Intraoperative detection of a previously undiagnosed patent foramen ovale (PFO) presents the dilemma to the cardiac surgeon of whether to close or not to close the PFO. The presence of a PFO may be of limited clinical importance, because complications from PFO are rare and not well characterized (1). However, associations between PFO and paradoxical embolism, as well as PFO and hypoxemia have been described. In a case report, Akhter and Lajos (2) demonstrated the occurrence of intraoperative oxygen desaturation secondary to the development of right-to-left shunting. This shunting and desaturation occurred when the heart had been elevated for surgical exposure of the posterior branches of the coronary arteries.
There has been a trend toward an increase in the use of perioperative transesophageal echocardiography (TEE) (3) as well as the use of the off-pump approach to coronary artery bypass graft (CABG) surgery (4). Because PFO is a common finding in approximately 25% of postmortem examinations (5,6), and TEE is able to detect PFO with almost the same frequency in the clinical setting (7), it is therefore likely that a new diagnosis of PFO will be made intraoperatively in many patients undergoing CABG surgery. Clinicians are faced with the following question in this situation: should the planned procedure (off-pump CABG) be changed to on-pump CABG (with or without) PFO closure? If the off-pump CABG approach is used, it is unclear whether the direction or magnitude of an interatrial shunt changes in this setting. Given the abovementioned considerations, it has been our routine practice to perform an intraoperative examination for right-to-left interatrial shunt with contrast TEE as well as to perform a series of blood gas analyses. We report our findings of 11 patients diagnosed intraoperatively as having a PFO in whom off-pump CABG was performed.
IRB approval was obtained to retrospectively collect these data for presentation. Data were collected between April 2000 and May 2001. During this time period, 658 CABGs were performed, 264 (40%) off-pump. All patients were screened for PFO with contrast study and color Doppler. Concomitant PFO closure was performed in 18 patients.
After the induction of anesthesia, tracheal intubation, and placement of a pulmonary artery catheter, a complete TEE examination was performed by using a Sonos 1000, Sonos 1500, or Sonos 5500 ultrasound system (Philips Medical Systems) with biplane or omniplane transesophageal probes. The following sequence was used for PFO detection. The fossa ovalis area of the interatrial septum was visualized from the midesophageal level longitudinal (90° angle) view by using two-dimensional examination, and color flow mapping of this area was performed. Next, 10 mL of agitated saline was injected IV, and a positive airway pressure (20–30 cm H2O) release (provocative maneuver) was performed as soon as the contrast material was visualized in the right atrium (RA). Transient bulging of the intraatrial septum toward the left atrium (LA) after airway pressure release confirmed that a transient right-to-left pressure gradient had developed.
A PFO was diagnosed if contrast material (air bubbles) was seen in the LA within 3–5 cardiac cycles after release of positive airway pressure or if a left-to-right shunt was clearly identified by color Doppler mapping in the region of the fossa ovalis. Each PFO was characterized as large or small based on whether >20 bubbles crossed the intraatrial septum. The fraction of inspired oxygen was 1.0 in all patients throughout the operation.
If a PFO was diagnosed, the following variables were assessed as per our routine at three time points (before sternotomy, after heart elevation for surgical access, and at the end of surgery): 1) the presence of right-to-left passage of agitated saline without a provocative maneuver (defined as a right-to-left shunt), and 2) the arterial O2 saturation (SaO2) and partial pressure of O2 (Pao2) at an Fio2 of 1 using an i-STAT portable clinical analyzer (i-STAT Corp., Princeton, NJ). Three anesthesiologists were involved in performing TEE and blood gas analyses.
Eleven patients with a PFO diagnosed by intraoperative TEE (mean age, 75 yr; range, 43–90 yr; ASA physical status III–VE) were monitored for right-to-left shunt and intraoperative hypoxemia during off-pump CABG. Seven patients had stable coronary syndromes, one patient had suffered an acute myocardial infarction, and two patients had unstable angina. Two patients had a history of heart failure. One patient had a history of paroxysmal atrial flutter/fibrillation. The mean left ventricular ejection fraction was 50% (range, 25%–60%). Seven patients had mild-to-moderate valvular heart disease. None of the patients had an atrial septal aneurysm. One patient had a history of significant chronic obstructive pulmonary disease. One of 11 patients underwent re-do CABG (Table 1).
Surgery was performed through a standard median sternotomy in all but one case. The left internal mammary artery was harvested in eight cases, the right internal mammary artery was harvested in four cases, and saphenous vein segments were harvested in nine patients. Target coronary arteries included the left anterior descending (LAD), the obtuse marginal (OM), the posterolateral, and the posterior descending (PDA) arteries. In one patient with a history of previous CABG, a low sternotomy was used and the gastroepiploic artery was divided and anastomosed to the PDA via diaphragmal incision (Table 1). The target coronary arteries were stabilized by using a Cardiothoracic Systems multivessel stabilizer (Cardiothoracic Systems, Inc., Cupertino, CA) or Octopus tissue stabilization system (Medtronic, Minneapolis, MN). In 10 patients, the left ventricle was elevated by placing a wet sponge under the heart for exposure of the OM, posterolateral, or PDA. In the patient in whom a gastroepiploic artery was used, the heart was twisted rather than elevated for PDA exposure. One to three distal anastomoses (mean 2.5) were performed per patient.
During heart elevation, hemodynamic status was stabilized by a combination of norepinephrine and nitroglycerine infusions with the goal of maintaining mean systemic arterial blood pressure >60 mm Hg and the diastolic pulmonary artery pressure <30 mm Hg.
Proximal anastomoses were performed under partial cross clamping of the aorta. Ventricular pacing wires were placed in every patient, and the chest was closed with sternal wires.
Four patients had a small PFO and seven patients had a large PFO (Tables 2, 3). Before sternotomy, a left-to-right shunt was present in five patients and a right-to-left shunt in the absence of a provocative maneuver was present in two patients (in one patient, the shunt was bidirectional). After heart elevation, none of the patients developed oxygen desaturation. Changes with heart elevation were statistically significant for systemic diastolic pressure and central venous pressure only (Wilcoxon’s signed rank test, P < 0.05). In one patient, the LA could not be clearly visualized by using TEE when the heart was elevated. Two patients developed a new right-to-left shunt, whereas in another patient, the right-to-left shunt persisted. In yet another patient, the right-to-left shunt disappeared and appeared again after the heart was returned to its normal position. One of two patients with new right-to-left shunting became hemodynamically unstable with an unreliable pulse oximetry tracing during initial heart manipulation and positioning for LAD anastomosis. A norepinephrine infusion was administered for 4 min, which resulted in the systemic arterial blood pressure returning to “prelifting” values of 110/70 mm Hg. In this patient, central venous pressure increased from 8 to 15 mm Hg, whereas pulmonary artery pressure (35/17 mm Hg) did not change significantly from the prelifting level. Deviation of the interatrial septum toward the LA was noted. Agitated saline was injected into the central venous catheter, and a right-to-left shunt was identified. The partial pressure of oxygen, however, was 246 mm Hg (332 mm Hg at baseline) and the surgeon decided to continue with the off-pump approach. Subsequently, however, when the LAD was reperfused and the heart was elevated for performance of the OM and PDA distal anastomoses, repeat contrast studies (without positive airway pressure release) revealed almost complete disappearance of the right-to-left shunt (1–2 bubbles crossed) and the partial pressure of oxygen was 382 mm Hg. The remainder of the operation was uneventful.
In no patient was there an increase in the severity of shunt or change in its direction at the end of the operation when compared with the baseline evaluation.
One patient died in the early postoperative period from perioperative myocardial infarction (patient no. 3, Tables 1–3). Another patient had to be reintubated shortly after tracheal extubation for acute hypoxemia and CO2 retention. He was successfully extubated after a short period of mechanical ventilation (patient no. 2, Tables 1, 2). His postoperative course was also significant for an episode of new onset atrial fibrillation. Two other patients had paroxysms of new onset atrial fibrillation that required medical treatment, but all patients were discharged in sinus rhythm (patient nos. 6 and 8, Tables 1, 2). None of the patients developed persistent hypoxemia or a cerebrovascular accident.
We present our observations from a series of 11 patients with PFO who underwent off-pump CABG. A new right-to-left shunt developed in two of these patients; however, this did not result in any clinically important sequelae (e.g., no patient developed significant hypoxemia). The preexisting right-to-left shunt persisted during heart elevation in one patient and temporarily disappeared in another. This variable response, in our opinion, may represent a relative dysfunction of the right or left ventricle, which translates into an increase of RA or LA pressure respectively with an altered interatrial pressure gradient. We believe that an increase in RA pressure was balanced in most of the cases by an increase in LA pressure, as suggested by the concomitant increase in pulmonary artery diastolic pressure observed.
PFO is a common finding in approximately 25% of postmortem examinations (5,6). With recent advances in technology and techniques of TEE, it is possible to detect PFO with almost the same frequency in the clinical setting (7). PFO is usually a benign and silent lesion, but it can cause hypoxemia and embolic phenomena under circumstances when RA pressure exceeds LA pressure. These circumstances may occur during the perioperative period as a result of the effect of mechanical ventilation (8) and thromboembolism (9,10).
PFO is a frequent incidental finding during cardiac surgery because TEE is routinely used in these settings. An incidental finding of PFO on perioperative TEE examinations presents a difficult dilemma for the cardiac surgeon because data from clinical trials are not currently available. Published case reports, reviewed by us elsewhere (1), however, suggest that this condition may cause clinically relevant problems in the perioperative period during increase of RA pressure. These circumstances may develop from a pericardial effusion, a loculated thrombus, or from right ventricular dysfunction secondary to inadequate cardiac protection, ischemia (11), or heart elevation as in off-pump CABG surgery (2).
Intraoperative right-to-left interatrial shunting was described by Akhter and Lajos (2) in a case report of a patient with unstable angina and triple vessel coronary artery disease in whom off-pump CABG was attempted. When the heart was elevated by placing a wet lap sponge under the diaphragmatic surface, oxygen saturation decreased to 80%, systolic arterial blood pressure decreased to 80 mm Hg, and the RA pressure increased from 8 to 16 mm Hg. Removal of the sponge reversed the hypoxemia and hypotension. TEE was performed and showed no defect. However, when the sponge was placed under the diaphragmatic surface again, RA pressure increased and a significant right-to-left shunt through the foramen ovale was demonstrated by TEE. Cardiopulmonary bypass was initiated, revascularization was completed, and the PFO was closed. In contrast to this case report, in our series, none of the patients developed oxygen desaturation, suggesting that clinically relevant right-to-left shunt through PFO during heart lifting is a relatively infrequent event. It should be recognized, however, that our series does not have sufficient power for us to state the exact incidence of PFO-related instability during off-pump CABG.
Limitations of this study include its retrospective design and the limited number of observations made. Although we did not find significant changes in arterial oxygen tension in any of our patients, the study was not designed to statistically prove that off-pump CABG is safe in all patients with PFO. The absence of significant arterial desaturation does not imply the absence of long-term risk for paradoxical embolism. Prospective data are needed to identify risk factors for PFO-related complications and to develop preventive strategies in these patients.
In conclusion, our series suggest that off-pump CABG can be safely performed in most patients with PFO. We believe that the use of intraoperative TEE can be very helpful, not only for the detection of a PFO but also for right-to-left shunt monitoring in patients with a known PFO, because changes in shunt direction may be variable.
The authors acknowledge Dr. Desmond Jordan for his help in retrieving patient-related data from the hospital’s database.
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