Anesthesia monitoring equipment (Proceedings)
Monitoring, for these purposes will consist of checking vitals on a regular basis. The vitals being monitored may be changed based upon the patient's status and the procedure performed. By watching for changes in trends we are able to catch and potentially stop a crisis from happening..
Why? Why do we monitor patients under anesthesia? By monitoring vitals of patients under anesthesia, we are able to facilitate optimum depth of anesthesia with minimal physiological impairment. When patients are deeper under anesthesia they are suffering from greater physiological impairment. By decreasing the concentration of inhalant, we are able to decrease the negative impact from the inhalant. The American College of Veterinary Anesthesiologists is now promoting new goals of monitoring under anesthesia which are less anesthetic morbidity rather than simply less mortality.
How? How we monitor animals under anesthesia will depend on 1) the procedure, 2) the patient, and mostly 3) the equipment available. We will typically monitor heart rate and rhythm, respiratory rate, temperature, blood pressure, oxygen saturation, end tidal carbon dioxide, capillary refill time, and mucous membrane color on all of our patients. We have the ability to run arterial blood gas samples when attainable and necessary.Blood Pressure
Blood pressure is a product of cardiac output, vascular capacity and blood volume. A decrease in any one will cause a drop in blood pressure. Measurement and support of blood pressure is extremely important because cardiovascular homeostasis is greatly compromised by anesthetic drugs. Compromising cardiovascular homeostasis causes hypotension and bradycardia. Excessive hypotension is a common cause of perioperative morbidity. Since the new goal of the ACVA is to decrease anesthetic morbidity, monitoring blood pressure is a vital part of "safer anesthesia". The goal of blood pressure measurement and support is to keep the mean arterial pressure (MAP) above 60 mmHg to assure adequate perfusion to kidneys and other organs. There are three methods of blood pressure measurement, oscillometric, Doppler, and direct (through an arterial catheter).
Oscillometric blood pressure monitoring uses a machine with a device that detects pulsation under an occlusion cuff placed over an artery. The machine will measure the mean then calculate the systolic and diastolic pressures. The blood pressure cuff used should be measured on the limb it will be used and should be approximately 40% of the circumference of the limb. This machine may not work consistently or even at all on small pets. The petMAP is a type of oscillometric blood pressure monitor. It is a sphygmomanometer that reads out systolic, diastolic, and mean blood pressures. There are two forms of this device the newest one is digital.
Measurement of blood pressure using a Doppler is ideal for small pets, but works on any size animal (assuming appropriate cuff sizes available). The Doppler uses an ultrasonic probe attached to a speaker to provide an audible sound for each pulse beat. Use of a blood pressure cuff and a sphygmomanometer with the Doppler will give systolic blood pressure readings. Sometimes the Doppler can be difficult to hear, this could be due to the patient (vasoconstriction) or the equipment (low battery, faulty probe). Sometimes the Doppler gets feedback from cautery, or from cell phones. For continuous BP measurements during a procedure, the Doppler probe can be attached to patient with tape. Common places where the Doppler will work is any limb or on the tail artery. A Doppler probe can be attached near the heart of exotic patients in order to monitor heart rate, but will not function as a blood pressure monitor.
Arterial catheterization is necessary for direct blood pressure, also known as invasive blood pressure (IBP), monitoring. This requires skills for placement of the catheter, a monitor with IBP capabilities, and a pressure transducer. Similar to oscillometric blood pressure monitoring, IBP will provide the systolic, mean, and diastolic arterial blood pressures. However, this monitor will actually measure the systolic and diastolic blood pressures then calculate the mean. Invasive blood pressure monitoring is different from oscillometric in that it is read out in "real time" not intermittently. Direct blood pressure monitoring is considered the most accurate form of blood pressure. On a multiparameter monitor, the direct blood pressure wave form should match the ECG complexes as well as the pulse oximeter plethysmograph. Other than blood pressure monitoring, the arterial catheter can be used to obtain blood samples.
Above all, with blood pressure monitoring consistency is most important. Switching between different monitors may result in completely different values. Watching trends in the blood pressure is better than just looking at a single value occasionally because it will give a better picture of what is going to happen in the future. The heart rate should be considered when monitoring blood pressure. If the heart rate is high and the blood pressure is low the patient may be hypovolemic. Intervention may be indicated with low blood pressures; an anesthetist should not continue to watch BP trend down without a plan of action. Turning down inhalant gas, providing fluid therapy, and starting a CRI of a positive inotrope are all good responses to low blood pressure. Which treatment to start with, and how much to give/turn down will depend on the patient and the procedure performed.