Purpose: To investigate the possibility of esophageal phonocardiography as a monitor for invasively measured pulse pressure (PP) and its respiratory variation (PPV) in patients undergoing liver transplantation.
Methods: In 24 liver transplantation recipients, all hemodynamic parameters, including PP and PPV, were measured during five predetermined surgical phases. Simultaneously, signals of esophageal heart sounds (S1, S2) were identified, and S1-S2 interval (phonocardiographic systolic time, PST) and its respiratory variation (PSV) within a 20-s window were calculated. Beat-to-beat correlation between PP and its corresponding PST was assessed during each time window, according to the surgical phases. To compare PPV and PSV along with 5 phases (a total of 120 data pairs), Pearson correlation was conducted.
Results: Beat-to-beat PST values were closely correlated with their corresponding 3360 pairs of PP values (median r = 0.568 [IQR 0.246-0.803]). Compared with the initial phase of surgery, correlation coefficients were significantly lower during the reperfusion period (median r = 0.717 [IQR 0.532-0.886] vs. median r = 0.346 [IQR 0.037-0.677]; p = 0.002). The correlation between PSV and PPV showed similar variation according to the surgical phases (r = 0.576 to 0.689, p < 0.05, for pre-reperfusion; 0.290 to 0.429 for the post-reperfusion period).
Conclusions: Continuous monitoring of intraoperative PST with an esophageal stethoscope has the potential to act as an indirect estimator of beat-to-beat arterial PP. Moreover, PSV appears to exhibit a trend similar to that of PPV with moderate accuracy. However, variation according to the surgical phase limits the merit of the current results, thereby necessitating cautious interpretation.
Keywords: heart sound; liver transplantation; non-invasive monitor; pulse pressure.