The pulmonary artery PA catheter

The PA catheter is an attempt to measure left heart function more directly than the CVP - but there are still many variables. It was introduced into clinical practice in the 1970s by Swan and Ganz. The balloon-tipped catheter is 70 cm long, with markings every 10 cm. The catheter has several channels including some for infusions, inflating the balloon, and connections to a thermistor. PA catheters enable therapy to be flow-based by measuring the following:

• right atrial, ventricular, and pulmonary artery occlusion pressures

• cardiac output/index

• derived data for systemic vascular resistance

• mixed venous O2 saturation used to assess oxygen delivery and uptake.

Complications of the PA catheter (in addition to those of the CVP) include:

• arrhythmias (50%; sustained or clinically important arrhythmias are uncommon)

• catheter knotting

• damage to valves or myocardium (for example, mural thrombus 30-60%)

• the reliance on numbers rather than clinical assessment.

An introducer is inserted into the right internal jugular or left subclavian vein. After the catheter has been prepared, it is connected to a pressure transducer. The catheter is inserted to 20 cm, beyond the length of the introducer and in the large veins. The balloon is then inflated and the catheter inserted further. The balloon helps the catheter tip move with blood

Pressures*

Introducer And Catheter

Figure 6.2 PA catheter appearance on a chest x ray film. The PA catheter is still indirect. The catheter curls in the right ventricle and the tip sits in the pulmonary artery (lungs) but is trying to measure pressure changes in the left ventricle - via the whole of the lungs, left atrium, and mitral valve. The PA catheter tip must sit in a West zone 3 of the lung. West described physiological lung zones; zone 3 is where pulmonary venous pressure is greater than alveolar pressure. If the tip is in a non-zone 3, the wedge pressure may reflect alveolar rather than left atrial pressure. Luckily, balloon catheters tend to enter zone 3 because it is the area with highest blood flow. Although West zones are anatomical in normal lungs, they are actually physiological zones which can be altered by disease. Absence of a normal waveform, a wedge pressure that fluctuates widely with respiration or a rise in wedge pressure greater than half of any PEEP increase are clues to a non-zone 3 placement

*Pressure in the pulmonary arteries (Pa), alveolus (PA) and veins (PV).

flow to the right place (Figure 6.2) - hence the expression "floating a PA catheter".

When the catheter tip is wedged, it directly communicates with the left atrium via the pulmonary vessels (Figure 6.3). The pulmonary artery occlusion pressure (PAOP) is approximately the same as left atrial pressure. At end-diastole, left atrial pressure approximates to left ventricular pressure, which in turn is assumed to reflect left ventricular end-diastolic

RA RV PA PAOP

RA RV PA PAOP

Arterial Pressure Trace Dampened

Figure 6.3 PA catheter trace patterns. (a) Waving the catheter tip in mid-air before insertion produces a trace like this. Air bubbles damp the trace as on the right hand side and should be flushed out. (b) First a characteristic central venous pressure (CVP) trace is seen, followed by a right ventricular (RV) trace as soon as the catheter tip enters the right ventricle (note the higher pressures). When the catheter tip enters the pulmonary artery (PA), the diastolic pressure increases and the trace changes to have a dichotic notch. When the balloon wedges, a damped trace is seen. The catheter must not be left wedged as pulmonary infarction may occur - this is for measurements only. In a healthy unventilated patient CVP = PAOP. PAOP, pulmonary artery occlusion pressure; RA, right atrium

Figure 6.3 PA catheter trace patterns. (a) Waving the catheter tip in mid-air before insertion produces a trace like this. Air bubbles damp the trace as on the right hand side and should be flushed out. (b) First a characteristic central venous pressure (CVP) trace is seen, followed by a right ventricular (RV) trace as soon as the catheter tip enters the right ventricle (note the higher pressures). When the catheter tip enters the pulmonary artery (PA), the diastolic pressure increases and the trace changes to have a dichotic notch. When the balloon wedges, a damped trace is seen. The catheter must not be left wedged as pulmonary infarction may occur - this is for measurements only. In a healthy unventilated patient CVP = PAOP. PAOP, pulmonary artery occlusion pressure; RA, right atrium volume. Readings are taken at end-expiration when intrathoracic pressure is nearest to zero. Lung and mitral valve disease are two major factors affecting the assumption that PAOP is equivalent to left ventricular end-diastolic volume. Pulmonary vascular resistance is increased in mitral stenosis, hypoxaemia, acidosis, pulmonary embolism, and acute respiratory distress syndrome (ARDS). A stiff left ventricle alters the relationship between pressure and volume. Furthermore, many values from the PA catheter are calculated

Table 6.4 Normal PA catheter values and intracardiac pressures

Value Normal range

Right ventricle 0-8 mmHg diastolic, 15-30 mmHg systolic

PAOP 5-15 mmHg

Left atrium 4-12 mmHg

Left ventricle 4-12 mmHg diastolic, 90-140 mmHg systolic

Stroke volume 55-100 ml

CO 4-8 litres per minute

CI 1-5-4 litres per minute m-2

SVR 770-1500 dyn.s cm-5

CI, cardiac index; CO, cardiac output; CVP, central venous pressure; PAOP, pulmonary artery occlusion pressure; SVR, systemic vascular resistance.

from other measurements rather than measured directly, so entering the incorrect weight and height of the patient can also introduce error.

Again, trends are more important than a single reading and should be used in conjunction with a thorough clinical assessment. Clinical assessment still remains far more important than numbers alone. PA catheter values are shown in Table 6.4.

Cardiac output is measured by the thermodilution method. Cold fluid is injected rapidly via the proximal PA catheter lumen into the right atrium at end-expiration. The thermistor measures the temperature change downstream and a computer calculates CO using the indicator dilution equation.

PA catheters are indicated when there is an operator experienced in their use and the risk:benefit ratio to the patient is acceptable. Although practice varies, examples of PA catheter use are:

• to monitor the effects of therapy, for example in septic shock where more than one vasoactive drug may be required;

• in right-sided myocardial infarctions where right and left ventricular pressures are very different;

• to distinguish cardiogenic from non-cardiogenic pulmonary oedema in severe sepsis.

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