Is Doppler-based calculation of pulmonary artery pressure valid in critically ill patients on mechanical ventilation?

Pulmonary artery pressure (PAP) is an important parameter in mechanically ventilated patients. In cardiology practice, the pulmonary artery systolic pressure (PASP) is measured by transthoracic echocardiography by continuous wave Doppler interrogation of the tricuspid regurgitation (TR) jet. The measurement is based on the following equations:

Tricuspid pressure gradient (Right ventricular systolic pressure – right atrial pressure) =4V² (where V = maximal velocity of the TR jet)

Right ventricular systolic pressure (RVSP) = Tricuspid gradient + CVP. The RVSP is assumed to be identical to the pulmonary artery systolic pressure (PASP).

If the CVP is not measured, conventionally, 10 mm Hg is added to the tricuspid gradient to obtain the PASP. This method of measurement of PASP has been validated in spontaneously breathing patients in cardiology practice. However, the question remains, would it be valid in patients on positive pressure ventilation and PEEP? The evidence so far has been inconclusive.

In a single-center study from France, published online first in the Critical Care Medicine, the investigators assessed the reliability of Doppler-derived PAP measurements compared to invasive measurements using a pulmonary artery catheter in mechanically ventilated patients (Mercado et al., 2018). The PASP was calculated by two formulas (1) PASP = tricuspid pressure gradient + CVP and (2) PASP = tricuspid pressure gradient + 10 mm Hg. The mean PAP was calculated (1) using the isovolemic relaxation time and (2) using the Chemla equation: 0.61 × PASP + 2. The Doppler-based calculations were compared with direct measurements from a pulmonary artery catheter.

The PASP calculated by adding the CVP to the tricuspid pressure gradient correlated best with invasive PASP measured using a pulmonary artery catheter, with a Spearman correlation coefficient of 0.87. When pulmonary artery hypertension was defined as a mean PAP more than 25 mm Hg, a PASP of more than 39 mm Hg, derived using this method, revealed 100% sensitivity and specificity. The mean PAP calculated using the Chemla equation showed a similar correlation (0.87) with invasive measurements. A cut-off value of 26 mm Hg revealed a sensitivity and specificity of 100% for the diagnosis of pulmonary hypertension, defined as mean PAP > 25 mm Hg by invasive measurement. When the PASP was calculated by adding 10 mm Hg to the tricuspid pressure gradient, the correlation coefficient was 0.79 on comparison with invasive measurements from a pulmonary artery catheter.

This study validates TR jet-based pulmonary artery pressure measurements in critically ill patients, who are on mechanical ventilation. The most precise method, with the best correlation, was by addition of the CVP to the tricuspid gradient. However, if the central venous pressure is not available, adding 10 mm Hg to the tricuspid gradient would be a close approximation. This study also showed that contrary to widely held belief, the maximal or minimal diameter of the inferior vena cava nor the variation in diameter bore no correlation with invasively measured CVP.

This study firmly establishes the precision of continuous-wave Doppler-derived assessment of pulmonary artery pressures in mechanically ventilated patients in the intensive care unit. However, a tricuspid regurgitation jet may not be evident in some patients to enable measurement of the gradient. In the present study, the authors report that a measurable jet was present in 60% of patients. Besides, an optimal 4-chamber view, with proper alignment of the Doppler cursor with the axis of the regurgitant flow may not be possible to obtain in some patients.

References:

Mercado, P., Maizel, J., Beyls, C., Kontar, L., Orde, S., Huang, S., Slama, M. (2018). Reassessment of the Accuracy of Cardiac Doppler Pulmonary Artery Pressure Measurements in Ventilated ICU Patients: A Simultaneous Doppler-Catheterization Study. Critical Care Medicine, Online First. https://doi.org/10.1097/CCM.0000000000003422