Post-Operative MAP Measurements Are Not Reliable for Estimating Pre-Operative Baseline BPs in Cervical Spine Surgery
Post-operative mean arterial pressure (MAP) measurements should not be used to estimate pre-operative baseline blood pressures in patients undergoing cervical spine surgery, as they fail to provide an accurate representation of the patient's true baseline hemodynamic status. 1
Why Post-Operative MAPs Are Unreliable as Baseline Estimates
Physiological Reasons for Unreliability
Altered Hemodynamics: The perioperative period involves significant hemodynamic changes that persist into the post-operative phase:
- Residual effects of anesthetics and analgesics
- Inflammatory response to surgical trauma
- Pain-related sympathetic activation
- Fluid shifts and potential blood loss
Lack of Systematic Baseline Assessment: Current perioperative practice rarely captures accurate baseline blood pressure in a systematic manner 1:
- Single clinic measurements are rarely reliable
80% of patients have preoperative systolic BP >120 mmHg in perioperative settings
- Pre-induction MAP cannot be used as a surrogate for normal daytime MAP derived from ambulatory measurements
Evidence Against Post-Operative BP as Baseline
The British Journal of Anaesthesia guidelines (2022) explicitly state that "no studies examining perioperative management of blood pressure have defined preoperative blood pressure in a systematic manner" 1. This highlights a fundamental problem in using any perioperative BP measurement as a reliable baseline.
Proper Approach to Blood Pressure Management
Establishing Proper Baseline
Patient-specific targets: Create individualized postoperative BP target ranges based on true preoperative baseline measurements taken before the surgical encounter 1
Ambulatory BP monitoring: When possible, use ambulatory BP monitoring which:
- Eliminates important clinical confounders
- Accounts for diurnal variation
- Predicts cardiovascular events independently of clinic measurements 1
Multiple measurements: Obtain multiple preoperative readings over time rather than relying on a single measurement
Postoperative BP Management
Reasonable therapeutic targets for patients with normal preoperative baseline BP 1, 2:
- Systolic BP >90 and <160 mmHg
- For patients with abnormal baseline values, adapt targets accordingly (e.g., maintain >70% of preoperative baseline)
Trigger values for clinical assessment 1:
- Systolic BP <100 mmHg (or <75% of baseline, whichever is higher)
- Systolic BP >160 mmHg (or >140% of baseline, whichever is lower)
Clinical Implications and Pitfalls
Common Pitfalls
"White coat hypertension": Elevated BP on day of surgery may represent situational response rather than true baseline 1
Assuming post-operative BP reflects baseline: This can lead to inappropriate medication adjustments and treatment targets
Ignoring patient-specific factors: Patients with chronic hypertension have altered autoregulation and may require higher BP targets 2
Critical Considerations
Hypotension risk: Intraoperative and postoperative hypotension (MAP <65 mmHg or SBP <90 mmHg for >15 minutes) is associated with myocardial injury, acute kidney injury, and mortality 1
Monitoring frequency: Increased frequency of postoperative BP monitoring is recommended for patients with concerning trends 1
Medication management: Resume preoperative antihypertensive medications as soon as clinically feasible to reduce 30-day mortality risk 2
Conclusion
Rather than attempting to use post-operative MAP measurements to estimate pre-operative baseline, clinicians should establish accurate baseline measurements before surgery and use these to guide individualized perioperative BP management. This approach will better protect patients from the risks associated with both hypotension and hypertension during and after cervical spine surgery.