Interatrial Right-to-Left Shunting after SCUBA Diving

Dear Editor,

I read with interest the article entitled “Diving-induced venous gas emboli do not increase pulmonary artery pressure” published in Int J Sports Med and whose authors are Valic et al. [[6]]. The aim of this study is particularly interesting but I would like to submit some points for consideration.

A right-to-left shunting can occur through a patent foramen ovale despite normal cardiac pressure. Indeed, despite the fact that the mean right atrial pressure is normally lower than the mean left atrial pressure, a physiologically transient spontaneous reversal of the left-to-right atrial pressure differential is present during early diastole and during isovolumetric contraction of the right ventricle of each cardiac cycle [[4]]. This reversal gradient may drastically increase under circumstances that increase the right atrial pressure such as a rise of the pulmonary arterial pressure secondary to a pulmonary embolism. Therefore, to estimate the risk of systemic air embolism after a SCUBA dive inducing venous gas emboli, it is important to assess the pulmonary artery pressure.

In their results, Valic et al. found an increase in acceleration time/right ventricular ejection time ratio (AcT/RVET). This modification was attributed by the authors to a decrease in PAP secondary to a reduction in blood flow after the dive. This interpretation is correct owing to the negative correlation between the AcT/RVET and the mean pulmonary artery pressure [[3]]. Furthermore, in a previous study Dujic et al. have shown a decrease in cardiac index and stroke volume after an open sea SCUBA dive [[2]]. In a recent study using pulsed Doppler echocardiography, a significant reduction of cardiac diameters and stroke volume was found by our team 1 hour after a SCUBA dive [[1]]. All these results are consistent and suggest that a decrease in cardiac preload is induced by the dive. The conclusion of the authors that “the pulmonary artery pressure does not increase after a dive profile leading to a significant bubble load” is therefore logical.

However, the concluding statement that the hemodynamic changes triggered by SCUBA diving do not increase the risk of arterial gas embolism is questionable. Indeed, the right-to-left shunting is promoted by an increase in the right-to-left atrium pressure gradient. After a dive, the decrease in cardiac preload induces probably a decrease in right and left cavities pressure. But if the decrease of the right atrial pressure is less important than the decrease of the left atrial pressure, the risk of right-to-left shunting could be increased despite normal pulmonary artery pressure. This hypothesis is conceivable because experimental venous gas embolism causes a pulmonary vasoconstriction via the release of vasoactive mediators such as Thromboxane A2 [[5]]. This pulmonary vasoreactivity could be responsible for a less marked reduction in the right sided pressure than in the left sided pressure after a dive inducing high grade venous gas emboli. Consequently, further studies assessing the modification of the interatrial pressure gradient, and not only the pulmonary artery pressure, are necessary to conclude on the risk of right-to-left shunting after SCUBA diving.

References

• 1 Boussuges A, Blanc F, Carturan D. Hemodynamic changes induced by recreational SCUBA diving.  Chest. 2006; 
• 2 Dujic Ž, Bakovic D, Marinovic-Terzic I, Eterovic D. Acute effects of a single open air sea dive and post-dive posture on cardiac output and pulmonary gas exchange in recreational divers.  Br J Sports Med. 2005;  39 e24
• 3 Kitabatake A, Inoue M, Asao M, Masuyama T, Tanouchi J, Morita T, Mishima M, Uematsu M, Shimazu T, Hori M, Abe H. Non invasive evaluation of pulmonary hypertension by a pulsed Doppler technique.  Circul. 1983;  68 302-309
• 4 Langholz D, Louie E K, Konstadt S, Rao T L, Scanlon P J. Transesophageal echocardiographic demonstration of distinct mechanisms for right to left shunting across a patent foramen ovale in the absence of pulmonary hypertension.  J Am Coll Cardiol. 1991;  18 1112-1117
• 5 Tanus-Santos J E, Gordo W M, Udelsmann A, Moreno Jr H. The hemodynamic effects of endothelin receptor antagonism during a venous air infusion in dogs.  Anesth Analg. 2000;  90 102-106
• 6 Valic Z, Duplancic D, Bakovic D, Ivancev V, Eterovic D, Wisloff U, Brubakk A O, Dujic Ž. Diving-induced venous gas emboli do not increase pulmonary artery pressure.  Int J Sports Med. 2005;  26 626-631