The direction of shunt flow through the PFO is determined by the pressure gradient between the left and right atria. Shunt flow does not always occur through the PFO. As the left atrial pressure (LAP) is usually marginally exceeds right atrial pressure (RAP), most small PFOs are functionally closed and clinically insignificant. However, during the perioperative period, hemodynamic changes can considerably affect the delicate pressure balance [5, 6].

The Valsalva maneuver allows for detecting PFO with higher sensitivity [7]. This maneuver increases intrathoracic pressure, decreasing blood return from the systemic circulation to the right atrium. When pressure is released, venous return surges, elevating RAP above LAP, potentially causing a right-to-left shunt flow through the PFO [8, 9].

Surgical repositioning of the heart during OPCAB can also alter atrial pressure (Table 1). Anastomosis of the posterior descending artery is typically performed by lifting the apex of the heart using a heart positioner. This maneuver compresses both ventricles and disrupts diastolic filling, resulting in an increase in both RAP and LAP, with a more pronounced rise in RAP compared to LAP [10]. These changes can reverse the pressure gradient between RAP and LAP, leading to a right-to-left shunt through the PFO. Several case reports document the appearance of right-to-left shunt during anastomosis of the right coronary artery [3,4,5]. The extent of heart displacement varies with surgical factors (anastomotic location, graft length, surgeon preference, etc.), potentially influencing the appearance and direction of shunt flow [11]. In the present case, hypoxemia resulting from the right-to-left shunt was resolved with increasing FIO2, suggesting a relatively small shunt flow fraction. The degree of hypoxemia due to right-to-left shunt flow through the PFO depends on the volume of shunt blood flow. When shunt blood flow is significant, increasing FIO2is less effective in improving arterial oxygenation [12]. In such cases, it may be necessary to perform coronary artery bypass grafting under cardiopulmonary bypass or to close the PFO under cardiac arrest [4].

Notably, in the present case, bidirectional flow was observed under high intrathoracic pressure (inspiratory pressure, 15 cmH2O; PEEP, 10 cmH2O), despite the absence of shunting at lower intrathoracic pressure (inspiratory pressure, 10 cmH2O; PEEP, 5 cmH2O). These changes in shunt blood flow have not been reported in other case reports. In a study of patients undergoing noncardiac surgery under general anesthesia, who were gradually increased to a PEEP of 19 cmH2O during mechanical ventilation, TEE examination revealed a right-to-left shunt through the PFO that appeared at a PEEP of 10 cmH2O or higher, but was not seen at 0 or 5 cmH2O [13]. This finding is consistent with the present case, indicating that high PEEP levels around 10 cmH2O can potentially reverse the pressure gradient between RAP and LAP. An increase in intrathoracic pressure would elevate RAP, while a decrease in venous return would reduce blood filling to the left ventricular system, thereby decreasing LAP (Table 1). Although LAP was not measured, we speculated that a right-to-left shunt appeared in this patient when the mean CVP (= RAP) during right coronary artery anastomosis was 14 mmHg, which substantially exceeded the LAP. In addition, when CVP is less than 9 mmHg during normal heart position, LAP is always greater than RAP, so no shunt occurred. When the airway pressure is increased and CVP elevated to 11 mmHg, RAP and LAP became almost equal, and a bidirectional shunt appeared because the pressure difference between RAP and LAP can easily reverse during the cardiac cycle. On TEE examination, it appears that a right-to-left shunt occurs during systole and a left-to-right shunt occurs during diastole, but it is unclear why the interatrial pressure gradient changed during the cardiac cycle in this way.

In conclusion, we encountered a patient with a right-to-left shunt through the PFO detected by TEE, which resulted in hypoxemia exacerbated by cardiac displacement during OPCAB, and shunt direction dynamically changed with high airway pressure after chest closure. It is important to recognize that in patients who have a PFO, surgical manipulation or positive pressure ventilation can lead to development of a right-to-left shunt and hypoxemia. Whenever an event such as cardiac displacement or increased airway pressure occurs, assessing the direction and volume of the shunt with TEE is essential. Furthermore, high airway pressure during mechanical ventilation should be avoided to prevent the development of a right-to-left shunt and hypoxemia.