Specialty Spotlight: Managing Hypotension Under General Anesthesia

March 26th, 2025 | Posted in General, Medical Articles

Managing Hypotension Under General Anesthesia

Cheyenne J. Cannarozzo, DVM, Practice Limited to Anesthesiology & Pain Medicine

Hypotension is one of the most commonly encountered complications under general anesthesia¹. While certain disease states and co-morbidities predispose animals to hypotension under general anesthesia, the inhalational agents commonly used for maintenance of anesthesia (eg. Isoflurane, Sevoflurane) and the hypnotic agents commonly used for induction (eg. propofol) induce vasodilation and consequently lower blood pressure. Additionally, the volatile anesthetic agents commonly used are myocardial depressants and reduce cardiac output.

Perfusion is vital for the delivery of oxygen and nutrients to tissues and for the removal of waste products. Prolonged or severe hypotension can cause organ damage. Hypotension also interferes with the body’s autoregulatory mechanisms, which preserve perfusion to vital organs such as the brain and kidneys over a wide range of blood pressures. The kidneys are particularly susceptible to hypotensive episodes². While blood pressure is not a direct measurement of cardiac output or perfusion, pressure is required for forward flow of blood to tissues and vital organs, making it a vital parameter for monitoring in anesthetized patients.

What is blood pressure?

Blood pressure is arterial hydrostatic pressure relative to atmospheric pressure. It refers to the force of blood exerted against the walls of the arteries and is traditionally expressed as three values: the systolic arterial pressure (SAP), the diastolic arterial pressure (DAP), and mean arterial pressure (MAP). Heart rate, contractility, preload, and afterload are all determinants of blood pressure and each of these variables can be influenced by a combination of anesthetic drugs, surgical procedures, and patient disease states.

How do we measure blood pressure?

There are both non-invasive and invasive methods for the measurement of blood pressure. Non-invasive methods of blood pressure measurement include Doppler and oscillometric methods. In small animals, Doppler blood pressure best represents the systolic arterial pressure. Oscillometric methods provide SAP, DAP, and MAP. For the anesthetist, MAP is the most important blood pressure value as it represents the average upstream pressure for tissue perfusion and average afterload to the heart.

Invasive methods require the placement of an arterial catheter in a peripheral artery, which is connected to a fluid-filled pressure transducer for monitoring. This is the gold standard of blood pressure measurement, but because it requires special skill for catheter placement and specialized equipment for monitoring, it is not frequently used in small animals outside of specialty centers.

What should blood pressure be under anesthesia?

Normal blood pressure ranges vary by species and by patient factors. It is crucial to obtain baseline blood pressure values prior to GA. Pulse palpation is not reliable for BP assessment, as anesthetic drugs can alter vasomotor tone. Ideally, MAP should be 70-80mmHg. A MAP under 65mmHg is consistent with hypotension and should not be tolerated; interventions should be made promptly.³

How should we manage hypotension under general anesthesia?

When managing hypotension under anesthesia, it is important to identify the underlying cause of hypotension and treat accordingly. Below is a basic “algorithm” for blood pressure management under anesthesia.

First, assess your patient’s depth. Indicators of adequate depth include a ventromedial eye position, negative palpebral reflex, and loose (but still present) jaw tone. If your patient lacks jaw tone, has a negative palpebral reflex, and their eye position is central, this indicates that your plane of anesthesia may be too deep. If you can decrease your inhalational anesthetic agent, you should do so, keeping in mind the minimum alveolar concentration (MAC) of the agent you are using. MAC is defined as the concentration of inhalant that prevents purposeful movement in response to a supramaximal stimulus in 50% of patients. For isoflurane, the MAC is 1.3%, and for sevoflurane, the MAC is 2.4%. Drugs used for pre-medication and drugs administered as infusions (opioids, alpha 2-agonists, acepromazine, ketamine, benzodiazapines, etc.) can help to reduce MAC, meaning they lessen the amount of inhalant needed to maintain an appropriate anesthetic plane. If you can add in an infusion or bolus a MAC-sparing agent, this can help decrease your inhalant concentration, decrease the degree of inhalant-induced vasodilation, and improve blood pressure.

Next, assess your heart rate. If your patient is bradycardic and hypotensive, you should treat the heart rate with an anticholinergic agent (atropine or glycopyrrolate). Increasing heart rate will increase cardiac output, which will often improve blood pressure. The anticholinergic agents do not work well in the presence of hypothermia, so a response may not be seen in hypothermic patients. Remember, young patients are more heart rate dependent for cardiac output, so maintaining heart rate in this population is paramount to successful blood pressure management.

Assess whether your patient is fluid responsive. If your patient is tachycardic, hemoconcentrated, has an elevated total solids or lactate, or shows evidence of pre-renal azotemia, a fluid bolus should be administered. Caution should be exercised in patients with underlying cardiac disease who are at risk for volume overload and heart failure. A crystalloid bolus of 10mL/kg IV over 20 minutes is often adequate, but this will vary depending on the severity of bloodwork changes and the patient’s disease state. Typically, fluid boluses will be ineffective at improving blood pressure in euvolemic patients without concurrent lightening of the anesthetic plane.

Finally, if these interventions fail or are not applicable, a vasoactive substance should be chosen. Vasoactive substances differ in the receptors they target. Those that target alpha adrenergic receptors will result in vasoconstriction, and those that target beta adrenergic receptors will increase heart rate and myocardial contractility. These agents should be chosen based on the underlying cause of hypotension and administered as infusions to effect with careful monitoring. Commonly used agents include phenylephrine, norepinephrine, dobutamine, and dopamine.^4-7

Depending on your patient’s disease state and the procedure they are undergoing, multiple interventions may be necessary concurrently (eg. septic patients who are vasodilated and hypovolemic will require fluid resuscitation and vasoactive support).

– Cheyenne J. Cannarozzo, DVM, Practice Limited to Anesthesiology & Pain Medicine

References

1. Costa RS, Raisis AL, Hosgood G, et al. Preoperative factors associated with hypotension in young, anaesthetised dogs undergoing elective desexing. Aust Vet J. 2015; 93, 99- 104.
2. Chen B, Davis J, Rossi G, et al. (In Press) Exploring urinary biomarkers of early acute kidney injury in a clinical model of canine intra-operative hypotension: an observational cohort study. Vet Anaesth Analg 2025.
3. Ruffato M, Novello L, Clark L. What is the definition of intraoperative hypotension in dogs? Results from a survey of diplomates of the ACVAA and ECVAA. Vet Anaesth Analg 2015; 42, 55-64.
4. Silverstein DC, Beer KA. Controversies regarding choice of vasopressor therapy for management of septic shock in animals. J Vet Emerg Crit Care (San Antonio) 2015;25:48-54.
5. Foulon P, De Backer D. The hemodynamic effects of norepinephrine: far more than an increase in blood pressure! Ann Transl Med. China, 2018;S25.
6. Huuskonen V, Restitutti F, Raekallio M, et al. Cardiovascular effects of dobutamine, norepinephrine and phenylephrine in isoflurane-anaesthetized dogs administered dexmedetomidine-vatinoxan. Vet Anaesth Analg 2022;49:546-555.
7. Murphy KM, Rishniw M, Silverstein DC. Use of vasopressors for treatment of vasodilatory hypotension in dogs and cats by Diplomates of the American College of Veterinary Emergency and Critical Care. J Vet Emerg Crit Care (San Antonio) 2022;32:714-722.