Postoperative Pacing for Cardiac Efficiency (POSTPACE)

Participants first undergo intrinsic sinus rhythm without protocol-directed atrial pacing, followed by temporary atrial pacing at 20 beats per minute above the intrinsic sinus rate and then 10 beats per minute above the intrinsic sinus rate. Each condition is maintained for approximately three hours.

Temporary atrial pacing is delivered using temporary epicardial atrial pacing wires placed during cardiac surgery. During the protocol-directed pacing period, participants receive atrial pacing at either 10 or 20 beats per minute above their intrinsic sinus heart rate, according to their randomly assigned sequence. Each pacing condition is maintained for approximately three hours. The comparison condition is intrinsic sinus rhythm without protocol-directed atrial pacing. The protocol-directed pacing period occurs during approximately the first 10 postoperative hours. Pacing may be temporarily interrupted, skipped, or discontinued at the discretion of the treating clinical team when clinically indicated.

Participants first undergo temporary atrial pacing at 20 beats per minute above the intrinsic sinus rate, followed by atrial pacing at 10 beats per minute above the intrinsic sinus rate and then intrinsic sinus rhythm without protocol-directed atrial pacing. Each condition is maintained for approximately three hours.

Temporary atrial pacing is delivered using temporary epicardial atrial pacing wires placed during cardiac surgery. During the protocol-directed pacing period, participants receive atrial pacing at either 10 or 20 beats per minute above their intrinsic sinus heart rate, according to their randomly assigned sequence. Each pacing condition is maintained for approximately three hours. The comparison condition is intrinsic sinus rhythm without protocol-directed atrial pacing. The protocol-directed pacing period occurs during approximately the first 10 postoperative hours. Pacing may be temporarily interrupted, skipped, or discontinued at the discretion of the treating clinical team when clinically indicated.

Renal near-infrared spectroscopy (NIRS) is a noninvasive monitor that estimates the amount of oxygen in blood within the tissues of the kidney. Renal NIRS will be continuously measured during each of the three study conditions: intrinsic sinus rhythm without protocol-directed pacing, atrial pacing at 10 beats per minute above the intrinsic sinus rate, and atrial pacing at 20 beats per minute above the intrinsic sinus rate. The mean renal NIRS value during the final 30 minutes of each approximately 3-hour condition will be used for analysis. The primary outcome will assess the change in mean renal NIRS across the three pacing conditions, with each participant serving as their own control. Expected values for renal NIRS in post-operative cardiac surgery patients with congenital heart disease range from 45% to > 95%.

Cerebral near-infrared spectroscopy (NIRS) is a noninvasive monitor that estimates the amount of oxygen in blood within brain tissue. Cerebral NIRS will be measured during the three study conditions: intrinsic sinus rhythm without protocol-directed pacing, atrial pacing at 10 beats per minute above the intrinsic sinus rate, and atrial pacing at 20 beats per minute above the intrinsic sinus rate. Expected values for cerebral NIRS in post-operative cardiac surgery patients with congenital heart disease range from 45% to > 95%.

Mean arterial pressure is an estimate of the average pressure in the arteries during a heartbeat and is routinely monitored after cardiac surgery. Mean arterial pressure will be recorded during the three study conditions. The mean value during the final 30 minutes of each approximately 3-hour condition will be compared across intrinsic sinus rhythm, atrial pacing at 10 beats per minute above the intrinsic sinus rate, and atrial pacing at 20 beats per minute above the intrinsic sinus rate. Expected ranges vary by age, sex and height (e.g., PMID: 28827377) and are adjusted for post-surgical status as opposed to ambulatory screening and management.

Arterial oxygen saturation is the percentage of hemoglobin in the blood that is carrying oxygen. It will be monitored during the three study conditions. The mean arterial oxygen saturation during the final 30 minutes of each approximately 3-hour condition will be compared across intrinsic sinus rhythm, atrial pacing at 10 beats per minute above the intrinsic sinus rate, and atrial pacing at 20 beats per minute above the intrinsic sinus rate. Expected values for mean arterial oxygen saturation in post-operative cardiac surgery patients with congenital heart disease range from 60% to > 95%.

Central venous pressure is the pressure measured in the central veins returning blood to the heart. Central venous pressure will be monitored as part of routine postoperative care. The mean central venous pressure during the final 30 minutes of each approximately 3-hour condition will be compared across intrinsic sinus rhythm, atrial pacing at 10 beats per minute above the intrinsic sinus rate, and atrial pacing at 20 beats per minute above the intrinsic sinus rate. Central venous pressure is highly dependent on the type of congenital heart disease repaired and will be compared to patients with similar physiology. Across a range of physiologic states, values are typically between 3 mmHg and 25 mmHg.

Mean airway pressure is the average pressure in the airways during a respiratory cycle. It will be recorded when available as part of routine postoperative care. The mean airway pressure during the final 30 minutes of each approximately 3-hour condition will be compared across intrinsic sinus rhythm, atrial pacing at 10 beats per minute above the intrinsic sinus rate, and atrial pacing at 20 beats per minute above the intrinsic sinus rate. Normal values are between 5-10 cmH₂O for our participant population.

Serum lactate concentration will be obtained from laboratory testing performed as part of routine postoperative clinical care. Lactate values and their corresponding collection times will be recorded and used to evaluate changes in lactate concentration following surgery and in relation to the study pacing conditions. No additional blood samples are required for the study. Expected values are lower than 10 mmol/L for most patients during recovery, although higher values are possible during marginal physiologic states. There is no meaningful interpretation of a low lactate level during surgical recovery and a level below the threshold of assay detection will be interpreted as a normal value.

Arterial pH measures the acidity or alkalinity of arterial blood. Arterial pH values obtained from blood gas testing performed as part of routine postoperative clinical care will be recorded with their corresponding collection times. Changes in arterial pH will be evaluated during the postoperative period in relation to the study pacing conditions. No additional blood samples are required for the study. While physiologic values between 7.35 and 7.45 are target values, we anticipate that patients will transiently have values between 7.2 and 7.35. Values lower than 7.2 will be evaluated by our clinical team. It is unusual to have significant non-iatrogenic alkalosis in the post-surgical state.

Arterial bicarbonate concentration is a measure obtained from arterial blood gas testing that provides information about the body's acid-base balance. Bicarbonate values obtained as part of routine postoperative clinical care will be recorded with their corresponding collection times. Changes in arterial bicarbonate concentration will be evaluated during the postoperative period in relation to the study pacing conditions. No additional blood samples are required for the study. We expected values between 21-27 mmol/L for our participant population.

Intrinsic heart rate is the participant's natural heart rate without protocol-directed atrial pacing. Heart rate will be continuously monitored during each study condition. Intrinsic heart rate measurements will be used to characterize the participant's underlying heart rate during the pacing phases and to evaluate changes associated with the different pacing conditions. Normal values are between 100-180 bpm for our participant population.

Pacing capture thresholds are the minimum electrical stimulus strengths required to consistently capture the heart and will be measured across multiple pulse durations using temporary epicardial pacing wires and routine temporary pacing equipment. These measurements will be used to construct postoperative strength-duration curves and derive rheobase and chronaxie, which describe the electrical stimulus characteristics required for cardiac capture. Measurements will be obtained at baseline and daily through postoperative day 3 to evaluate changes in pacing requirements and electrophysiologic properties over the postoperative period.