Of the patients with a true-positive outcome of increased postoperative clinic blood pressure, 5.34% were assigned a risk <50% in model 1 and were reclassified to the greatest-risk category (≥50%) in the most complex model, but 2.46% of true-positive patients who had been assigned the greatest risk in model 1 were reclassified to a lower risk in the more complex model. For patients with a true-negative outcome (i.e., nonincreased clinic blood pressure in the 12 months postoperatively), the more complex model correctly reclassified 1.05% to a lower-risk category who had been deemed high risk in the simple model but also incorrectly reclassified 1.49% of the true-negative patients into the high-risk category who had been more accurately assigned a lower-risk category in the simple model. In sum, if 100 hypertensive and 100 normotensive patients were put into model 4 versus model 1 and referred for follow-up based on a predicted 50% or higher likelihood of postoperative hypertension, it is estimated that an additional 2.88 of the 100 patients who were truly positive for postoperative elevated blood pressure in the 12 months after surgery would have been correctly referred, but this would have come at the cost of an additional 0.44 of the 100 patients without an elevated blood pressure being referred. The absolute net improvement in correct referral decisions of this hypothetical cohort using model 4 instead of model 1 would have been an improvement of 2.44 of 200 or 1.2% of referral decisions.
Given our desire to develop an easy-to-use clinical prediction tool and the apparently marginal improvement of the most complex model as compared with a simple model using preoperative blood pressure to guide referral decisions, we next sought to measure the sensitivity, specificity, PPV, and NPV of several easy-to-remember referral thresholds based on mean preoperative SBP and DBP of 140/90 mm Hg, 150/95 mm Hg, and 160/100 mm Hg (Table 2). A mean preoperative blood pressure referral threshold of ≥150/95 mm Hg demonstrated 33.7% sensitivity (95% CI, 33.3–34.1), 89.1% specificity (95% CI, 88.9–89.2), 51.5% PPV (95% CI, 51.0–52.0), and 79.6% NPV (95% CI, 79.4–79.7). This threshold would have resulted in a decision rule leading to 16.8% (95% CI, 16.6–16.9) of the cohort being referred. Such a decision rule would have achieved the results of (1) referring a group of patients who were more likely than not to in fact demonstrate poorly controlled outpatient clinic blood pressure and (2) not referring a group in which 4 of 5 were indeed normotensive during follow-up appointments.
In a large national cohort of surgical patients treated in VHA hospitals, poorly controlled outpatient clinic blood pressure in the year after surgery occurred in 25.7% of all patients, including 14.2% of patients with no known preoperative history of hypertension or antihypertensive treatment. Regarding the tradeoff between model performance and ease-of-use in identifying which patients are likely to demonstrate increased postoperative clinic blood pressures, our predictive modeling demonstrated marginal, and likely clinically trivial, improvements in predictive modeling when broad ranges of clinical and administrative data were added to a model that used preoperative blood pressures alone. A simple decision rule using a blood pressure referral threshold >150/95 mm Hg from 2 preoperative readings was able to identify a subset of between 16.6% and 16.9% of the national cohort who, as a group, were more likely than not to demonstrate increased outpatient clinic blood pressures (PPV lower 95% confidence limit: 51.0%) in the year after surgery. Importantly, almost 4 of 5 patients not meeting this screening criterion indeed demonstrated normal ambulatory clinic blood pressures (NPV lower 95% confidence limit: 79.4%). These findings are consistent with the notions that (1) even in the preoperative context, blood pressure does in fact perform reasonably well to predict blood pressure; and (2) despite adding large amounts of clinical and administrative data, models to predict increased postoperative clinic blood pressures demonstrate only marginal improvement in guiding referral decisions with the disadvantage of creating a much less user-friendly decision-support tool.
Despite a health care landscape that advocates for incentivizing prevention science44 and associated campaigns such as the Millions Hearts Initiative,45 the surgical literature has, until recently, remained largely focused on outcomes directly attributable to the surgical encounter, thereby unintentionally separating the perioperative health care experience from broader national efforts to improve public health through the provision of high-quality preventive medical care. Our finding that >1 in 4 patients demonstrated increased ambulatory clinic blood pressures in the year after surgery provides evidence to support the notion that the public health opportunity for anesthesiologists to reduce long-term morbidity by assuring timely follow-up care for poorly controlled blood pressure is significant.
Our work adds to the growing body of literature defining the emerging concept of the Perioperative Surgical Home. This concept has motivated several groups of researchers to examine ways in which care coordination around the time of surgery may enable safer and more efficient care of patients in need of surgical interventions.46–49 Our findings also reinforce research from other investigators who have found that consistently increased blood pressures, even within a high-stress health care environment such as the emergency department, are likely to reflect true blood pressure elevation, rather than merely a transient effect of being in a stressful environment.50 Prospective studies of counseling and referral efforts to improve the long-term preventive medical care of surgical populations are clearly warranted.
Several limitations of the present study deserve to be noted. First, it is not known to what extent the performance of a blood pressure referral threshold developed among US veterans would generalize to other settings.51,52 US veterans demonstrate a bimodal distribution in age, following historical variations in the numbers of active-duty US military personnel. They are also more likely to be male and are more likely than the general US population to carry a diagnosis of substance-use disorders, posttraumatic stress disorder, and other psychiatric comorbidities. However, even in the unlikely event that our findings were entirely limited to US veterans, our data would still apply to a growing population of several million people who together comprise the patient population of the largest single health care system in the United States.
In addition to its large patient population, the VHA also provides the advantage of being one of the few health care systems that is national in scope and that follows patients longitudinally across inpatient and outpatient settings within a single, integrated EHR. Second, our models may perform differently in populations who lack timely postoperative follow-up, as the present observational study was by necessity limited to patients who did have follow-up blood pressures available for analysis. Also, although blood pressures from structured fields in the VHA EHR compare quite well with manually extracted blood pressures,33 the variability in cuff sizes, patient positioning, and provider technique was unavoidable in this retrospective study.
As would be expected, in our analysis we identified significant bidirectional variability in the relationship between preoperative and ambulatory clinic blood pressure measurements, which, although similar to what has been previously reported,16 may be reduced in future prospective studies using standardized blood pressure collection methods. Among such methods, home blood pressure monitoring and ambulatory blood pressure monitoring53 performed outside of the medical clinic increasingly are used as part of primary care treatment decisions regarding hypertension and are likely to be useful adjuncts in the present population as well. In addition, other clinical and administrative data in the VHA EHR also are prone to varying levels of inaccuracy, and the associated misclassification of comorbidities and other clinical and administrative data is a factor that has been shown to introduce bias into results from large-scale EHR data research.32 Finally, it is not known what type of blood pressure counseling or referral intervention would find acceptance from physicians and patients already encumbered with arguably more acute concerns of the perioperative period. This final limitation is an additional vital avenue of inquiry to be pursued in further prospective clinical trials.
Despite these limitations, our findings provide evidence that by identifying patients with increased blood pressure in the perioperative period, the surgical care episode may be harnessed toward promoting long-term preventive medicine efforts. Similar work already has been pursued among anesthesiologists to promote long-term risk factor reduction in the case of smoking cessation.54–58 Specifically, regarding increased blood pressure, several multidisciplinary cooperative efforts among nurses, pharmacists, and other physician specialists, including surgeons, have demonstrated the potential feasibility of this idea for addressing the urgent and persistent public health need of improving the longitudinal control of elevated outpatient blood pressure.59–62
In summary, we found that among surgical patients, poorly controlled postoperative ambulatory clinic blood pressure is common and may present an opportunity for anesthesiologists to improve public health through care coordination efforts focused on improving follow-up care for undertreated blood pressure elevation. Among veterans presenting for surgery, the use of a simple approach to referral for blood pressure control based on a mean preoperative blood pressure ≥150/95 mm Hg provides a level of predictive performance that may find acceptance among clinicians and patients. Care coordination efforts by anesthesiologists, if they should succeed in improving blood pressure control in surgical patients, would be highly likely to markedly reduce long-term morbidity and mortality for this population.
a Although JNC-8 guidelines have since argued for a greater systolic blood pressure goal among patients older than 60 years of age,15 they were published after the institution of the present protocol and have not been endorsed by the American Heart Association or the National Heart Lung and Blood Institute.
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