Anesthesia-Related Hypotension

Hypotension is commonly observed in patients under general anesthesia and can have harmful consequences; therefore, it is critical to monitor for and manage low blood pressure.

Importance of Measuring Blood Pressure During Anesthesia

Normal cardiovascular function is essential to maintain adequate oxygen delivery to tissue. Oxygen delivery is defined by the relationship between cardiac output and oxygen content but cannot be directly measured. Cardiac output is not typically measured in patients under anesthesia, but blood pressure measurement can be used to indirectly assess oxygen delivery and estimate organ perfusion.

Following Ohm’s law (see Mean Arterial Pressure as an Application of Ohm's Law), which relates pressure to flow and resistance, there are many potential causes of altered blood pressure. It is important to understand the influence of disease, anesthetic and nonanesthetic medications, supportive measures, surgical procedures, and other interventions in order to manage patients with hypotension.

Identifying Blood Pressure Values That Are Too Low

Normal arterial blood pressure values in nonanesthetized patients range from 100 to 160 mm Hg for systolic arterial pressure (SAP), 60 to 100 mm Hg for diastolic arterial pressure (DAP), and 80 to 120 mm Hg for mean arterial pressure (MAP).¹ These values are likely to fall in the lower part of these ranges in calm patients that are awake or nonanesthetized or in patients accustomed to having blood pressure measured. Values commonly decrease in patients under anesthesia.

Hypotension is considered the most common complication in patients under general anesthesia.² Lower limits of SAP (80-90 mm Hg) and MAP (60-70 mm Hg) are important for maintaining adequate organ perfusion in healthy adult dogs and cats.^3,4 In patients with chronic disease (eg, renal insufficiency) that results in hypertension, blood pressure should be maintained in higher ranges during anesthesia. Conversely, in puppies and kittens that maintain lower normal blood pressure in the first few months of life, lower values may be tolerated during anesthesia if other cardiovascular parameters are in normal ranges.⁵

Pressure may not directly equate with perfusion, and certain medications may cause vasoconstriction, which improves blood pressure; however, perfusion can continue to be compromised. Initial attempts to improve flow (cardiac output) versus vascular resistance are therefore preferred, but other approaches may also have value under other circumstances.

Options for Monitoring Blood Pressure

Indirect Methods

Pulse palpation can provide qualitative information on stroke volume (ie, difference between SAP and DAP) but is not reliable for estimating blood pressure values. 

Oscillometric and ultrasonic techniques (ie, Doppler) measure pressure in the cuff (hence, indirect) following cuff inflation and deflation. Oscillometric technology provides values for SAP, DAP, and MAP; Doppler is useful for monitoring trends in SAP. Accuracy varies and is influenced in part by location of the cuff, shape of the extremity, rapidity of deflation, size of the cuff, and distance of the monitoring site above or below the level of the heart. Recorded pressures increase or decrease, respectively, as the recording site falls below or rises above the heart level due to the hydrostatic pressure gradient; 0.7 mm Hg should be subtracted from or added to the recorded value for each cm below or above the heart. Cuff width should be ≈40% of the circumference of the extremity. Cuffs that are too wide or too narrow give erroneously low or high readings, respectively. 

Accuracy can also be influenced by species. Measuring blood pressure in cats can be challenging with either indirect technique, as oscillometric techniques may be unreliable, and Doppler can underestimate SAP.⁶ Vasoconstriction and bradycardia can also influence readings in dogs.⁷ Although oscillometric technology allows for automated readings, Doppler requires manual cuff inflation but provides an audible pulse signal.

Direct Methods

Aneroid manometer and strain gauge are both considered more accurate than noninvasive monitoring. These methods require cannulation of a peripheral artery and connection of the catheter to a measurement device. Accuracy depends on appropriate use (eg, maintaining the external zero-reference point level with the base of the heart). Aneroid manometer measures MAP and is inexpensive. A strain gauge or transducer records SAP, DAP, and MAP and displays values and a waveform on a physiologic monitor. Direct blood pressure monitoring is advocated in physiologically compromised patients to allow for rapid assessment and intervention. Analysis of the pressure waveform can also help determine the cause (eg, poor contractility) and treatment strategies.

Managing Perianesthetic Hypotension

Perioperative hypotension management is covered extensively in the literature.^8-10 Treatment should be directed at the cause, when known. If vascular volume is depleted (eg, dehydration, fasting, blood loss), IV fluids or appropriate colloids are indicated to increase preload; however, caution is recommended in some patients (eg, those with anemia or propensity for cardiac failure due to vascular volume overload). Vascular volume support is also generally indicated for drug- (eg, acepromazine, propofol) or toxin-induced vasodilation. Some circumstances, however, may require use of vasoconstricting medications (eg, phenylephrine, norepinephrine).

Depressed myocardial contractility is common with increased doses of inhaled anesthetics, some injectable anesthetic agents, and certain cardiac conditions (eg, dilated cardiomyopathy). If myocardial contractility is depressed, administration of positive inotropic drugs (eg, dopamine, dobutamine) and decreasing the anesthetic dose when possible are suggested. 

Anesthesia-supportive measures (eg, positive-pressure ventilation) may result in decreased venous return. Surgical conditions (eg, abdominal distension during laparoscopy) may have a similar influence. Surgery may also result in blood loss or release of endotoxins that should be managed appropriately. 

Cardiac rhythm changes suspected to cause hypotension should be treated. Heart rate is a significant determinant of cardiac output; therefore, patients with drug-induced bradycardia should be treated with reversal agents (eg, alpha-2 adrenergic receptor agonists [eg, after administration of dexmedetomidine]) or anticholinergics (eg, after administration of opioids). Tachydysrhythmias should be similarly treated with appropriate medication (eg, lidocaine for ventricular tachycardia). Acid-base and electrolyte abnormalities occasionally contribute to these conditions (eg, hyperkalemia, which can result in bradycardia and arrhythmias) and require treatment (Table).

Balanced anesthetic techniques that employ cardiovascular-sparing medications are advocated for patients in which hypotension is anticipated because of disease or planned intervention. Although opioids are most commonly used, low doses of other drugs (eg, ketamine, lidocaine [dogs only]) may also provide inhaled anesthetic-sparing effects and analgesic benefits.

Dosages Commonly Used For Cardiovascular Support During Anesthesia

Case Examples

Case 1

An otherwise healthy, 6-year-old, 70-lb (32-kg) spayed Labrador retriever was presented for tibial plateau leveling osteotomy. Physical examination, CBC, serum chemistry profile, and urinalysis were normal.

Treatment included premedication (acepromazine, 0.01 mg/kg, combined with hydromorphone, 0.1 mg/kg IM); anesthetic induction (propofol, 3 mg/kg IV to effect, and midazolam, 0.2 mg/kg IV); anesthetic maintenance (isoflurane in oxygen; femoral/sciatic nerve block [0.5% bupivacaine, 1 mg/kg]); monitoring (electrocardiography to monitor heart rate and rhythm, oscillometric blood pressure measurement, capnography, pulse oximetry, and use of a thermistor [ie, esophageal temperature probe]); and support (IV fluids, 5 mL/kg/hour; forced air-warming device).

SAP, MAP, and DAP were 94 mm Hg, 68 mm Hg, and 50 mm Hg, respectively, shortly after induction with an initial isoflurane setting of 2%. Heart rate was 86 bpm.

MAP was low but not at the lower limit. The patient was determined to be deeper than needed, and isoflurane was decreased to 1.5%. There was no significant change ≈5 minutes later, and MAP was 66 mm Hg. 

The patient needed to be moved; thus, the vaporizer setting was not decreased further. A fluid bolus of up to 10 mL/kg was started to compensate for the vasodilating properties of acepromazine and propofol, as they may cause a decrease in preload. This method was considered appropriate because the patient was healthy and had a normal packed cell volume (PCV) and total protein. 

SAP, MAP, and DAP increased to 100 mm Hg, 76 mm Hg, and 61 mm Hg, respectively, after the fluid bolus was given. The patient was clipped, the block was performed, and the patient was moved to the operating room. While waiting for surgery, her blood pressure started trending down, but because MAP was in the 64- to 68-mm Hg range, the decision was made to wait and see if surgical stimulation increased blood pressure. Heart rate ranged between 58 and 62 bpm. 

No change in blood pressure or heart rate were observed as surgery was initiated. Reassessment was made based on lack of response to surgical stimulation. It was determined that the local block was effective and should allow the vaporizer setting to be reduced; isoflurane setting was decreased to 1.25%. An anticholinergic had not yet been administered. Atropine (0.02 mg/kg IM) was administered because the heart rate continued to decrease. After ≈10 minutes, heart rate increased to 80 to 84 bpm, and MAP was 74 mm Hg. The remainder of the procedure was uneventful.

Case 2

A 12-year-old, 9.3-lb (4.2-kg) neutered male domestic shorthair cat was presented for dental cleaning and evaluation. CBC and serum chemistry profile revealed elevated BUN (40 mg/dL; range, 18-35 mg/dL) and creatinine (2.5 mg/dL; range, 0.8-2.2 mg/dL), low PCV (28%; range, 32%-47%), normal total protein (7 g/dL; range, 6.3-8 g/dL), and urine specific gravity of 1.020. Results were consistent with previously diagnosed chronic kidney insufficiency. SAP was 155 to 175 mm Hg when the patient was awake. 

Although the effects of gabapentin might be prolonged due to renal compromise in this patient, the owner was asked to administer 100 mg PO prior to the visit, as gabapentin had previously been effective in calming the cat. On physical examination, he was quiet and appeared slightly dehydrated; however, an IV catheter to administer fluids prior to premedication was unable to be placed. 

Premedication with methadone (0.3 mg/kg IM) and alfaxalone (2 mg/kg IM) was administered to facilitate IV catheter placement. Other opioids (eg, hydromorphone) or combinations (eg, butorphanol and midazolam, if no extractions are planned) may be substituted based on drug availability. A low dose of ketamine IM can be substituted for alfaxalone, but there is potential for prolongation of the drug’s effects in cats with renal disease. Anesthetic induction with additional alfaxalone (2 mg/kg IV to effect) was administered after 3 mL/kg of IV fluids were given. Anesthesia was maintained with isoflurane in oxygen because isoflurane could be decreased with planned dental blocks. Monitoring (ie, electrocardiography to monitor heart rate and rhythm, Doppler blood pressure measurement, capnography, pulse oximetry, a thermistor) and support (IV fluids, 3 mL/kg/hour after initial bolus; a forced air-warming device; mannitol, 0.1 g/kg/hour after hydration; fluids to be continued into recovery; and inotropes as needed) were also provided.

Doppler value (SAP) was 60 mm Hg during initial dental radiography, and heart rate was 160 bpm. Doppler might underestimate the SAP values, but this is not always the case, and it is preferred that Doppler readings remain ≥80 mm Hg, preferably 90 to 100 mm Hg, based on renal function. 

The isoflurane dose was reduced, but dental radiography was limited due to signs (ie, swallowing, thoracic limb movement) of lightening anesthetic plane. 

Although additional fluid was a concern based on initial PCV (because the cat was anemic, there was risk for hemodilution), the patient was slightly dehydrated on presentation, and another 3 mL/kg IV bolus was administered. Doppler readings increased slightly, but values remained in the low- to mid-60s. Dopamine (5 µg/kg/minute IV) was administered via syringe pump, and maintenance fluids were continued. Methadone (0.1 mg/kg/hour CRI) was also given to help reduce the isoflurane dose.

The patient responded favorably (Doppler SAP, 110-120 mm Hg), and the procedure was completed.