Author: Michael Vlessides
Transesophageal echocardiography (TEE) measurements made during general endotracheal anesthesia with mechanical ventilation underestimate the severity of aortic stenosis in patients undergoing cardiac surgery, according to a new study. Pressure gradients in these situations also were significantly greater during the inspiratory phase than the expiratory phase.
Given these findings, the investigators suggested that pressure gradient measurements should be taken at similar points in the respiratory cycle, and that the severity of aortic stenosis should be interpreted with caution during general endotracheal anesthesia with mechanical ventilation.
“We know that aortic valve replacement is one of the commonest valve surgeries,” said Muralidhar Kanchi, MD, a senior consultant and professor of anesthesia and critical care at the Narayana Institute of Cardiac Sciences in Bangalore, India, who collaborated with researchers at the University of Minnesota in Minneapolis, for this study. “Most patients with severe aortic stenosis present for aortic valve replacement.”
Yet as Dr. Kanchi described, both general endotracheal anesthesia and mechanical ventilation may influence the TEE measurement of the pressure gradient across the aortic valve during aortic valve replacement. This phenomenon may ultimately provide misleading and inconsistent findings due to stroke volume variability and alteration in loading conditions.
Dr. Kanchi and his colleagues enrolled 70 adult patients with severe aortic stenosis undergoing aortic valve replacement into the trial. All participants had undergone TTE within one month of surgery. Patients with aortic regurgitation that was greater than mild, those with associated valvular lesions, and those undergoing combined procedures were excluded from the study. Each participant underwent a standard general endotracheal anesthesia protocol and was mechanically ventilated after relaxation with pancuronium.
For preoperative TTE, the flow across the aortic valve was examined with multiple views and the highest pressure gradient across the valve recorded. Mean pressure gradient was obtained by integrating a continuous wave Doppler flow tracing across the aortic valve.
All patients underwent TEE evaluation prior to cardiopulmonary bypass during steady-state conditions (systolic blood pressure, 100-140 mm Hg), using three different controlled tidal volumes—8, 10 and 12 mL/kg body weight—with a respiratory frequency adjusted to ensure normocarbia (end-tidal carbon dioxide, 35-40 mm Hg).
In pre–cardiopulmonary bypass TEE, the deep transgastric aortic valve long-axis view was used to align the sample volume as close to parallel to the blood flow as possible; mean pressure gradient was measured during both inspiration and exhalation.
TTE Values Higher Than TEE Values
As Dr. Kanchi reported at the 2018 annual meeting of the American Society of Anesthesiologists (abstract A2091), the final study cohort included 43 men and 27 women, with a mean age of 58.3 years. Preoperative systolic pressure gradient across the aortic valve on TTE measurement was 96.9±26.5 mm Hg; the mean pressure gradient at that point was 60.7±18.1 mm Hg.
There was a significant decrease in the pre–cardiopulmonary bypass pressure gradient when measured by TEE (Table).
|Table. Pre-CPB Pressure Gradient Across Aortic Valve During General Endotracheal Anesthesia Using Three Tidal Volumes at Inspiration and Expiration|
|Tidal Volume||Pressure Gradient||Inspiration (mean ± SD)||Expiration (mean ± SD)||P Value|
|8 mL/kg||Peak pressure gradient||66.13±23.27||54.51±18.09||<0.01|
|Mean pressure gradient||37.20±13.44||30.60±10.83||<0.01|
|10 mL/kg||Peak pressure gradient||66.86±22.26||50.05±20.18||<0.001|
|Mean pressure gradient||38.43±14.16||30.01±13.35||<0.01|
|12 mL/kg||Peak pressure gradient||66.91±20.94||52.91±19.32||<0.001|
|Mean pressure gradient||39.10±13.35||31.67±13.82||<0.01|
|CPB, cardiopulmonary bypass|
“There was also a difference between inspiration and expiration in the mean and peak gradient pressure at all tidal volumes where the inspiratory pressure gradients were higher than the expiratory pressure gradients, at all levels,” Dr. Kanchi said.
Nevertheless, changing the tidal volume itself did not contribute to any significant change in observed pressure gradients.
“To summarize,” he said, “we found that the TTE-derived values were higher than the TEE-derived values, which means once the patient is anesthetized, the pressure gradients are underestimated. Furthermore, the pressure gradient measured during inspiration was greater than the mean during expiration in a paralyzed patient who was anesthetized under stable conditions. Finally, the variation in tidal volume from 8 to 12 mL/kg did not affect the pressure gradient.
“Given these findings, we would like to suggest that when we measure the pressure gradients after an anesthetic, it will be underestimated when compared to preanesthetic conditions,” Dr. Kanchi added.
Ying Hui Low, MD, an assistant professor of anesthesiology at Dartmouth-Hitchcock Medical Center, in Lebanon, N.H., noted that the study’s findings are consistent with previous retrospective studies showing that aortic stenosis severity is underestimated in patients undergoing general anesthesia, due to changes in loading conditions (Br J Anaesth2017;118:699-704; Anesth Analg 2016;122:953-958). “This study reinforces that knowledge and may be the first to evaluate the effects of general anesthesia on aortic stenosis measurements in a prospective manner.
“Knowing that TEE measurements of aortic stenosis change with loading conditions, the authors also demonstrate a more nuanced finding that these expected changes are also observed with changes in respiratory phases,” Dr. Low said. “Future studies that will be useful in guiding the analysis of intraoperative TEE measurements include examining whether awake loading conditions can be mimicked more closely by changing blood pressure through vasopressor use or ventilation settings, or evaluating appropriate cutoff values for the classification of aortic stenosis severity under general anesthesia by correlating them with awake measurements obtained during recent TTEs or angiography.”
Yet as Dr. Low added, the applicability of the findings may be somewhat limited by the tidal volumes used by the researchers. “The fact that they demonstrated changes with the larger tidal volumes means the findings may be less relevant to those who use common ventilation settings,” he said.