Monitoring Pulmonary Artery Pressure

Q: What is the proper reference point to use when leveling (referencing) and zeroing hemodynamic monitoring systems?The phlebostatic axis provides an external reference point that approximates the anatomic level of the left and right atria and the pulmonary artery. Leveling (referencing) and zeroing the hemodynamic monitoring system to the tip of the catheter that lies within the pulmonary artery ensures that hemodynamic values obtained with the catheter are accurate.1 The air-fluid interface (zeroing stopcock), not the transducer, should be used when the hemostatic monitoring system is being leveled (referenced) to the phlebostatic axis. Recent research with computed tomography has confirmed that the phlebostatic axis approximates the level of the left atrium at the point midway between the anterior and posterior surfaces of the chest at the fourth intercostal space2,3 when the patient is supine (Figure 1).Q: What is the best position in which to place the patient to ensure accuracy when monitoring pulmonary artery pressure?A variety of backrest positions may be used that do not compromise the accuracy of measurements of pulmonary artery pressure. Back-rest elevations of 0°, 30°, 45°, and 60° with the patient supine (back flat against bed surface) do not affect the accuracy of measurements of pulmonary artery pressure if the hemodynamic monitoring system has been properly referenced and zeroed.4–10 As the patient moves from flat to higher levels of backrest elevation, the reference level must remain horizontal to the phlebostatic axis in order for measurements of pulmonary artery pressure to remain accurate (Figure 2).Q: Are pulmonary artery pressures measured in patients who are positioned on the side (lateral or side-lying position) accurate?Monitoring pulmonary artery pressures in patients who are in lateral or side-lying positions (eg, 20°, 30°, 45° to the side) is generally not recommended. Most research results are conflicting and inconclusive because of the lack of a standardized point of reference for leveling and zeroing in patients in side-lying positions.11–18 The only lateral position for which measurements of pulmonary artery pressure seem to be accurate is with patients lying on the side in a 90° lateral position with the backrest flat.3 The landmarks for leveling and zeroing with patients in the right and left lateral positions areas follows: (a) right lateral position (at 90° with backrest flat)—the intersection of the fourth intercostal space and the midsternum; (b) left lateral position (at 90° with backrest flat)—the intersection of the fourth intercostal space and the left parasternal border. Because most critically ill patients who require monitoring of pulmonary artery pressure are rarely positioned in a 90° side-lying position, until external landmarks for the correct reference location in 20° to 45° side-lying positions are determined by further research, pulmonary artery pressure should not be monitored while patients are lying on the side (Figure 3).Q: Is it normal for measurements of pulmonary artery pressure to fluctuate?Fluctuations in pulmonary artery pressure are normal in critically ill adults but should be assessed for individual variations and clinical significance. Limited clinical studies indicate that normal fluctuations in pulmonary artery pressure of 4 mm Hg occur for pulmonary artery diastolic pressure and pulmonary artery wedge pressure (PAWP), and normal fluctuations of 5 mm Hg occur for pulmonary artery systolic pressure.19Q: Are measurements of pulmonary artery pressure accurate in patients being treated with positive end-expiratory pressure (PEEP) during mechanical ventilation?For patients being treated with PEEP of less than 10 cm H2O, correlation between the PAWP and left atrial pressure is good, in the absence of hypovolemia. Accuracy also depends upon having the tip of the pulmonary artery catheter below the level of the left atrium or lung zone 3 (dependent blood flow). If the tip of the catheter is in lung zone 1 (upper lung, no blood flow) or lung zone 2 (intermittent blood flow), the PAWP may reflect alveolar airway pressure rather than vascular pressure, resulting in a falsely elevated PAWP20–25 (Figure 4). For patients being treated with PEEP greater than 10 cm H2O, the accuracy of measurements of pulmonary artery pressure may be compromised, and data should be interpreted cautiously. PEEP greater than 10 cm H2O increases alveolar and intrathoracic pressure, compresses the pulmonary vasculature, and affects the accuracy of PAWP measurement. Graphic recording of the hemodynamic waveform is essential for accurate measurement of PAWP, particularly if respiratory variation is significant and the patient is being treated with high levels of PEEP.23,24,26–28 The accuracy of measurements of pulmonary artery pressure should be questioned if one of the following occurs:The following correction calculation can be used to estimate PAWP and correct for high levels of PEEP27:For example, in a patient with a measured PAWP of 24 mm Hg who is receiving 16 cm H2O applied PEEP, the correction calculation would be as follows:Q: Which is the most accurate and reliable method of measuring pulmonary artery pressure, graphic recording or digital data taken directly from the monitor?Studies have shown that measurements of pulmonary artery pressure are more accurate and reliable when taken from a graphic strip recorder than when based on digital data taken directly from the monitor. Use of graphic strip recordings enhances detection of phases of respiration and makes determination of end-expiration much more accurate. These factors are particularly important if respiratory variation is significant or high PEEP is used in a patient receiving mechanical ventilation. Significant discrepancies in measurements of pulmonary artery pressure can occur, resulting in falsely high measurements inpatients receiving mechanical ventilation and falsely low pressures in spontaneously breathing patients when digital data are taken directly from the monitor.29–31This article was first published in Critical Care Nurse December 1999.