Intraoperative Invasive Blood Pressure Monitoring and the Potential Pitfalls
Invasive intraarterial blood pressure measurement is currently the gold standard for intraoperative hemodynamic monitoring but accurate systolic blood pressure (SBP) measurement is difficult in everyday clinical practice, mostly because of problems with hyper-resonance or damping within the measurement system, which can lead to erroneous treatment decisions if these phenomena are not recognized. A hyper-resonant blood pressure trace significantly overestimates true systolic blood pressure while underestimating the diastolic pressure. Invasively measured systolic blood pressure is also significantly more affected than mean blood pressure by the site of measurement within the arterial system. Patients in the intraoperative period should be treated based on the invasively measured mean blood pressure rather than the systolic blood pressure. In this review, we discuss the pros/cons, mechanisms of and the interpretation of the invasively measured systolic blood pressure value.
Introduction & Background
Disposable IBP Transducer Kit-Single Channel is the gold standard of arterial pressure measurement in 10-20% of high-risk patients [1-2]. In the remaining 80%-90% of surgical patients, the standard intermittent non-invasive blood pressure (BP) that is obtained using oscillometry with a brachial cuff has been shown to have only poor agreement with IBP in critically ill patients [3-4]. These observed measurement differences are clinically significant because they would have triggered a change in treatment in as many as 20% of the critical care patients. Non-invasive oscillometric BP measurement with a brachial cuff tends to, on average, overestimate BP during hypotension and underestimate BP during hypertension, with a significant bias and considerable scatter. Invasive BP measurement with an arterial catheter, providing continuous BP measurements, detected nearly twice as many episodes of hypotension as intermittent oscillometric measurements with a brachial cuff [5]. Continuous rather than intermittent hemodynamic monitoring is highly desirable in high-risk patients. Even when continuous BP monitoring was accomplished in medium-risk patients with non-invasive techniques, the number of episodes of intraoperative hypotension was still reduced by half when compared to intermittent monitoring with a brachial cuff [6]. Although non-invasive continuous monitoring has fewer complications than arterial cannulation, it has not yet as the gold standard in high-risk patients, but rather serves as an alternative in low and medium-risk patients where IBP measurements are not warranted [7].
An adequate blood pressure level is a means to achieve the ultimate goal of the circulation, which is adequate end-organ perfusion and tissue oxygenation. Adequate organ perfusion is mostly regulated locally, in the organs, by changing the local vascular resistance, which, when seen over multiple organs and the entire circulation, works as a re-distribution of the total flow or cardiac output (CO) [8]. In addition, the total flow or CO is also regulated centrally if this re-distribution is not enough. The local flow control via regulation of resistance of the arterioles only functions properly under the condition of adequate perfusion pressure, in which the mean systemic arterial pressure plays a central role. Continuous monitoring of local organ circulation, global flow, or CO and arterial pressure is, therefore, the key. Monitoring the microcirculation has been shown to be useful when determining the optimal BP range that is associated with adequate regulation of local blood flow and tissue oxygenation for an individual patient [9-10]. Pulse contour analysis provides a means of assessing global flow or CO because it has long been recognized that an apparently adequate BP level may not necessarily be associated with an adequate total blood flow to all the tissues [11-12]. Different organs have a different range of perfusion pressures that allow for adequate local control of organ flow. While the coronary circulation can increase flow fivefold as long as heart rate is maintained at 70 bpm, diastolic arterial pressure is maintained at adequate levels and coronary obstructive lesions are absent, the kidney is much more sensitive to decreases in perfusion pressure [13]. The average lower limit of cerebral blood flow autoregulation in normotensive adult humans is around a mean arterial pressure (MAP) of 70 mmHg [14]. Hence, the heart has a greater range of adequate perfusion pressures than both the brain and the kidneys. Blood pressure goals as adequate perfusion pressure ranges, therefore, need to be specifically determined and adjusted for every individual clinical situation by considering the patient’s specific comorbidities as well as the planned surgical procedure.
