiac function are discordant, PCOP is a valuable tool with which to measure left sided heart function by way of filling pressures independently from right sided cardiac function.

Limitations to Pulmonary Capillary Occlusion Pressure:

       The assertion that PCOP is an accurate reflection of left cardiac function is based on certain assumptions regarding the patient’s cardiopulmonary function.  PCWP is accurate as a predictor of left ventricular end diastolic volume only IF the vascular system between the catheter tip and the left ventricle is free from any pathology which could influence the pressures detected by the catheter.  Typically, pulmonary artery wedge pressures are equivalent to left ventricular end diastolic volumes over a range of 5-25 mmHg. 

• Instances where PCWP overestimates LVED pressure include those which create an interfering pressure gradient, but do not represent the function of the left ventricle:
  • Chronic Mitral Stenosis
  • PEEP (Positive end expiration pressure ventilation)
  • Left atrial myxoma
  • Pulmonary Hypertension

• Instances where PCWP underestimates LVED pressure include those that increase the pressure in the left ventricle which the catheter tip cannot detect:
  • Stiff Left Ventricle
  • LVED pressure > 25mmHg
  • Aortic Insufficency

Note that pulmonary artery rupture is one of the most serious complications arising from use of a pulmonary artery catheter.  This usually results from the catheter tip being too distal and “overwedging” the balloon in an attempt to obtain pulmonary capillary wedge pressure.  A way to decrease the incidence of this occurrence is to wedge the catheter less frequently.  Often, while the balloon is deflated, the pulmonary artery end diastolic pressure is used to estimate PCWP; this provides a constant estimate of left ventricular end diastolic volume while decreasing the risk of pulmonary artery rupture.  In the absence of increased pulmonary vascular resistance, the difference between pulmonary artery end diastolic pressure and pulmonary capillary wedge pressure is 1-4 mmHg .

Thermodilution and cardiac output :

   Another important measurement that can be obtained with a pulmonary artery catheter is cardiac output. (Remember, CO(L/min) = stroke volume x heart rate.)  This can be a vital piece of information to help with a diagnosis, make decisions concerning theraputic interventions, and assess prognosis.  The PAC measures cardiac output with a technique called thermodilution.

 How is this performed?

• A small quantity (2.5-10ml) of cool fluid is injected into the right atrium from the proximal opening in the PAC.
• A thermistor embedded in the tip of the PAC lying in the pulmonary artery detects a temperature change as the cooler blood flows by.  The degree of change in the temperature is inversely proportional to the cardiac output.
  • Increased blood flow (and C.O.) = Minimal temperature change
  • Decreased blood flow (and C.O.) = Pronounced temperature change

• Plotting this temperature change against the time it took for the cooler fluid to reach the thermistor gives us a thermodilution curve .  The equation for this curve is given by the Stewart-Hamilton equation:

             (V₁ (T_b – T₁) K₁ K₂ ) 

 Q      =  _________________

            (T_b (t) dt)

Where:

Q = cardiac output

V₁ = injectate volume

T_b = blood temperature

T₁ = injectate temperature

K₁   = density factor

K₂  = a constant

T_b(t) dt = change in blood temperature as a function of time

        Thankfully, a computer built into the monitor calculates this for the anesthesiologist, integrates the area under the thermodilution curve, and gives a digital readout of the cardiac output in L/min.  Isn’t technology great?

Further Calculations :

    There are many useful hemodynamic variables that can be calculated from the cardiac output along with other information obtained by a pulmonary artery catheter:

       Obviously, the information obtained from pulmonary artery catheters can be very useful when dealing with complex cardiac and hemodynamic situations.

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