Edited by: Cecile Tissot, Clinique des Grangettes Geneva, Switzerland
Reviewed by: Dunbar Ivy, University of Colorado Denver School of Medicine, United States; Maurice Beghetti, University Hospitals of Geneva, Switzerland
Specialty section: This article was submitted to Pediatric Cardiology, a section of the journal Frontiers in Pediatrics
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There is a growing awareness of the role that increased pulmonary vascular resistance (PVR) plays in many pathologies; therefore, assessment of pulmonary artery pressure (PAP) is an increasingly requested investigation in the critical care environment. This article will go through the basic concepts regarding PAP and PVR, then will go on to outline the various echocardiographic parameters which are used to assess them. Finally, an outline of how to undertake this assessment will be presented.
To start with, it is very important to understand the differences and interdependencies of three commonly quoted (and confused) variables (Table
Important variables in the assessment of pulmonary vascular resistance.
Variable | Units of measurement |
---|---|
Flow | L/min (indexed to body surface area as L/min/m2) notation: “Q” |
Resistance | Woods unit·m2 (WU·m2) |
Pressure | Torr (mmHg) |
Pressure is the product of flow and resistance and is related to flow and resistance by the hydraulic Ohm’s law analogy:
In order to account for the pressure distal to the intrapulmonary vasculature, we need to add in the effect of left atrial pressure (LAp):
The gold standard for measurement of pulmonary vascular resistance (PVR) is cardiac catheterization (
In most critical care patients in whom there is a concern about “pulmonary hypertension,” the underlying pathological process of interest is increased PVR. We cannot directly measure PVR on echocardiography, so we have to use another measurement as a surrogate for it: i.e., PAP. Unfortunately, there are other pathological processes which affect this; therefore, it is always vital to remember that:
Looking at Eq. Increased PVR. Increased Qp. Increased LAp. Any combination of the above.
Any assessment of PVR requires the assessment of all of these possibilities. Pulmonary arterial hypertension (PAH) from increased PVR can only be assumed if there is no increased flow (e.g. left-to-right shunt) and/or there is no suggestion of increased LAp (e.g. due to left ventricular failure or mitral stenosis).
In adults, pulmonary hypertension is defined as a mean PAP > 25 mmHg (
In adult practice there are well-defined thresholds for indicating the degree of PAH (see Table
Quantification of PAH in adults.
Mean pulmonary artery pressure (PAP) | (≈Systolic PAP) | Degree of PAH |
---|---|---|
<25 mmHg |
<40 mmHg | Normal |
25–40 mmHg | 40–60 mmHg | Mild |
41–55 mmHg | 60–90 mmHg | Moderate |
>55 mmHg | >90 mmHg | Severe |
Quantification of PAH in children.
Systolic PA pressure/systemic systolic BP | Degree of PAH |
---|---|
<1/3 | Normal |
1/3–2/3 | Mild |
2/3–1 | Moderate |
>1 | Severe |
The most common method used for assessing PAP is to perform a continuous wave spectral Doppler analysis of the TR jet (
The modified Bernoulli equation states that the pressure gradient between either side of a fixed obstruction with no significant length, is proportional to the velocity (
Measuring the velocity of the TR jet can, therefore, give an estimate of the pressure gradient between the right ventricle and right atrium at peak systole. Therefore:
Central venous pressure monitoring is often used in the critical care environment, and can usually be treated as analogous to the right atrial pressure. If this is not available, the RAp component is often ignored, as its value usually only contributes a small amount to the overall result (especially in the context of increased RVSP).
Many patients will have no or only trivial TR. Given the motion of the AV valve plane during systole, it can be impossible to obtain a spectral Doppler TR trace for the duration of systole. The spectral Doppler trace of TR is quite characteristic, and values should not be quoted unless there is a clear trace, otherwise, there is the risk of a significant underestimate. This means that this technique cannot be used for all patients (see Figures
The Doppler interrogation beam should be within 20° of the jet direction, or there will be significant underestimation of the velocity. This is not always possible with some eccentric jets.
Many confuse the quantity of TR with the pressure gradient. It is possible to have severe TR with a low velocity or mild TR with a high velocity. The two are not directly related.
In severe TR, the right atrial pressure will increase rapidly during systole, resulting in an underestimation of the RVSP.
If there is any right ventricular outflow obstruction (e.g., pulmonary stenosis), then it is important to remember that the RVSP will not reflect the systolic PAP. It is very important to check the whole of the right ventricular outflow (subpulmonary area, pulmonary valve, main and branch pulmonary arteries) for evidence of obstruction.
Large left-to-right shunts [e.g., large ventricular septal defect (VSD), aortopulmonary window, PDA] will cause equalization of right ventricular/PAP with left ventricular/aortic pressure. There is little point in measuring the TR velocity in this context as it will just reflect the systemic systolic blood pressure.
A VSD ejecting into the region of the tricuspid valve may “contaminate” the spectral Doppler trace, making it uninterpretable.
Similar to TR, most (but not all) patients will have some degree of PR. Analysis of the spectral Doppler pattern from this can also give important information regarding the PAP (
Not all patients will have a PR jet.
It is vitally important to capture the full spectral Doppler envelope and be well aligned with it.
This technique is not reliable in the context of severe PR.
Given that the magnitude of the mean and diastolic PAP is less than the RVSP/systolic PAP, the RAp has more of an influence on this measurement. It is, therefore, more prone to underestimation if this is not known.
The geometry of the interventricular septum is dependent on the relative pressures within the ventricles. Usually, as the left ventricle is at much higher pressure than the right ventricle, the interventricular septum is pushed rightwards. This gives the characteristic circular profile of the left ventricle on the short-axis view. As right ventricular pressure increases, the septum is pushed more toward the left resulting in the left ventricle becoming “
The end systolic eccentricity index (EIs) allows some categorization of the RV pressure (
Categorisation of RV pressure based on EIs (
End systolic eccentricity index | RV pressure |
---|---|
<1.15 | Normal/mildly increased |
1.15–1.29 | Moderately increased |
≥1.3 | Severely increased |
Serial measurement of the EIs over time may allow monitoring of the progression of the RV pressure.
Interventricular conduction abnormalities (i.e., left or right bundle branch block) will alter the relative timing of peak left and right ventricular pressure, making this method unreliable.
The presence of a large VSD will affect the motion of the interventricular septum, making this method unusable.
Given this method indirectly measures the
The presence of a shunt between the systemic and pulmonary circulations also allows estimation of the relative pressures of the two systems. Please note that pre-tricuspid valve level shunts (e.g., systemic arteriovenous malformation, partial anomalous pulmonary venous connection, atrial septal defect) do not communicate between the high-pressure portions of the circulation, so the points below do not apply.
As previously stated, large non-restrictive shunts (in the absence of any outflow tract stenosis) will result in equalization of the systemic and pulmonary pressures. The direction and quantity of flow across the shunt will reflect the relative difference between the systemic vascular resistance and PVR—this can be assessed clinically as well as on echocardiography (NB: here, we are evaluating flow as opposed to pressure, see Table
Evaluation of PVR in the presence of a large shunt.
Shunt direction on echo | Clinical findings | Pulmonary vascular resistance (PVR) vs SVR |
---|---|---|
Left-to-right | Normal saturations Respiratory distress Pulmonary congestion Hepatomegaly Heart failure |
PVR < SVR |
No net shunt (bidirectional) | Normal/variable saturations (may have labile PVR) Asymptomatic from shunt |
PVR ≈ SVR |
Right-to-left shunt | Low saturations (if PDA, then may have differential upper/lower limb saturations) Difficult to oxygenate Asymptomatic from shunt |
PVR > SVR |
If the size of the shunt is small, then it will not greatly affect the pressures between the two sides of the circulation. Therefore, using the modified Bernoulli equation will again allow estimation of the relative difference between the systemic and pulmonary pressures.
For example, if there is a small VSD, the peak systolic velocity across it will reflect the pressure gradient between the ventricles at peak systole. Therefore:
The above principle applies to a PDA, although this also gives information regarding the differential between the relative diastolic pressures as well. Given that the diastolic PAP is less influenced by flow, it is often thought to better reflect the PVR and is less affected by shunts. Patterns of ductal flow can be used to infer PVR status (see Table
Evaluation of pulmonary artery pressure based on PDA spectral Doppler pattern.
Spectral Doppler trace (blue = right-to-left flow, red = left-to-right flow) | Comment |
---|---|
Right-to-left shunt in systole and diastole Pathological trace—indicates very elevated pulmonary artery pressure Likely to have differential upper and lower limb saturations |
|
Right-to-left shunt in systole, no shunt in diastole Indicates significantly elevated pulmonary artery pressure |
|
Right-to-left shunt in systole, left-to-right shunt in diastole Systolic pulmonary artery pressure (PAP) > systolic BP Normal trace in newborns [physiologically high pulmonary vascular resistance (PVR)] May reflect increased PVR or large shunt |
|
Shunt predominantly left-to-right, some right-to-left flow in systole Transitional trace in newborn, indicating falling PVR Indicates elevated systolic PAP |
|
Continuous left-to-right shunt Systolic and diastolic PAP < systolic and diastolic systemic BP Peak velocity will reflect difference between systolic BP and systolic PAP |
Given the high incidence of PDA in the neonatal population, this is often the best way of assessing PAP in this group.
There is no one perfect method of assessing PAP or PVR. Figure
Flowchart demonstrating approach to evaluating pulmonary hypertension.
Pulmonary hypertension and/or elevated PVR is a complicated area, with highly specialized knowledge required to effectively diagnose or manage. The above guide provides a framework to allow the pediatric or neonatal echocardiographer to suspect the presence of these pathologies; but it would be highly recommended that if they are suspected, then advice from a specialist should be sought.
The author confirms being the sole contributor of this work and approved it for publication.
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.