Evaluation of hypotension prediction index software in patients undergoing orthotopic liver transplantation: retrospective observational study

Abstract

Background: Extreme hemodynamic changes, especially intraoperative hypotension (IOH), are common and often prolonged during Liver Transplant (LT) surgery and during initial hours of recovery. Hypotension Prediction Index (HPI) software is one of the tools which can help in proactive hemodynamic management. The accuracy of the advanced hemodynamic parameters such as Cardiac Output (CO) and Systemic Vascular Resistance (SVR) obtained from HPI software and prediction performance of the HPI in LT surgery remains unknown.

Methods: This was a retrospective observational study conducted in a tertiary academic center with a large liver transplant program. We enrolled 23 adult LT patients who received both Pulmonary Artery Catheter (PAC) and HPI software monitoring. Primarily, we evaluated agreement between PAC and HPI software measured CO and SVR. A priori, we defined a relative difference of less than 20% between measurements as an adequate agreement for a pair of measurements and estimated the Lin's Concordance Correlation Coefficient and Bland-Altman Limits of Agreement (LOA). Clinically acceptable LOA was defined as ± 1 L.min^-1 for CO and ± 200 dynes s.cm^-5 for SVR. Secondary outcome was the ability of the HPI to predict future hypotension, defined as Mean Arterial Pressure (MAP) less than 65 mmHg lasting at least one minute. We estimated sensitivity, positive predictive value, and time from alert to hypotensive events for HPI software.

Results: Overall, 125 pairs of CO and 122 pairs of SVR records were obtained from 23 patients. Based on our predefined criteria, only 42% (95% CI 30%, 55%) of CO records and 53% (95% CI 28%, 72%) of SVR records from HPI software were considered to agree with those from PAC. Across all patients, there were a total of 1860 HPI alerts (HPI ≥ 85) and 642 hypotensive events (MAP < 65 mmHg). Out of the 642 hypotensive events, 618 events were predicted by HPI alert with sensitivity of 0.96 (95% CI: 0.95). Many times, the HPI value remained above alert level and was followed by multiple hypotensive events. Thus, to evaluate PPV and time to hypotension metric, we considered only the first HPI alert followed by a hypotensive event ("true alerts"). The "true alert" was the first alert when there were several alerts before a hypotension. There were 614 "true alerts" and the PPV for HPI was 0.33 (95% CI 0.31, 0.35). The median time from HPI alert to hypotension was 3.3 [Q1, Q3: 1, 9.3] mins.

Conclusion: There was poor agreement between the pulmonary artery catheter and HPI software calculated advanced hemodynamic parameters (CO and SVR), in the patients undergoing LT surgery. HPI software had high sensitivity but poor specificity for hypotension prediction, resulting in a high burden of false alarms.

Keywords: Cardiac output; Hypotension prediction index; Intraoperative hypotension; Liver transplantation; Systemic vascular resistance.

Figure 1

Prediction alert and hypotension episode. Hypotension episode: MAP < 65 mmHg for at least 1 minute continuously. Started when MAP < 65 mmHg and ended when MAP ≥ 65 mmHg. Alert episode: at least two continuous records of HPI ≥ 85. Started when HPI ≥ 85 and ended when HPI < 85. From the start of an alert episode, if there was hypotension within 15 mins, we considered this alert to have successfully predicted the subsequent hypotension and called it a “true alert”. A “false alert”, by contrast, was defined when no hypotension occurred within 15 mins. If there were multiple subsequent alert episodes (A1, A2, and A3) before the first hypotension (H1), the subsequent alert episodes (A2 and A3) were ignored. If there were multiple subsequent hypotension episodes (H2 and H3) within 15 mins (from T5, the start of A4), all hypotension episodes (H2 and H3) were predicted by this single HPI alert (A4). For alerts lasting more than 15 mins or when hypotension occurred within the alert episode, we split the alert episode either at 15 mins (T7) when no hypotension occurred; or at the end of the subsequent hypotension episode (T8) when a Hypotension episode (H4) occurred within 15 mins. Time to hypotension was defined as the duration from the start of the true alert episode to the start of the predicted hypotension episode (Th1–T1). For alerts that predicted multiple hypotension episodes, time to the first hypotension was counted (Th2–T5).

Figure 2

Scatter plots of HPI software against the reference pulmonary artery catheter measurement on cardiac output and systemic vascular resistance. The Red dashed line is the 45-degree diagonal line, which indicates the perfect concordance of the two measurements. Plots were based on intraoperative measurements from 23 patients, with 125 pairs of cardiac output records and 122 pairs of systemic vascular resistance records.

Figure 3

Bland-Altman plots for HPI software vs. the reference Pulmonary Artery Catheter (PAC) measurement on cardiac output and systemic vascular resistance. The X-axis is the average of the two measurements, the Y-axis is the difference between the two measurements, specifically, PAC – HPI software. The solid line is the mean bias; red dash lines are 95% Limits of Agreement (LOAs); shaded areas represent the 95% Confidence Interval (95% CI) for limits of agreement. Plots were based on intraoperative measurements from 23 patients, with 125 pairs of cardiac output records and 122 pairs of systemic vascular resistance records. Bias and LOAs were calculated using Bland-Altman limits of agreement for repeated measures, adjusting for within-patient correlation. The 95% CIs for limits were estimated using the Method of Variance Estimates Recovery (MOVER).