Measurements and errors

Pulmonary artery pressures

Pressures should be obtained after proper calibration and zeroing to atmospheric pressure, with the mid-chest level as reference. Technical errors are frequent (20 per cent) and may not be detected. The difference between the diastolic pulmonary artery pressure and the pulmonary artery wedge pressure (PAWP) can be regarded as a measure of the pulmonary vascular resistance. The gradient may increase in sepsis and pulmonary embolism.

Pulmonary artery wedge pressure

Proof of a true wedge position of the catheter tip is a high oxygen saturation (higher than in arterial blood) measured in blood aspirated from the distal port of the pulmonary artery catheter with an inflated balloon, but this is not done routinely. Other criteria for the true wedge position are given in Table.2. It is relatively hard to judge the validity of PAWP readings even if the waveform is recorded on paper. Errors in measurements include damping of the curve, 'overwedging', and incomplete wedging.

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Table 2 Criteria for the wedge position in West zone III

PAWP only reflects left atrial and thus, in general, left ventricular filling pressure, if there is continuity with the left atrium. Left atrial reflects left ventricular end-diastolic pressure, in the absence of major mitral valve disease (TabJe..,3). PAWP and changes in it may not reflect left ventricular end-diastolic volume, which is a true measure of left ventricular preload, since distensibility of the left ventricle may vary between patients and between clinical situations.

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Table 3 PAWP, left ventricular end-diastolic pressure (LVEDP), and capillary pressure

During mechanical ventilation, positive airway pressure is introduced which may be partially transmitted into the alveoli and pulmonary vessels, and may affect PAWP

measurements if the catheter tip is in West zones I and II (O'QMinn.aDd.M.aiin! 1983). In contrast to zone III, airway pressure exceeds venous (and arterial) pulmonary pressure in zones I and II, so that PAWP may be influenced by intra-alveolar rather than intravascular pressure. Clinically, zone III conditions can be assumed if the tip of the pulmonary artery catheter is below the level of the right atrium (this can be confirmed on a lateral chest radiograph). Sometimes, it is necessary to reposition the catheter tip, although in the majority of cases the catheter tip moves into the dorsal region of the lungs, where blood flow is higher than in the ventral region during insertion in the supine patient (O'QM!Dn...,a^d...,MaIiOi..,1983.). Alternatively, changing the patient's position from supine to lateral (catheter tip below the heart) and re-zeroing to atmospheric pressure at the mid-chest level may improve the accuracy of PAWP as a measure of pulmonary venous pressure. Mechanical ventilation with positive end-expiratory pressure (PEEP) may enlarge zones I and II by increasing the intra-alveolar pressure. This effect is greatest during hypovolemia or other conditions associated with pulmonary hypoperfusion. Hence, during incremental PEEP, PAWP may rise more than the pulmonary artery pressure, central venous pressure, and thus left atrial pressure, if the catheter tip is not situated in West zone III.

During spontaneous and artificial ventilation, the proper phase in the ventilatory cycle to judge the PAWP is at end-expiration, when air flow is zero and the intrathoracic and atmospheric pressures are equal unless PEEP is applied. This will avoid the need for complicated estimates or measurements of transmural pressures (i.e. intrathoracic intravascular pressure minus intra-alveolar pressure). Conversely, the electronically derived mean pressure (over the respiratory cycle)

may not conform to the pressure at end-expiration (O1QMnD.a.n.d,Ma^D..i 1983). Hence, manual measurements from graphic recordings at end-expiration are preferable to automated measurements in digital displays of PAWP, which reflect mean values over the respiratory cycle.

With PEEP, measurements of pressures at end-expiration with the atmospheric pressure as reference may not indicate transmural pressures in the thorax ( O'..Q.uln.n, and M..a.rini,1983). This can be corrected by subtracting half the PEEP (in mmHg), reflecting transmitted airway pressure in the presence of normal respiratory compliance, from the measured PAWP at end-expiration (Smj..seíh.,eí..a.l 1996). Measurement of pleural or esophageal pressure (or pericardial pressure after cardiac surgery) to calculate transmural pressure is not routine. An alternative approach is provided by measuring the intrathoracic intravascular pressure after a sudden discontinuation of PEEP, assuming that the delay between the reduction in PEEP and the increase in venous return prevents changes in cardiac loading. Finally, some authors suggest that central venous pressure (right atrial pressure) may equal pericardial pressure, so that referencing PAWP to central venous pressure may yield a transmural pressure when pericardial pressure is elevated by a high airway pressure ( Smiseth,,et,a.l 1996). However, the validity of this idea has been questioned.

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