Right ventricular function is identified to be an independent risk factor for mortality in various diseases as chronic obstructive pulmonary disease (COPD), pulmonary arterial hypertension (PAH) (RV failure is the end-result of PAH and the cause of at least 70% of all PAH deaths), adult respiratory distress syndrome (ARDS), etc [8]. Also pulmonary hypertension secondary to dilated cardiomyopathy constitutes a risk factor for heart transplantation procedure because of the dysfunction of the right ventricle of the graft [9]. Dysfunction of the right ventricle (RV) can occur in a number of clinical scenarios, including pressure overload, cardiomyopathies, ischemic, congenital, or valvular heart disease, arrhythmias, and sepsis. Pressure overload can occur in an acute or chronic setting [10].
Often the development of a RVF exhibits the final phase of the disease. In cardiothoracic surgery, RVF seems to be a frequent cause for postoperative cardiogenic shock associated with high mortality [11–13]. Different surgical techniques has been proposed for RVF, as atrial septostomy [3], extracorporeal right to left atrial bypass with a centrifuge blood pump and a membrane oxygenator [14], an experimental atrial septostomy with veno-venous extracorporeal membrane oxygenation (VV-ECMO) [15], or a creation of a peripheral shunt [16]. Nevertheless, the implantation of a right side assist device is associated with a high mortality [17].
The first idea of a pulmonary artery to left atrium shunt was introduced 50 years ago, and belongs to Bilgutay and Lillehei [18]. Gupta evaluate in 1972 a PA-left atrium shunt in pulmonary hypertension in an experimental model [19]. The most important side effect of Gupta's model, but also in recent practice of atrial septostomy, is severe hypoxemia from excessive right-to-left shunting. Our recordings confirmed the decrease of arterial oxygen in both groups, but it was not statistical significant (Figure 4).
Besides several other mechanisms which lead to low cardiac output in RVF, a major feature is a reduced trans-pulmonary blood flow with a reduced left atrial respectively ventricular filling result, which is called serial ventricular interdependence. Our aim was to evaluate hemodynamic status of a pulmonary artery to left atrium shunt which can have many advantages and comparison of this shunt with an interatrial shunt.
Pulmonary artery banding in pigs reproducibly results in right side circulatory failure detectable as an increase in right ventricular and mean pulmonary artery pressures and a decrease in left ventricular end-diastolic pressure. In our study, in both groups after shunting it was detectable an increase in heart rate at 10 and 20 minute and a decrease of mean arterial pressure but there was statistically significant difference of mean arterial pressure between the two groups at 20 minute (p = 0.054) being more prominent in group 1 (PA-LA) shunt. This result can be explained from the concomitant decrease in this group of SVR at 20 minutes. Τhere is statistical significant difference between groups concerning the percentage change from baseline to 10 minute of the SVR variable and a statistically significant difference between the two groups at 20 minute (p = 0.075). Our recordings of a low MAP and low SVR in both groups are consistent with the results described by other investigators [20–22].
The right ventricular pressure was statistically significant higher in the group of RA-LA. Right ventricular overload - pressure lead often to life threatening ventricular tachycardias. From this point of view the PA-LA shunt has a significant advantage. We observed that right atrial pressure in both groups was not increased as expected, because the experiment was acute and the tricuspid valve by epicardial echocardiography had sufficient competence. However, an interatrial shunt is likely beneficial only if sufficient right-to-left shunting occurs to increase cardiac output.
The results of lower mean arterial pressure and SVR in favor of PA-LA shunt insinuate easier manipulation of heart function in order to optimize heart performance by simple maneuvers like volume infusion or medical intervention in cases of real conditions of right ventricle overload.
Atrial septostomy has been associated with a risk of intraprocedural and postprocedural mortality up to 30% in several series [3, 5, 23–25], most commonly, secondary to progressive hypoxia, right heart failure and ventricular arrhythmias. For this reason, Zierer et al [26] had tried to determine the qualitative and quantitative impact of low-flow vs. high-flow shunting. In this study, low-flow shunting (15% of cardiac output) improved RV diastolic compliance by 42% and caused a shift of the RA reservoir-to-conduit ratio toward physiological conditions. In our study, the cardiac output was not significantly different between the two groups. This can be attributed to the Frank-Starling mechanism. According to the Frank-Starling mechanism, as the heart is stretched in response to increased preload, it augments its contraction force at the expense of increased myocardial oxygen consumption. But in our study we observed that flow in LAD had statistically significant difference between the groups concerning the percentage change from baseline to 10 minutes and statistically significant difference between the two groups at 20 minutes (p < 0.0005) in favor of the PA-LA shunt (Figure 5,6).
According the Hagen-Poiseuille law
the PA - LA shunt has 10 fold higher volumetric flow rate, where Q: volumetric flow rate, π: mathematical constant, η: dynamic fluid viscosity [pascal - second (Pa·s)], P
i
: inlet pressure, P
o
: outlet pressure, L: total length of the tube in the x direction (meters), R: is the radius.
Because of the anatomical contiguity between pulmonary artery and left atrium, the length of the PA-LA graft is always shorter than the RA-LA graft. The pressure gradient PA-LA is always higher than the RA-LA. These two issues constitute an inherent advantage of PA-LA shunt and are rendering PA-LA shunt more effectively in that it can provide wider range of achievable flows through the shunt. Given the fact that the current technology allows the pulmonary artery banding to be adjustable, we can assume that in the future we may be able to calculate the ideal flow in an individualized manner.
To our surprise, systemic arterial de-saturation following the PA-LA shunt was not increased dramatically with devastating consequences such as systemic oxygen delivery. The advantages of a pulmonary artery to left atrium shunt are the following:
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1.
Can be performed without extracorporeal circulation
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2.
Can be used with a telemetrically controlled adjustable occlusion device, as the Flo-Watch pulmonary artery banding device (EndoArt, Lausanne, Switzerland), which has been successfully introduced in clinical practice of banding [20].
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3.
Can be easily occluded with the current devices, as the Gianturco-Grifka vascular occlusion device which is an appropriate closure system to occlude the shunt because of the large size (9 mm) [21]
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4.
Can be easily performed in conjunction with a pumpless lung assist device as Novalung in parallel with the PA shunt or in a serial setting [22].