Refractory Hypoxemia after Mitral Valve Surgery: An Unusual
Cause and Its Successful Percutaneous Treatment

A variety of approaches are used for mitral valve repair and replacement operations. Rare complications specific to the surgical method may need to be considered if patients encounter unexplained problems postoperatively. We discuss such an example here. The operative approach to the mitral valve in this case was transseptal (from the right atrium), a method which has been widely used as an alternative to the more frequently employed left atriotomy.^1,2 There are certain situations in which the transseptal approach may be particularly useful — for example, allowing better visualization of the mitral valve than a standard left atrial incision in cases where the left atrium is relatively small (such as in acute mitral regurgitation).^3,4 Additionally, in cases of reoperation, a transseptal approach can prevent the need for extensive dissection of adhesions around the left atrium.^3,4 Nevertheless, the transseptal approach is associated with potential problems of its own. Disorders of cardiac rhythm are a concern, partly because the right atrial incision will compromise the artery to the sinus node and partly because the incision may disrupt normal atrial conduction pathways.^5,6 Comparative studies do indicate a higher incidence of postoperative arrhythmias, particularly junctional rhythms, with the transseptal approach, though these tend to be transient.^6–8 Another possible issue after an interatrial incision is shunting through any residual septal defect postoperatively, though published series suggest that the incidence of this is actually very low.^7,8Case Report. A 69-year-old male with severe mitral regurgitation secondary to posterior mitral valve leaflet prolapse underwent attempted mitral valve repair via a superior transseptal approach through the right atrium. The right atrium was incised parallel to the anterior aspect of the atrioventricular groove. A separate incision was made in the interatrial septum, starting from the inferior end of the fossa ovalis and extending superiorly. The right atrial incision was extended superiorly, medial to the right atrial appendage, to meet the interatrial incision. The roof of the left atrium was opened at the point where these two incisions met. Excellent visualization of the mitral valve was thereby achieved. Quadrangular resection of the posterior leaflet was performed but excessive regurgitation from the anterior valve leaflet persisted, hence the operation was converted to valve replacement using a metallic prosthesis (Standard 33 mm, CarboMedics, Burnaby, B.C., Canada). The atrial septal incision was subsequently closed using two continuous prolene sutures. The patient came off cardiopulmonary bypass on sequential epicardial pacing alone, but returned to the operating room later the same day due to a bleeding external (right) atrial suture line. Postoperative progress was slow. He became acutely unwell on removal of the epicardial pacing wires 5 days postoperatively, with sudden hypotension and tachycardia. Cardiac tamponade was suspected and he underwent emergency open drainage of the pericardium. A small perforation of the right ventricular free wall at the site of the epicardial pacing wire was identified and oversewn with prolene. The patient’s postoperative progress remained slow, with a persistently high inspired oxygen requirement and an inability to wean from the ventilator. Initial transthoracic echocardiography did not reveal any additional pathology. However, a subsequent transesophageal echocardiogram showed a sizeable atrial septal defect (ASD), with a large flap of atrial tissue billowing to-and-fro across the atrial septum. The defect measured 14 mm in a midesophageal bicaval view, and 24 mm in a midesophageal 4-chamber view, with a minimal caudal rim (Figure 1). Color Doppler indicated bidirectional interatrial shunting. It was agreed that the patient’s deteriorating clinical condition meant that he would be unlikely to survive further surgery to correct the defect. Percutaneous ASD closure was therefore attempted under general anesthesia. The right femoral vein was cannulated and a multipurpose catheter passed to the pulmonary artery. This showed a pulmonary artery pressure of 34/13 (mean 22 mmHg) and an oxygen saturation of 84% (on 60% inspired oxygen). The catheter was then withdrawn to the right atrium and passed across the defect to the left atrium; the pressure in this latter chamber was measured as 17/11 (mean 13 mmHg). The defect was sized and a 26 mm Amplatzer ASD closure device was deployed. The device was positioned across the defect with good closure and minimal residual flow (Figures 2 and 3) assessed by intraprocedural transesophageal echocardiography. An immediate improvement in systemic arterial oxygen saturation (on 60% inspired oxygen) from 92% to 97% was observed, suggesting that there had indeed been significant right-to-left shunting, presumably due to a “baffle” effect from the residual atrial septal tissue. Pulmonary arterial saturation fell immediately from 84% to 73%, while pulmonary artery pressure fell to 28/15 (mean 19 mmHg). The patient recovered quickly following this intervention, was successfully extubated and was finally discharged home 13 days later. A 6-month course of aspirin 75 mg o.d. was prescribed (in addition to warfarin therapy required for the metallic valve) to reduce the risk of thromboembolic problems relating to the ASD closure device. It was felt that the potential benefits of using a second antiplatelet agent (clopidogrel) were outweighed by the risks of major bleeding in this context. The patient has subsequently been reviewed on an outpatient basis several months later, and remains well with good exercise capacity. No residual shunt is apparent on echocardiography, and the mitral valve prosthesis is working well. Discussion The superior transseptal approach to access the mitral valve is a commonly used technique, allowing excellent exposure of the valve and offering clear advantages in certain situations, as discussed above. An uncommon complication of the approach is seen in this case, namely the dehiscence of the atrial septal suture line. This was manifest in a particularly unusual manner, with bidirectional shunting at the atrial level, resulting in refractory arterial hypoxemia and a consequent inability to wean from ventilatory support. Right-to-left shunting was not initially suspected, as pulmonary arterial pressures were not elevated. We feel that the explanation for this phenomenon is likely to be a “baffle” effect of the residual atrial septal tissue. This tissue, seen billowing freely in the right atrium on echocardiography, appeared to be directing vena caval return into the left atrium, in the absence of a driving pressure gradient. This postulated mechanism is probably similar to that underlying at least some cases of the rare, but recognized platypnea-orthodeoxia syndrome (POS).^9,10 POS is characterized by dyspnea and arterial hypoxemia, which are accentuated by the upright position and relieved by recumbency. It is due to intermittent (posturally related) right-to-left intracardiac shunting (most often due to an interatrial communication, usually a patent foramen ovale).¹¹ POS occurs in the presence of normal pulmonary artery pressures. Given the relatively common occurrence of a patent foramen ovale in the general population, additional anatomic factors must be responsible for the rare POS. These factors remain elusive in many cases, though a number of different anatomic substrates have been hypothesized.^9,12 Closure of an identified intracardiac communication usually results in successful treatment of POS.^13,14 Percutaneous device-based closure of the ASD proved possible in our patient without complication, despite the minimal margin between the defect and the atrioventricular valves. The Amplatzer septal occluder is a frequently used device for ASD closures and is associated with a low incidence of residual atrial shunting and embolization.¹⁵ It is also particularly effective in the closure of large defects such as is seen in this case.¹⁵ Occlusion of the atrial septal defect produced a dramatic improvement in our patient’s perilous postoperative state and allowed a successful recovery which has been maintained at medium-term follow up.

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