Aspiring Medical Student and current Critical Care RN. Enjoys everything outdoors but can also be found inside nerding out on her medical education artwork
The Pre-brief
We already know how ECHO can be used to assess hemodynamics (see this post). Pulmonary artery (PA) pressures can provide useful information in the evaluation of patients with a wide range of critical illnesses, including cardiogenic shock, pulmonary embolism, right ventricular failure, and respiratory failure. Traditionally, a pulmonary artery catheter was needed to measure the pressure within the pulmonary vasculature, but fortunately, echocardiography is an acceptable surrogate.
ΔCalculating PA systolic and diastolic pressures can be challenging and somewhat time-intensive, but can provide useful data in the diagnosis and management of certain life-threatening illnesses.
- Pulmonary embolism: PA systolic pressure < 60 mmHg AND pulmonary acceleration time < 60 msec can aid in the diagnosis of acute PE (60/60 sign).
- Cardiogenic shock: PA diastolic pressure can be a surrogate for wedge pressure assuming no elevated pulmonary vascular resistance. If you rely on wedge pressure from a PA catheter to gauge the response to diuresis, consider utilizing echo-calculated PA diastolic pressures. PA diastolic pressure is usually ~5 mm Hg > than wedge pressure.
- Right heart failure: PA pressures can help differentiate a primary right ventricle (RV) failure process (i.e acute myocardial infarction) with a secondary process due to pulmonary hypertension (cor pulmonale). Trending of PA pressures in an ARDS patient may help the clinician identify the development of acute cor pulmonale (CCN ACP). This, in turn, can guide pulmonary vasculature dilation therapy.
- Critical illness: The presence of pulmonary hypertension of any etiology correlates with increased patient mortality.
How to measure PA systolic pressure
If you feel immediate nausea when discussing physics, welp, buckle up. Measuring PA pressures relies on the Bernoulli principle, a concept in fluid dynamics which, to summarize for our purposes, states that fluid traveling between two chambers through an area of resistance will increase in velocity. The difference in pressure between these two chambers is related to this velocity in the modified Bernoulli equation ΔP=4V^2.
In measuring the PA systolic pressure, the two chambers of interest are the RV and the RA (right atrium). We can calculate the difference in pressure between the RV and the RA utilizing the modified Bernoulli equation. The velocity described in the equation is from fluid traveling between the two chambers, which is the tricuspid regurgitation jet. Pulmonary artery systolic pressure is approximately equal to right ventricular systolic pressure (assuming no pulmonary artery stenosis, which is rare). Therefore, the difference in pressure between RA and RV is then added to the RA pressure (RAP), which provides the systolic pressure of the RV, which in turn is equal to the systolic pressure in the PA. The RAP is calculated by measuring IVC size and collapsibility.
Step 1: Measure velocity of tricuspid regurgitation jet
 The maximum velocity of the tricuspid regurgitation jet is measured by continuous-wave doppler in one of three echo views. The key is to measure the maximum velocity, which is obtained when the echo doppler is as parallel to the regurgitation jet as possible. In the picture below of a continuous wave Doppler measurement in an apical 4 chamber view, TR velocities are changing because the patient’s respirations are affecting the angle of the doppler with the jet.
The three views. The easiest and most useful view is the apical 4 chamber. Below, the continuous wave doppler is placed over the regurgitation jet seen with a color doppler providing the displayed wave pattern. The maximum velocity is measured. Note that on this machine the velocity is in cm/s. The modified Bernoulli equation requires the velocity to be in m/s (divide by 100). Also note that this machine is actually calculating the pressure gradient for us (PG). Not all ultrasound machines will do this, which is why knowing the equation is helpful.
Two other echo views used to visualize the tricuspid valve are the RV inflow view and the parasternal short view.
RV inflow view: Start in the parasternal long view and tilt the ultrasound beam toward the patient’s right hip. You may need to make a few small adjustments to optimize the view:
Parasternal short: Start in parasternal short axis and slide the probe medially and/or tilt the ultrasound beam medially until the image below appears
Once the maximum TR velocity has been measured, the Bernoulli equation is applied: ΔP=4V^2s
Step 2: Calculate RA pressure via IVC diameter and respirovariability
One of the very few uses of the IVC in ultrasonography is to measure CVP, which is essentially equal to RAP. This simple step involves measuring the maximum diameter of the IVC and the approximate percent collapse of the vessel during respiration. A chart can then be referenced to estimate the RAP.
Step 3: Math
Add the P from the modified Bernoulli equation to the measured RAP, which equals the RV systolic pressure, which is equal to the PA systolic pressure. In the example above: the maximum TR velocity was measured at 2.55 m/s. ΔP=4 x 2.552 = 26.01. Assume the IVC maximum diameter was 1.5 cm with >50% collapse corresponding to a RAP of 3 mmHg.
RAP + ΔP = RV systolic pressure = PA systolic pressure
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In our example…
3 + 26.01 = PA systolic pressure of 29.01 mm Hg
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How to measure PA diastolic pressureÂ
PA diastolic pressure is measured in a similar fashion except for the two chambers of interest are the RV and PA artery and the velocity measured is that of the pulmonary valve regurgitation jet (PVR). Of all the steps in measuring PA pressures via echo, obtaining a quality PVR consistently is one of the most difficult. The best means of viewing this valve is in the RV outflow view. This view is found by starting in the parasternal long-axis and tilting the ultrasound beam toward the left shoulder. Depending on patient anatomy, you may need to slightly rotate or slide the probe to get in between ribs and obtain the image shown below:
An alternative method is to start in a parasternal short view at the level of the mitral valve, tilt the ultrasound beam toward the base of the heart (toward right shoulder / medially) to visualize the aortic valve in cross section. The RV inflow, outflow, with pulmonary and tricuspid valves should come into view (with some small micro-adjustments likely needed).
The third method of obtaining a view of the pulmonary valve is to start in the subcostal view, rotate the probe 90 degrees counterclockwise, and fan the probe until the RV, PV, and PA all come into view as seen here (easier said than done, but potentially effective in some patients).
Once the valve is visualized, add color and place continuous doppler along with the PVR. The following waveform should be seen.
For diastolic PA pressure, we want to measure the end-diastolic velocity of the regurgitation jet, which is shown below in a patient with pulmonary hypertension.
Pitfalls
Transthoracic echocardiography is not as accurate as of right heart catheterization in the measurement of pulmonary artery pressures. Inaccuracies are especially apparent in patients with severe TR, compensated right ventricular dysfunction, pulmonary valve stenosis, and severe pulmonary regurgitation. In atrial fibrillation, multiple velocities need to be obtained and averaged. The diastolic pulmonary artery pressure can be difficult, if not impossible to obtain in some patients due to the technical difficulty of visualizing the pulmonic valve. Regardless of these shortcomings, bedside echocardiography provides acceptable PA pressure values for critically ill patients and is less time-consuming and less invasive than right heart catheterization.
The DeBrief
- Measuring pulmonary artery pressures via echocardiography can provide useful data in the management of critically ill patients
- Utilizing echocardiography may help avoid invasive procedures such as PA catheter placement
- Echocardiography measurement of PA pressures is not as accurate as right heart catheterization but generates data acceptable for use in patients with pulmonary embolism, cardiogenic shock, right heart failure, and respiratory failure.
References
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