Anesthesia is risky because it upsets physiologic homeostasis. If that cat spay only takes 10 minutes, a healthy animal is likely to withstand being hypotensive and bounce back without experiencing a clinical disaster. But, if you want to anesthetize a 15 year old cat with renal disease and a heart murmur for a 2 hour dental procedure, you have to think about upping your monitoring and supportive care.
There are 5 macrovariables that can be assessed easily during anesthesia, including heart rate/rhythm, pulse oximetry, temperature, blood pressure, and capnography.
Measurement of blood pressure is an objective indicator of cardiovascular function, and, although pressure is not equal to flow, there does have to be a pressure difference in order for blood to move forward to the tissues. Indirect blood pressure measurement is the most commonly used method, including both Doppler and oscillometric devices. Unfortunately, none of the available monitors meet all the requirements for accuracy in all conditions. This can be really frustrating!
In one clinical study in dogs, sensitivity of Doppler BP for hypotension was about 67% and specificity was 87%. (1) To me this means that if the Doppler reading is low, it's a pretty safe bet that blood pressure actually is low, whereas a normotensive reading - especially a borderline reading - may actually mean the patient is hypotensive.
Following trends is usually your best bet. Get a baseline reading before induction and compare subsequent readings to get an idea of how things are changing in the patient in front of you.
Once you've documented hypotension in your anesthetized patient, it's time think about what is causing it and how to manage it.
Causes of Hypotension
Treatment of hypotension is best directed by addressing the underlying cause. Arterial blood pressure is determined by cardiac output and systemic vascular resistance (SVR). Cardiac output is dependent on heart rate and stroke volume, which, in turn, is determined by cardiac contractility, preload and afterload. Mean arterial pressure represents the driving force for organ perfusion.
If you look closely at the determinants of blood pressure, you will see that hypotension can only be caused by a decrease in cardiac output or vasodilation.
During anesthesia in healthy patients, there are a few likely culprits in the development of hypotension. The big one is dose-dependent vasodilation (ie decreased SVR) due to general anesthetics. Bradycardia can also decrease blood pressure by reducing cardiac output. Stroke volume may be slightly decreased if the patient is not volume-replete or loses blood due to surgical interventions. Typically, cardiac contractility is maintained at clinically relevant doses of inhalational agents in healthy animals.
Here's a quick overview of how I think about managing hypotension during anesthesia:
1. Decrease delivery of inhalational anesthetic
Is the patient too deep? Would a small dose of an analgesic agent allow you to reduce inhalant delivery? Can partial injectable techniques be used?
2. Ascertain that heart rate is appropriate
Because blood pressure is dependent on cardiac output (BP=CO x SVR) and cardiac output is dependent on heart rate (CO=HR x SV), increasing HR, if the patient is bradycardic, may increase BP. An anticholinergic may be indicated; however, if the animal has received an alpha-2 agonist such as dexmedetomidine in the past 30 minutes or so, reversal with atipamezole should be considered before the use of an anticholinergic.
3. Consider volume status
Healthy anesthetic patients may be marginally volume-depleted due to deprivation of water and fluid shifts that may occur during anesthesia and surgery. Assuming normal cardiac status, boluses of 3 to 10 mL/kg of crystalloids may be attempted, keeping in mind the total fluid volume delivered. A healthy patient who isn't losing blood does not require a ton of fluids and 20 - 30 mL/kg total in the perianesthetic time period should be plenty.
These first 3 steps should control blood pressure in most healthy animals undergoing procedures of short length. In long procedures or for patients with systemic illness, you may need to:
4. Employ inotropes and/or vasopressors
The choice of drug depends greatly on the overall status of the patient. The choice of drug depends greatly on the overall status of the patient. For example, dopamine is often quite effective in increasing BP in healthy, volume-replete dogs and cats. However, patients with hypertrophic cardiomyopathy may be better served by using a vasopressor such as phenylephrine or ephedrine. Septic patients or those with SIRS may need a combination of pressors and inotropes to improve blood pressure.
Here's the Anesthesia Critical Events Checklist I use to deal with hypotension.
In a future post, we will discuss calculating infusions and mathematical shortcuts you can employ.
1. Bourazak & Hofmeister. Bias, sensitivity, and specificity of Doppler ultrasonic flow detector measurement of blood pressure for detecting and monitoring hypotension in anesthetized dogs. JAVMA 253(11):1433-1438.