In our sample of community anesthesia practice, there was a substantial occurrence of IOH using the previously-defined IOH measure, with 29% of cases having MAP < 65 mmHg for 15 minutes or longer. IOH has been reported in 5–99% of surgical cases.3,24,25 Limiting to studies that use a similar MAP threshold of 65 mmHg, the incidence of IOH ranges from 19–88% for MAP < 65 mmHg for at least 1 minute,3,8,26 45–49% for at least 5 minutes,3,26 and 31% for at least 10 minutes.3,26 Most previous studies are from major academic institutions. The incidence we observed was surprising given the presumed lower risk of our study population compared to previous studies.
Counter-intuitively, patients under age 40 experienced more IOH than those over 65. This aligns with some26 but not all8,27,31 published reports. Clinicians may be less likely to intervene on younger patients perceived to tolerate lower pressures.28 While older patients are more predisposed to hemodynamic disturbances, it is unclear whether this is predictive of adverse outcomes.8,29 IOH occurred more commonly in females than males, which aligns with published data.8,30,31 We did not report variation by BMI due to missing data. IOH occurred more commonly in patients with ASA physical status II, versus III or IV. This aligns with some published reports30 but contrasts with others.31 This pattern may also reflect sicker patients being managed by more experienced clinicians and in better-equipped settings. Practically, sicker patients are more likely to be monitored more frequently – or more invasively – and hypotensive episodes may be shorter-lived because of earlier intervention.5
Incidence of IOH increased with surgical duration, a finding in agreement with some5,32,33 but not all1,28,30 literatures. Perhaps surprisingly, the TWA of IOH was highest among surgeries lasting 1–2 hours. The majority (96.6%) of analyzed cases qualified as general anesthetics with 28.6% incidence of IOH. Reliable comparisons between different anesthetics cannot be made since anesthetic type was determined by the highest level anesthetic administered. Anesthesia base units were used as a practical surrogate of anesthetic complexity. Most cases were lower complexity (0–5 base units) or intermediate complexity (6–10 base units). High complexity cases (11 or greater base units) were least common but had the highest incidence of IOH (33.5%) compared to intermediate (29.1%) and low complexity (28.2%) cases.
Incidence of IOH was higher in ASCs than in hospitals. Our median surgery length was only 21 minutes shorter in ASCs than in hospitals (65 versus 86 minutes). (Supplemental material Table S.7) This result may reflect trends in management of IOH in ASCs, where patients are presumed to be healthier (less intensively monitored), or sites are less prepared to escalate IOH treatment.
The incidence of IOH varied across clinicians. Counter-intuitively, clinicians with worse scores on the risk-adjusted IOH measure were generally caring for younger patients with lower ASA physical status and were more likely in an ASC. This suggests that variation in IOH at least partially results from differences in clinician behavior or assessment of risk, as opposed to simply being driven by patient risk factors. Hence, IOH – generally attributed to pre-existing conditions and higher-risk procedures12,31 – may be an appropriate target for quality improvement.
This analysis has limitations. The data represent a convenience sample from two large anesthesia practices. Despite including 45 facilities in multiple U.S. regions, data may not be nationally representative. Our study population is limited to non-emergent non-cardiac surgical procedures and the facilities that contributed data do not use unified hemodynamic monitoring protocols. Considering the inherent limitations of retrospective and descriptive analyses, it is difficult to determine differences in IOH treatment among clinicians. We were unable to adjust the analysis based on combination anesthetics (i.e., general + regional) since the highest anesthetic intervention was allocated to each case.
BP values were either manually entered or automatically captured in the electronic record. It is reasonable to assume that manually recorded vitals would be artificially “smoothened,” but they may also omit obvious artifacts that an electronic system would include.28 However, there was not a clinically meaningful difference in IOH incidence across modes of data entry.
The IOH measure used for the clinician-level analysis is only risk-adjusted for five factors. To minimize reporting burden and limit chart reviews, the current model was limited to variables available to the anesthesia registries that calculate the measure: age, gender, BMI, ASA physical status, and surgical duration. Also, while some clinicians recommend using relative drops in blood pressure from a predefined, patient-specific baseline as a means to individualize hemodynamic management7; absolute thresholds have been shown to be similarly predictive of morbidity.12,15,34 It is plausible that MAP < 65 mmHg is equivalent to a ≥ 20% deviation from baseline for most patients. Based on these different definitions and thresholds, it is difficult to determine which of our four IOH defintions is the best target for intervention.
We did not have outcome data to study acknowledging that IOH is consistently associated with adverse outcomes. Based on published observational analyses, it is likely that the lower the blood pressure, and the longer the episode of hypotension, the greater the risk for adverse outcomes.2,5–8,14−18,23,31
Despite limitations, this study contributes by describing IOH rates in a community anesthesia setting and suggests that the burden of IOH is not limited to complex cases in academic hospitals. That IOH is more common in “lower risk” patients/settings and that it varies across clinicians, even after risk adjustment, suggests that it is modifiable (not solely determined by the patient and case risk) and a potential target for quality improvement. By instituting educational and systematic quality efforts, IOH could be amenable to mitigation, potentially improving outcomes for many patients. Policy changes may also provide incentives to reduce IOH. A recent government report on adverse events in hospitals found that hypotension was the most common harm event related to surgeries and procedures. The report recommended that CMS update its lists of hospital acquired conditions (HAC) for the HAC Reduction Program and the Deficit Reduction Act HAC list to capture common, preventable, and high-cost harm events, potentially including IOH.35