By Michael H. Crawford, MD

Professor of Medicine, Chief of Clinical Cardiology, University of California, San Francisco

Dr. Crawford reports no financial relationships relevant to this field of study.

SYNOPSIS: Researchers compared echocardiographically determined right atrial pressure and characteristics of the inferior vena cava to right heart catheterization-measured values. They concluded that echo estimates reached through this technique should not be used clinically to estimate pulmonary artery pressure.

SOURCE: Magnino C, Omedè P, Avenatti E, et al. Inaccuracy of right atrial pressure estimates through inferior vena cava indices. Am J Cardiol 2017;120:1667-1673.

Several echocardiographic-based methods for estimating right atrial pressure (RAP) have been proposed, but external validation of their accuracy is limited. Thus, investigators from Turin, Italy, conducted a prospective, blinded study of several echo methods as compared to the results of right heart catheterization in 190 patients. Exclusion criteria included vasoactive drug infusions, known pulmonic stenosis, and respiratory ventilator support. An echo was performed either just before (63%) or just after the catheterization during normal respiration recorded on a respirometer. When necessary, deeper respiration rather than sniffing was used to enhance the measurements. Six echo methods based on measuring the inferior vena cava (IVC) for estimating RAP were compared to the hemodynamic data.

IVC evaluation was not possible in 19% of patients. In the rest, end-expiration IVC diameter and IVC collapsibility were significantly correlated with invasive RAP, but the strength of the correlations was low (c = 0.35-0.40). All methods of RAP estimation were associated significantly with invasive RAP, but accuracy was low (average = 34%). No method was clearly superior to the others, and no clinical or echo data seemed to affect the observed error, including tricuspid regurgitation, the level of invasive RAP, the time between studies, the pulmonary artery pressure, or body size. The authors concluded that all currently described methods of estimating RAP are highly inaccurate and should not be used in the estimation of pulmonary artery pressures.

COMMENTARY

An estimate of RAP is desirable to provide a more accurate estimation of pulmonary artery pressure from the tricuspid regurgitation jet velocity-measured pressure gradient. Older studies showed that IVC imaging estimates of RAP are superior to using the physical examination-derived jugular venous pressure or using a fixed value (10 mmHg) for every patient. This concept has morphed out of the echo lab into other areas of the hospital with the availability of small, hand-held echo machines. The ED, ICUs, and enterprising residents all report RAP values, use the information to determine the fluid status of patients, and treat accordingly. It will be more pervasive soon, as some medical schools are issuing hand-held echo machines to all their students and as devices that interface with smartphones or other devices are perfected. Thus, knowing the accuracy of echo RAP estimation is important.

The few small studies that assessed the accuracy of this method of estimating RAP have shown conflicting results. Consequently, various modifications to the basic technique have been suggested, not all of which have been studied adequately. Thus, this study is of interest. Magnino et al compared all the recommended techniques to right heart catheterization and found that all were unreliable and none were superior to the others. Also, there was no cutoff that accurately distinguished high vs. low values of RAP. There was a general trend to underestimate the value. Additionally, there were no clinical features that, if excluded, significantly improved the group prediction, including significant tricuspid regurgitation and body surface area.

There were limitations to this study. Although larger than most studies in this area, the authors only studied 190 patients. These were patients with considerable comorbidities, so the results may not apply to a healthier population. The right heart cardiac catheterization and the echo were not performed simultaneously because it is more difficult to obtain excellent quality echoes in the cath lab setting. However, these tests were performed as close as feasible. Magnino et al used deep inspiration rather than a sniff to assess IVC collapsibility because sniffing cannot always be performed correctly and it can change the imaging plane. The most recent guidelines (2015) recommend measuring the IVC diameter by m-mode echocardiography (from a 2-D image) in the long axis subcostal view perpendicular to the IVC and 1-2 cm inferior to the junction of the IVC and the RA. The percent collapse with normal respiration or a sniff, if there is no collapse with quiet respiration, is calculated. The upper limit of normal is 2.1 cm and the normal collapse is > 50%. Measures below 2.1 cm and above 50% represent a RAP of 3 mmHg. Values > 2.1 cm and below 50% represent 15 mmHg. Measurements not conforming to these combinations are considered 8 mmHg. Excluded are athletes and those on a ventilator. Interestingly, these guideline recommendations from a committee have never been tested, and since this study did not use sniffing, Magnino et al did not really test them, either. So, how should echo labs act? The European Society of Cardiology Pulmonary Hypertension guidelines advise against using echo estimates of RAP to determine pulmonary pressures. My pulmonary hypertension group uses echo as a screening tool, but does not rely on it to make treatment decisions, preferring right heart catheterization. The rest of us should do the same.