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In the early days of Doppler echocardiography, cardiologists showed that measuring the tricuspid regurgitation velocity can estimate the pressure gradient across that valve. They concluded that the prediction of RV systolic pressure (RVSP) should be possible in patients with tricuspid regurgitation "by adding the Doppler-determined trans-tricuspid gradient to the right atrial pressure estimated clinically." In a 1984 study, they sought to test this method by comparing their estimate to right heart cath data. The RVSP was estimated with the following formula:
RV Systolic Pressure = delta P + JVP
JVP
The jugular venous pressure was measured in centimeters above the sternal angle at 45-degree elevation of the thorax, and right atrial pressure was then estimated by adding 5 cm to the jugular venous pressure measurement.
delta P
delta-P refers to the pressure difference between the right atrium and right ventricle. To estimate it, they first measured the maximum velocity of tricuspid regurgitation and took the average of 4 beats for sinus rhythm and 8 beats for atrial fibrillation. This trans-tricuspid gradient was then estimated using the modified Bernoulli equation:
delta P = 4V^2, where V = average maximum velocity in m/s
In this example, the average TR jet velocity is 2.9 m/s
delta P = 4 x 2.9 m/s ^2 = 33.6
If the meniscus is visualized 3 cm above the clavicle, then the RAP estimate would be:
5cm + 3cm = 8cm x 0.7359 = 5.8 mmHg
RVSP therefore would = 33.6 + 5.8 = 39.4 mmHg
However, a major limitation to this technique is how inaccurate this so-called method of Lewis is for estimating right atrial pressure. As we described in The POCUS Manifesto, and in a JVP deep dive previously, numerous studies suggest this eyeball method is very imprecise and the assumption that every patient's right atrial depth is 5 centimeters is simply not true.
As the researchers in this study explained, "clinical estimation of right atrial pressure from jugular venous pulse is a significant and unavoidable source of error." When comparing RA pressure from a cath, the clinical estimation of JVP showed "poor correlation between clinically estimated and catheterization measured values for right atrial pressure."
On the contrary, the delta P between the right atrium and right ventricle as estimated by echo is quite accurate compared to the right heart cath data:
Due to the JVP limitations, over time the right atrial pressure estimate was done by measuring the inferior vena cava (IVC) and assessing its respirophasic variation.
However, the sensitivity and specificity of these parameters are variable. One study of 102 patients found that a cutoff of 2cm IVC diameter was only 73% sensitive and 85% specific for RAP over 10mmHg. IVC collapsibility cutoff of 40% was 73% sensitive and 83% specific for high RAP. The traditional classification (as seen in the chart above) was only 43% accurate.
In another study of 200 patients, IVC estimates were compared to right atrial pressure from right heart caths and while the pressures did correlate, the accuracy was extremely poor:
"We confirmed that [invasive RAP] is positively associated with end- respiratory IVC diameter and is inversely associated with IVC collapsibility. However, even considering the best association found in the present study by measurements taken in the M-mode long-axis view, the strength of such an association is very low. [invasive RAP] values varied greatly within the same ranges of IVC diameter and collapsibility index, and there were no clear IVC cutoffs capable of discriminating between high and low [invasive RAPs] with acceptable accuracy. As a direct consequence, all 6 different methods that have been evaluated in the present study showed poor precision and wide limits of agreement. The accuracies of these methods were very low and the estimation error was substantial in the majority of patients."
As you can see, the best R^2 value of different IVC measures and invasive right atrial pressures is only 0.16. For a refresher, an R^2 value estimates the correlation between two variables. Values over 0.9 are considered very good. They concluded that "the range of error for any single RAP estimate was so wide that the common practice of estimating RAP during a standard echocardiography should be considered unreliable."
Using IVC leads to inaccurate pulmonary artery pressures
The use of IVC as a poor surrogate for RAP results in an inability to use echo to accurately assess for pulmonary hypertension (PH). This was illustrated in a study at the Johns Hopkins PH clinic. They enrolled 65 patients that underwent a right heart cath and a complete echocardiogram within one hour, and compared the results.
In this study, right atrial pressure was estimated using the IVC in the following manner:
RAP = 5 mmHg if IVC diameter is < 2cm, collapse > 50% with respiration
RAP = 10 mmHg if IVC diameter is < 2cm, collapse < 50% with respiration
RAP = 15 mmHg if IVC diameter is > 2cm, collapse > 50% with respiration
RAP = 20 mmHg if IVC diameter is > 2cm, collapse < 50% with respiration
Each patient then underwent a complete echo and continuous wave doppler was used to determine the peak velocity of the tricuspid regurgitant jet at end-expiration. They defined 'accurate' as within 10 mmHg, but only 48% of the echo estimates met this criteria. Six of the 16 patients had pressure overestimates greater than 20 mmHg, and 12/15 of the pressure underestimates were more than 20 mmHg off.
11 out of 20 patients had echo estimated RAP of 15 mmHg when it was actually less than 10, and in 4 out of 20 it was less than 5 mmHg.
In fact, half of the cases of PA systolic pressurestimation were related solely to right atrial pressure overestimation by echocardiography
The authors concluded that "these results indicate that the size of IVC, and its variation with respiration routinely used to estimate RAP, may not be as useful as commonly believed."
New Method for Right Atrial Pressure Estimation
As we've discussed previously, Dr. Istrail has completed a study (still in preprint) proposing a new method for right atrial pressure estimation. This is a direct measure of pressure in each person, and accounts for the patient specific changes in right atrial depth. It also overcomes the traditional limitations of BMI or thick necks. We've organized it into a decision tree for ease:
If you have tried this method, let us know how it goes.
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Get a copy of The POCUS Manifesto: Expanding the limits of our physical exam with point-of-care ultrasound.