Anesthetic hypotension is the most encountered peri anesthetic complication observed in veterinary medicine. If left uncorrected and sustained it puts major organs at risk for permanent or fatal damage. Recognition and correction are critical, especially in patients with existing co-morbidities. The most common causes of hypotension under anesthesia include- drugs administered (including inhalant anesthesia) hypovolemia, and patient co-morbidities. Pattern recognition enables the anesthetist to determine and mange hypotension.
When hypotension occurs cerebral and coronary flow can be compromised. The mean blood pressure should not go below 60mmHg and the systolic below 80mmHg. Mean arterial pressure is the most important as it represent tissue perfusion.
Systemic blood pressure is a product of cardiac output times systemic vascular resistance. Cardiac output is heart rate times stroke volume. The latter encompasses preload, contractility, and afterload. The factors that affect cardiac output will result in changes of systemic blood pressure.
Monitoring of Arterial Blood Pressure
Arterial blood pressure can be measured directly and indirectly. Gold standard is arterial catheterization and allows for real time measurements. Minute to minute information can dictate immediate changes in the treatment blood pressure. Indirect blood pressure monitoring includes doppler and oscillometric.
Doppler flow can only provide one blood pressure measurement whilst oscillometric can provide a digital reading of systolic, mean, and diastolic blood pressure. The doppler and osccillometric methods require little technical skill although the doppler is more labor intensive. The doppler can be used across a wide range of species. Doppler tends to provide artifactually low measurements in cats but is still useful in monitoring trends of blood pressure.
Palpation of pulse pressure is unreliable and inaccurate due the magnitude of difference that can occur between systolic and diastolic pressures. The palpation of a pulse should be used to assume the heart is providing forward flow.
Management of Hypotension
- Can the inhalant be turned down?
- Can my patient receive intravenous fluids?
- Use of anticholinergic agents
- Upgrading to inotropes and or vasopressors
Anticholinergics such as atropine and Glycopyrrolate will increase heart rate to enhance cardiac output.
Ephedrine causes direct stimulation of alpha and beta receptors and indirect stimulation of norepinephrine. It can be administered as a bolus with a long duration of action. The drug is a controlled regulated medication.
Dobutamine stimulates the beta one receptors which in turn increase contractility and thus cardiac output. Due to its short duration of action, it must be used as a continuous rate infusion.
Dopamine is a norepinephrine pre-cursor. At lower doses it will affect the dopaminergic, at mid-range doses the beta receptors are affected, and high doses affect the alpha one receptor. There is an increase in the incidence of tachyarrhythmias. Due to the sort duration of action it must be used as a continuous rate infusion.
Norepinephrine causes an increase in overall systemic vascular resistance. Th negative effects include tissue ischemia secondary to intense vasoconstriction and tachyarrhythmias.
Epinephrine can increase the mean arterial pressure and causes an increase in overall myocardial oxygen consumption and can predispose the patient to arrhythmias.
Calcium maintains cardiovascular stability and assists in cardiac and vascular smooth muscle contraction. Adverse effects of hypocalcemia include negative inotropic effects and hypotension. Calcium administration should be done with a patient attached to an ECG monitor – if bradycardia develops the infusion should be discontinued and started at a lower rate.
Hypotension is very common in veterinary anesthesia. Blood pressure equipment is relatively inexpensive and is considered standard of care for anesthetic episodes. Monitoring can provide early recognition of hypotension and prompt treatment. Careful anesthetic planning can also negate hypotension in high risk patients. Prompt treatment of anesthetic hypotension can improve anesthetic outcomes and improve overall mortality and morbidity.
RW Morris, LM Watterson, RN Westhrope, et al. Crisis Management During Anesthesia: Hypotension. BMJ Quality and Safety 2005;14:e11.
E Mazzaferro and AE Wagner. Hypotension During Anesthesia in Dogs and Cats: Recognition, Causes, and Treatment. Compendium of Small Animals. Vol. 23, No.8, August 2001. 728-737.