The oximeter measures only the level of oxygen saturation and heart rate, which may be elevated when the patient hyperventilates
in response to discomfort. Unfortunately, a hyperventilating patient also can inhale excessive anesthetic progressing to hypovolemia.
Pulse oximeters do not measure how forcefully the heart is beating.
Blood pressure measurement
In human medicine, blood pressure measurement is part of most examinations and constantly evaluated under general anesthesia.
In small animal practice, blood pressure measurement is equally important. The mean arterial pressure (MAP) should be greater
than 60 mm Hg under anesthesia. If blood pressure drops below this level, the anesthetic concentration should be lowered.
Noninvasive blood pressure measurement uses a cuff placed around the patient's limb or tail at the level of the heart. In
cats, the tail base may have to be clipped. The cuff diameter should be 40 percent of the circumference of the limb or tail
base (3 cm in cats, 4 cm for small dogs, and 5 cm and up for larger dogs). Normal readings for anesthetized dogs and cats
are systolic 90-105 mm Hg, diastolic 40-60 mm Hg, mean 60-70 mm Hg.
Commonly used types of noninvasive blood pressure monitors include:
- Doppler that measures systolic arterial pressure. The pitch of the sound reflected from the moving blood cells is proportional
to its velocity. A piezoelectric crystal microphone, amplifier, inflatable cuff, manometer and earphones are used. An ultrasonic
flow detector is placed over an artery and taped in place. A cuff is placed proximal to the crystal and inflated until blood
flow is occluded. The cuff is slowly deflated. The pressure at which blood flow becomes audible again is the systolic pressure.
Pressure-plethysmography provides systolic, diastolic and mean pressure using an inflatable cuff to occlude blood flow, and
a sensor is placed distal to the cuff to detect arterial pulsation. The cuff is wrapped above the carpus, tail or below the
hock, and the sensor placed on the same limb just below the cuff. Accurate placement over an artery is not essential. The
cuff automatically inflates to a pressure, which occludes the underlying arteries and then deflates gradually. When the cuff
pressure is equal to systolic arterial pressure, flow proceeds and arterial pulsation returns. After systolic and diastolic
arterial pressures have been determined, the computer calculates the mean pressure.
Carbon dioxide measurement
Carbon dioxide is produced by all cells, transported by the circulatory system and eliminated through the lungs. Alveoli are
sites of gas exchange. The highest concentration of carbon dioxide should occur at the end of expiration when the diluted
gases from the trachea and primary bronchi are no longer being sampled. Changes in carbon dioxide levels reflect changes in
metabolism, circulation and respiration. Measuring expired carbon dioxide allows an estimation of arterial CO2, which lets the veterinarian know whether the anesthetized patient is ventilating adequately.
- Increased CO2 readings can be seen due to the following:
- Mild to moderate airway obstruction
- Faulty check valves
- Exhausted soda lime
- Decreased CO2 readings can be seen due to the following:
- Disconnection from the breathing circuit
- Esophageal intubation
- Cardiac arrest.
Blood-gas acid/base monitoring
Blood-gas acid/base measurement can provide useful information in the anesthetized patient. While capnography and pulse oximetry
give an indirect indication of respiratory function, direct measurement of pO2 and pCO2 can be made using arterial blood-gas measurements (obtained from the femoral or lingual artery). Typically, these measurements
are done serially as rapid changes can occur with changes in the patient's respiratory function. As such, these parameters
are sensitive indicators of what is at times a challenge in the compromised patient under anesthesia.