Defining Preload Responsiveness with Transesophageal Echocardiography After Elective Coronary Artery Bypass Grafting: do threshold and cardiac cycle sample size matter?

J. S. Kenny¹, G. Clarke², I. Kerrebijn³, S. Atwi³, T. Savery², C. Horner³, M. Knott², M. Elfarnawany³, A. M. Eibl¹, J. K. Eibl⁴, B. Nalla⁴, R. Atoui^4
1Health Sciences North Research Institute, Sudbury, Ontario ²Flosonics Medical, Sudbury, Ontario ³Flosonics Medical, Toronto, Ontario ⁴Northern Ontario School of Medicine, Sudbury, Ontario

Purpose: Preload responsiveness (PR) is present when stroke volume (SV) increases significantly following preload. PR-guided resuscitation may improve surgical recovery in elective, coronary artery bypass grafted (CABG) patients; therefore, this paradigm is clinically-important. Transesophageal echocardiography (TEE) can assess PR, but PR threshold and number of averaged cardiac cycles are not well-defined.

Methods: The local Research Ethics Board approved the study; adult, elective CABG patients were enrolled. Exclusion criteria were: known severe carotid stenosis, lack of informed consent and contraindication to TEE.
Left ventricular outflow tract (LVOT) velocity time integral (VTI) was measured by a cardiac anesthesiologist using a Phillips Epiq (Cambridge, MA, USA) 2.9 MHz probe in the trans-gastric window (0o insonation angle, 4 mm sample window, Figure 1A).
LVOT-VTI was continuously recorded during a 60 second (sec) baseline and 90 sec Trendelenburg maneuver following CABG with the chest open.
For each subject, maximal LVOT VTI change (LVOT∆) was calculated using 1-to-20 consecutively-averaged, cardiac cycles between supine and Trendelenburg positions. We analyzed three different thresholds to define PR: +10%, +15% and +20% LVOT∆.
Our primary outcome was the number of consecutively-averaged cardiac cycles needed to achieve an even allocation between PR and unresponsive patients - an accepted distribution in the literature.

Results: Data from 42 patients and preload challenges comprising 6168 cardiac cycles were included. Table 1 and Figure 1 show baseline patient characteristics and ratio of responders under different PR thresholds over a growing number of averaged cardiac cycles, respectively.

Figure 1B-D shows the number of preload responsive and unresponsive patients for the +10%, +15% and +20% LVOT∆ thresholds. For all thresholds, averaging fewer cardiac cycles increased the preload responsive-to-unresponsive patient ratio. With increasing numbers of averaged cardiac cycles, this ratio reversed.

A 1:1 ratio between preload responsive and unresponsive CABG patients was achieved at approximately 10 and 5 consecutively-averaged cardiac cycles for the +10% and +15% LVOT∆ thresholds, respectively. The +20% LVOT∆ threshold did not achieve a 50/50 split between PR and unresponsive patients, though at 5 consecutively-averaged cardiac cycles, 40% of subjects were labeled as PR.

Conclusion: For all LVOT∆ thresholds, averaging too few or too many cardiac cycles over- and under- diagnosed preload responders, respectively. Further, the recommended 3-6 cardiac cycles was not adequate at +10% LVOT∆ but was at +15% LVOT∆ ; this indicates a critical need for further research into correct LVOT∆ PR thresholds.

Identify the source of the funding for this research project: NOAMA-A-21-01