Pulmonary artery catheter; indications and use by nursesnote

 Pulmonary artery catheter

Introduction

The use of the balloon-tipped, flow-guided catheter to measure the fulling pressure of the left side of the heart was first described in 1970 by Swan. The use of the device to measure the cardiac output by a thermodilution method was described in the same paper by Ganz. In the intervening 30 years various other devices have been developed to measure the cardiac output, but the pulmonary artery (Swan-Ganz) catheter has remained the standard against which the other devices have been judged.

Indications

Despite > 30 years of international experience with the pulmonary artery catheter, its use has never been validated in an adequately powered RCT. Indications for use are therefore based on expert opinion and consensus statements from a variety of international societies. Potential indications for use of the pulmonary artery catheter as descried by Swan and Ganz include:

• Establishing the aetiology of shock states (i.e. cardiogenic vs hypovolaemic vs septic vs obstructive shock)
• Diagnosis of pulmonary hypertension and assessment of the response to treatment.
• Differentiation between cardiac and non-cardiac causes of pulmonary oedema.
• Monitoring and management of AMI.
• Monitoring and management of cardiac performance when restoring spontaneous circulation after cardiopulmonary bypass.
• Monitoring of fluid balance in patients where this is difficult clinically , e.g. burns patients, sepsis with capilary leak.
• Assessment of response to inotropic drugs or vasopressors.
• Peri-operative optimization of oxygen delivery in high-risk surgical patients.

Modifications of the classic Swan-Ganz pulmonary artery catheter also allow.

• Continuous measurement of cardiac output
• Temporary cardiac pacing
• Continuous monitoring of mixed venous oxygen saturation to monitor adequacy of global oxygen delivery.

Use of the pulmonary artery catheter

Use of the pulmonary artery catheter allows the direct measurement of a variety of haemodynamic parameters:

• Right atrial pressure
• Pressures within the right ventricle
• pulmonary artery pressures
• Pulmonary artery wedge pressure 
• The cardiac output

From these data, various other haemodynamic parameters can be derived 

Derived haemodynamic parameters

The cardiac output is measured by the thermodilution method described below

Thermodilution method for the measurement of cardiac output

Cardiac output can be calculated by using the Stewart Hamilton equation. A bolus cold fluid of known volume and temperature is injected through the proximal port of the pulmonary artery catheter into the superior vena cava. This fluid then mixes with the blood in the right ventricle and causes a decrease in the temperature of the blood which is detected by a thermistor at the distal end of the pulmonary artery catheter, i.e. in the pulmonary artery.

The changes in blood temperature detected by the thermistor is plotted against time. The area under the curve is inversely proportional to the cardiac output. If the cardiac output is high, there is a large initial change in the blood temperature, which is short lived. If the cardiac output is poor, the initial change in blood temperature is smaller but the change is more prolonged. Usual practice is to perform three measurements in quick succession and to take the mean value of the measured cardiac output; this compensates for the small variations in cardiac output which are seen due to ectopic beats or the respiratory cycle. 

A modification of this technique can be used for the 'continuous' cardiac output measurement using a modified pulmonary artery catheter. A heating coil is incorporated into the pulmonary artery catheter to lie within the right atrium and right ventricle. Every 30-60s this heats a bolus of blood; this temperature change is again monitored by a thermistor at the distal end of the catheter, and the cardiac output is calculated from the temperature/time curve. This technique allows a more rapid assessment of the effects of treatment on cardiac output than the intermittent thermodilution technique.

There are several potential sources of error when using the thermodilution method to measure cardiac output:

• Intracardiac shunts
• Too slow injection of cold injectate
• Impairment of thermistor function by impingement against blood vessel wall.
• Tricuspid valve regurgitation 

Even if the absolute reading of cardiac output are rendered inaccurate by the presence of one of the above factors the trends of the readings may still be useful in guiding treatment.

The thermodilution technique directly measures the cardiac output of the right side of the heart. At equilibrium it is assumed that the output of the right side of the heart is equal to that of the left side.

Summary

The pulmonary artery catheter can be used to measure and derive a variety of cardio vascular parameters. The clinical valve of measuring and manipulating these parameters either by the pulmonary artery catheter or by newer cardiac output monitors such as the oesophageal Doppler remains controversial.