Factors Associated With Hypotension in Dogs Under General Anesthesia

In the Literature

Miller L, Duncan JC, Handel IG, Shaw DJ, McKenzie HE, Greenhalgh SN. Association between body mass and hypotension in dogs under general anaesthesia. J Small Anim Pract. 2023;64(11):687-695. doi:10.1111/jsap.13671

The Research …

Common complications of general anesthesia, including hypoventilation, hypothermia, and hypotension, should always be anticipated. Hypotension has a reported incidence of up to 38% and can cause delayed recovery, neurologic deficits, intestinal dehiscence, and renal ischemia and injury.^1-4

This study was designed to explore a reported association between body mass and hypotension and identify whether additional factors might be associated with increased risk. Anesthetic monitoring records of 1,789 dogs that underwent general anesthesia at a tertiary referral hospital were retrospectively evaluated. Hypotension was documented when ≥2 consecutive readings of mean arterial pressure <60 mm Hg were recorded at ≥5-minute intervals. Noninvasive and invasive blood pressure measurement techniques were used at the discretion of the anesthetist.

Brachycephalic breed, American Society of Anesthesiologists physical status classification >III, performance of a surgical (vs diagnostic) procedure, and bradycardia were independently associated with increased odds of hypotension; increasing body mass, administration of an alpha-2–adrenergic agonist as part of premedication, and increasing temperature were independently associated with decreased odds. Hypotension occurred at least once in 32% of cases, with a median time from induction to first indication of hypotension of 31 minutes. Risk for hypotension did not significantly increase with increased anesthetic duration.

Based on these results, hypotension continues to be a common complication in dogs under general anesthesia. Special attention should be given to dogs with low body mass, brachycephalic breeds, and dogs with a higher American Society of Anesthesiologists physical status classification. Appropriate monitoring in the pre-, peri-, and postanesthetic periods is vital to ensure appropriate perfusion and minimize risks and complications.

… The Takeaways

Key pearls to put into practice:

• Most episodes of hypotension occur within 30 minutes following induction.

• Small patients have a lower body weight:surface area ratio, resulting in predisposition to hypothermia. Risk for hypotension may be increased in hypothermic patients. Brachycephalic breeds have a higher resting vagal tone that can lead to bradycardia, possibly resulting in predisposition to decreased cardiac output and resultant hypotension.

• Determining blood pressure via noninvasive techniques may be more difficult in smaller patients and brachycephalic patients due to their small and/or conical-shaped limbs.

• Although noninvasive blood pressure measurement (eg, oscillometry, Doppler) does not provide consistent, accurate readings compared with invasive measurement (eg, arterial catheter placement and measurement via transducer), monitoring trends over time and correlating with other monitoring data can help predict the likelihood of hypotension. For example, concurrent decreasing end-tidal carbon dioxide readings can indicate decreased perfusion, and a decreasing heart rate and blood pressure trend over time may signify blood pressure has become heart rate dependent.