Several pathologic conditions can produce profound reductions in SVR, including sepsis, anaphylaxis, spinal shock, and reperfusion of ischemic organs. Most drugs administered during general anesthesia and neuraxial regional anesthesia decrease SVR. Decreasing DBP can have more dramatic effects on SBP when vascular compliance is poor. Increased pulse pressure can occur with an increased SV but most often occurs because of the poor aortic compliance that accompanies aging. The aorta is responsible for most of this compliance. The pulse pressure is created by the addition of stroke volume (SV) on top of the DBP within the compliant vascular tree. Where SVR is the systemic vascular resistance and CO is cardiac output. Although most of our focus is on understanding MAP alone, the other pressures (e.g., SBP, DBP, and pulse pressure ) also require attention. Physiologic Approach to Hypotension The logical treatment of acute hypotension categorizes MAP into its physiologic components:

Whether a change in anesthetic management will alter these risks needs future study. (4,5) In addition, the combination of hypotension, small volatile anesthetic concentrations, and low bispectral index scale (BIS) values have been associated with worse postoperative outcomes. In recent large retrospective studies, intraoperative hypotension of even 5 minutes’ duration (systolic blood pressure < 70 mm Hg, MAP < 50 mm Hg, diastolic blood pressure < 30 mm Hg) is associated with increased postoperative morbidity and mortality risks. Intraoperative hemodynamic instability has long been thought to result in worse outcomes after surgery. All have typical injury patterns associated with prolonged “shock.” Understanding the physiology behind hypotension is critical for diagnosis and treatment. Organs of most immediate concern are the heart and brain, followed by the kidneys, liver, and lungs. Hypotension can be of sufficient magnitude to jeopardize organ perfusion, causing injury and an adverse outcome. Hypotension varies from mild clinically insignificant reductions in MAP from general anesthesia or regional anesthesia to life-threatening emergencies. Although treatment of chronic systemic hypertension is sometimes necessary, acute hypotension is often a problem with many anesthetics. Systemic arterial blood pressure and mean arterial pressure (MAP) are commonly monitored by anesthesia providers via a blood pressure cuff or an indwelling arterial cannula.

Physiologic vs pathologic dead space