Physician anesthesiologist at Stanford at Associated Anesthesiologists Medical Group
Richard Novak, MD is a Stanford physician board certified in anesthesiology and internal medicine.Dr. Novak is an Adjunct Clinical Professor in the Department of Anesthesiology, Perioperative and Pain Medicine at Stanford University, the Medical Director at Waverley Surgery Center in Palo Alto, California, and a member of the Associated Anesthesiologists Medical Group in Palo Alto, California.

CLINICAL CASE: You’re scheduled to anesthetize a healthy 55-year-old female for an appendectomy. Her blood pressure is 150/90 on admission. In the operating room, you induce anesthesia with your standard recipe of 2 mg of midazolam, 100 mcg of fentanyl, 200 mg of propofol, and 40 mg of rocuronium, and intubate the trachea. Five minutes after induction and 15-30 minutes before the surgical incision will occur, her blood pressure drops to 85/45. Is this a problem? What will you do? What level of hypotension is acceptable to you?

Low blood pressure in surgery

DISCUSSION: During surgery, anesthesiologists balance their administration of drugs to the level of surgical stimulation the patient is experiencing. The placement of an endotracheal tube is an intense stimulus to an awake patient, but only a moderate stimulus to an anesthetized patient. After the placement of an endotracheal tube, a lag time of fifteen minutes to thirty minutes or more occurs prior to surgical incision. During this interval, the blood pressure sometimes sags.

Let’s look at the anesthesia literature to learn what has been described about this problem.

David Reich, et al of Mt. Sinai Hospital in New York queried the computerized anesthesia records of 4,096 patients undergoing general anesthesia and analyzed the incidence of hypotension in the period immediately after induction. (Predictors of hypotension after induction of general anesthesia Anesth Analg. 2005 Sep;101(3):622-8). The median blood pressure (MAP) was determined before anesthesia induction, during the first 5 minutes after induction, and also the period from 5-10 minutes after induction. Hypotension was defined as either (1) a mean arterial blood pressure (MAP) decrease of >40% and MAP

Statistically significant predictors of hypotension after anesthetic induction included: ASA III-V, baseline MAP

Dr. Reich wrote, “association with mortality alone was not reported in the manuscript but was nearly statistically significant (P = 0.066). The majority of our colleagues apparently believe that transient hypotension is inconsequential to outcomes. Although limited by the problems associated with retrospective studies, the results of our study provide preliminary evidence that runs counter to the prevailing wisdom regarding transient severe hypotension during general anesthesia.”

What level of hypotension is unsafe for patients?

The effects of hypotension in nonsurgical subjects was studied in 1954 (Finnerty, FA, Cerebral Hemodynamics during Cerebral Ischemia Induced by Acute Hypotension1 Clin Invest. 1954 Sep; 33(9): 1227–1232). Young and old experimental subjects were subjected to increasing degrees of hypotension until clinical signs of cerebral ischemia developed. Hypotension was induced by intravenous administration of the anti-hypertensive medication hexamethonium. The authors discovered a linear relation between pre-hypotensive blood pressure and the level of induced hypotension that produced clinical signs of cerebral ischemia such as yawning, sighing, staring, confusion, inability to concentrate, inability to perform simple commands, nausea, dizziness, and involuntary body movements. Their data revealed that the safe level of hypotension was no lower than about 2/3 of the resting blood pressure before inducing hypotension. At 2/3 of their pre-procedure MAP, patients reached a threshold of clinical cerebral ischemia, with onset of yawning, sighing, staring, confusion, inability to concentrate, and inability to carry out simple commands. Because these studies were done on unanesthetized humans, it’s impossible to equate the data to patients with surgical anesthesia. Surgical patients have a different etiology for their hypotension, as well as reduced cerebral oxygen consumption from general anesthetic drugs. This explains why most surgical patients fail to manifest any cerebral damage resulting from episodes of hypotension occasionally following the induction of anesthesia.

The problem of hypotension and refractory hypotension following induction of anesthesia is currently being studied in an ongoing clinical trial at the University of Iowa. ( identifier: NCT02416024, contact Kenichi Ueda, MD, Induction agents in this study will include 1.5 mg/kg propofol, 2 mcg/kg fentanyl, 100 mg lidocaine, and 0.6 mg/kg rocuronium. Inhaled anesthetic will be sevoflurane at 0.5 MAC with 5L/min of 100% oxygen starting at mask ventilation till 10 minutes after tracheal intubation. Blood pressure will be measured by a brachial cuff prior to induction and every minute after intubation for 10 minutes. If the systolic pressure drops below 90 mmHg or more than 25% from baseline, the patient will be classified in the study as “Hypotensive.” Conversely, if the patient’s systolic blood pressure does not drop below 90 mmHg more than 25% from baseline within 10 minutes of intubation, the patient will be classified as “Not Hypotensive.” In attempt to bring systolic blood pressure up to above 90 mmHg or more than 25% from baseline in “hypotensive” patients, the anesthetic provider will use 100 mcg of phenylephrine (or 5 mg ephedrine if heart rate < 50 bpm) within 10 minutes of intubation. If over 200 mcg of phenylephrine (or 10 mg ephedrine) has been used without a return of the systolic brachial blood pressure >90 mmHg or more than 25% from baseline, the patient will be classified in the study as “Refractory Hypotensive.” Look for the results of this trial to be published in years to come.

Based on the data reviewed in this column, it seems advisable to maintain a patient’s mean arterial pressure at or above a level of 2/3 of their baseline pressure. What if the patient’s baseline blood pressure in their outpatient clinic notes is 120/80 (MAP=93) yet in the pre-operative room on admission to surgery their blood pressure is 150/90 (MAP=110)? This is not an uncommon occurrence, as blood pressure often spikes secondary to the inevitable anxiety which accompanies a pending surgery. Is the anesthesia provider compelled to maintain the blood pressure at 2/3 of 110 = 73 after induction, or compelled to maintain the blood pressure at 2/3 of 93 = 62 after induction? I can find no specific data to answer this question. In my experience, after the administration of 2 mg of intravenous midazolam the hypertensive 150/90 often decreases to the 120/80 (MAP=93) range. With this MAP = 93 value as the baseline blood pressure, 2/3 X 93 = 62 would be the lowest level of MAP I’d feel comfortable with. We’re trained to treat post-induction hypotension with a vasopressor. Typically phenylephrine 100 mcg will increase the pressure to its preinduction level. Some patients require more than one dose of phenylephrine.

Let’s return to the management of your Clinical Case above.

  1. You choose to administer a dose of phenylephrine 100 mcg IV, and the blood pressure returns to 110/70. You maintain general anesthesia depth with the inhaled anesthetic sevoflurane at 0.5 MAC with 5L/min of 100% oxygen.
  2. Five minutes later the blood pressure drops to 85/45 again, and you repeat a dose of phenylephrine 100 mcg IV.
  3. When the surgery begins, the blood pressure increases to 150/90, and you treat by increasing anesthesia depth.
  4. Note that per the Reich data above, the incidence of hypotension increased with higher doses of fentanyl at induction (5-5.0 mcg/kg fentanyl vs. 0-1.5 mcg/kg fentanyl). I’ve found that the lower dose range of fentanyl, specifically zero fentanyl at induction, works very well for many patients. Incremental doses of propofol alone blunt the transient hypertensive response to laryngoscopy and intubation, and the lack of fentanyl leads to less hypotension in the ten minutes post-intubation. Appropriate levels of narcotics are then titrated in when surgery commences and the surgical stimulus increases. Also per Reich’s data, for patients age 50 or older who are ASA III-V, or for patients who present with a baseline pre-operative MAP.


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