Chapter 17
Troubleshooting Hypotension
A common complication
Lesley J. Smith
Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
- Q. What is considered “low” blood pressure in anesthetized animals?
- A. Mean arterial pressure (MAP) is the driving force for blood flow (perfusion) through capillaries that supply oxygen to organs and tissue beds of the body. In healthy dogs, cats, and other small mammals, mean arterial pressures <60 mmHg result in compromised perfusion of visceral organs and peripheral tissues, potentially leading to cellular or whole organ oxygen deprivation. In all species, mean arterial pressures <40 mmHg are associated with inadequate perfusion of vessel-rich organs such as the heart, lungs, and CNS. Prolonged hypotension can have obvious and devastating effects on organ function that are not apparent until the recovery period. Post-anesthetic blindness is an unfortunately common example.
- Q. How important is it to treat a mean arterial pressure < 60 mmHg?
- A. The answer to this depends a bit on your patient. A young healthy animal can probably tolerate a MAP < 60 mmHg for quite a bit of time (minutes, not hours) without suffering obvious or sub-clinical organ impairment. Very young animals (<6 months of age) have physiologically lower blood pressure, so a MAP of 60 mmHg in adolescent animals can be considered “normal.” In geriatric patients, sub-clinical organ impairment that may not yet reveal itself in abnormal chemistry values should have a high index of suspicion. For this reason, in geriatric patients a MAP < 60–70 mmHg should be treated earlier and more aggressively.
- Q. What are the major risks posed by hypotension?
- A. Significant hypotension that lasts for minutes to hours can lead to renal failure, delayed metabolism and clearance of drugs, ventilation/perfusion mismatch and hypoxemia, delayed recovery from anesthesia, and CNS abnormalities after recovery from anesthesia that may or may not resolve with time and supportive care. Untreated hypotension can lead to cardiac and respiratory arrest.
- Q. How can I estimate MAP from systolic blood pressure?
- A. Mean arterial pressure = diastolic pressure + 1/3(systolic pressure – diastolic pressure). Most indirect blood pressure monitors provide data for systolic pressure (e.g., Doppler method) or systolic, diastolic, and mean arterial pressure (e.g., oscillometric devices). Roughly, the mean arterial pressure is 20–30 mmHg less than the measured systolic pressure on a Doppler in most species. Thus, a Doppler reading of 80–90 mmHg correlates with a mean arterial pressure that would be considered hypotensive. The exception to this rule is cats, in which the Doppler reading correlates most closely with mean arterial pressure [1]. For more information on blood pressure monitoring, see Chapter 15.
- Q. What determines MAP?
- A. Mean arterial pressure = cardiac output (CO) × systemic vascular resistance (SVR). Cardiac output = heart rate (HR) × stroke volume (SV). Thus, a drug that reduces contractility (e.g., isoflurane) will lower stroke volume and can then contribute to a lower cardiac output, which may result in low mean arterial pressure if systemic vascular resistance has not increased. Figure 17.1 summarizes the factors that influence mean arterial pressure.
Figure 17.1 Factors that influence Mean Arterial Pressure (MAP)
- Q. Which factors decrease systemic vascular resistance and thus might lead to hypotension?
- A. acepromazine;
- thiobarbiturates;
- propofol;
- isoflurane;
- sevoflurane;
- desflurane;
- hemorrhage;
- any type of shock (cardiogenic, septic, neurogenic, anaphylactic);
- dehydration;
- inadequate volume administration or replacement;
- histamine release from any cause (e.g., mast cell degranulation);
- severe hypercapnia;
- significant hyperthermia.
- histamine release from any cause (e.g., mast cell degranulation);
All of the above listed drugs decrease SVR in a dose-dependent fashion. Thus, higher doses of acepromazine, as might be used for pre-anesthetic sedation in a nervous dog, could lead to hypotension because of a profound decrease in SVR. The injectable anesthetics decrease SVR transiently because of rapid redistribution. High inhalant settings will profoundly decrease SVR and lead to hypotension.
- Q. What common factors might decrease heart rate?
- A. These include:
- physiologic bradycardia (e.g., athletic dog);
- increased vagal tone (GI disease, brachycephalic breeds, oculo-cardiac reflex);
- hypothermia;
- intracranial disease, especially space-occupying lesions that elevate intracranial pressure;
- electrolyte imbalances;
- opioids;
- alpha-2 agonists;
- acetylcholinesterase inhibitors (e.g., edrophonium);
- anticholinesterases (transient, paradoxical; increase in heart rate usually follows).
As with drugs that decrease SVR, bradycardia caused by anesthetic drugs is usually dose-dependent and will be most profound in animals that have pre-existing bradycardia.
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- Q. What are the major risks posed by hypotension?
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