Chapter 15
Anesthetic Monitoring Basics
What you don’t know can hurt them!
Martin J. Kennedy
Anesthesiologist, MedVet Animal Medical and Cancer Center for Pets, Ohio, USA
- Q. Who should monitor the patient under anesthesia?
- A. The American College of Veterinary Anesthesia and Analgesia (ACVAA) Monitoring Guidelines recommends that a designated veterinarian or technician remain continuously with the patient throughout the anesthetic period and that person be solely dedicated to managing the anesthetized patient [1]. The status of an anesthetized patient can be quite dynamic and changes can occur with little or no warning. Thus, having a person dedicated to continuously monitor the patient can allow early detection of complications or sudden changes in anesthetic depth that could compromise patient care.
- Q. Why do I need to monitor?
- A. Vigilant monitoring during the peri-operative period (before, during, and after anesthesia) is crucial in order to maximize patient safety and wellbeing. Patient monitoring is needed to assess that there is an adequate depth of anesthesia and minimize any insult to normal homeostasis that can be caused by anesthetic drugs directly, as well as from underlying disease. Monitoring enables early detection of anesthetic complications in order to direct prompt intervention (e.g., fluid bolus, anticholinergic, etc.) to avoid more severe complications. There is no absolute correlation between anesthetic depth and physiologic impairment; a patient can be “too light” but still remain hypotensive or hypoxic, thus appropriate monitoring is always indicated.
- Q. What should I monitor?
- A. Anesthesia monitoring should primarily focus on the cardiovascular and pulmonary systems – the systems responsible for oxygen delivery to tissues and removal of metabolic by-products from the organism. Patient monitoring involves a combination of repeated assessment of physical signs and objective measurements of cardiovascular and respiratory variables. Unfortunately there is no single variable to monitor, thus it is best to monitor multiple variables and make conclusions based on all available information. At a minimum, any patient under general anesthesia should have pulse, respiratory rate, blood pressure, and hemoglobin saturation monitored and recorded every 5 min, as recommended in the ACVAA guidelines [1]. Ideally, end-tidal CO2 should also be considered part of basic monitoring. Do not forget to repeatedly assess the patient; your hands, eyes, and ears are your most valuable monitoring tools! A common mistake is for the person monitoring anesthesia to rely too heavily on monitoring equipment. The first thing you should do if you suspect your patient is not doing well is to confirm that there is a pulse and check mucous membrane color!
- Q. Do I need to keep an anesthetic record?
- A. Yes! The anesthesia monitoring record serves as a legal document that records events during the anesthetic period. The monitoring record documents trends in a patient’s physiologic parameters as well as the response to any interventions. It is important to record any drugs administered during the peri-anesthetic period and the patient’s response (e.g., premedication drugs and level of sedation). Monitored physiologic parameters should be recorded at least every 5 min, as described above. Any unexpected complications encountered during the anesthetic period should also be recorded (e.g., hemorrhage, inadvertent extubation, etc.). Keeping a detailed anesthetic record is important not only for legal reasons, but it can also serve as a useful reference if the patient requires anesthesia again in the future.
- Q. What physical signs should I monitor?
- A. Patient heart rate, respiratory rate and character, jaw tone, eye position, muscle tone, and ocular reflexes are important indicators of anesthetic depth. Heart rate can be obtained from pulse palpation or an esophageal or standard stethoscope. Respiratory rate can be determined by watching thoracic movements or movement of the rebreathing bag in medium to large patients. In small patients (e.g., cats) the tidal volume is small enough that movement of the rebreathing bag during inspiration and expiration may be difficult to detect. Keep in mind that jaw tone in some breeds of dogs may be more difficult to assess due to highly developed masseter muscles (e.g., American pit bull). Capillary refill time (CRT) can also offer a subjective assessment of peripheral perfusion; a prolonged CRT (>2 s) can be an indicator of poor peripheral perfusion or excessive vasoconstriction. Anesthetic depth has classically been described as progressing through four stages based on these physical signs [2].
Stage I is the stage of voluntary movement, this stage lasts from the initial administration of anesthetic drug(s) to the loss of consciousness. Patients frequently have high heart rates, increased or irregular breathing, dilated pupils, and can often salivate, urinate, and/or defecate. Muscle tone and reflexes remain normal.
Stage II is defined as the stage of delirium or involuntary movement. This stage begins with the loss of consciousness and continues until a regular pattern of breathing develops. The CNS is depressed but reflexes become exaggerated and patients can react to stimuli with violent struggling and/or vocalization. Respiratory pattern can vary from breath holding to hyperventilation. Heart rate is increased and arrhythmias are possible. The pupils are dilated and the ocular reflexes are prominent. Endotracheal intubation attempts may elicit vomiting, regurgitation, or laryngospasm and thus should be avoided until the patient has obtained Stage III.
Stage III is characterized by unconsciousness with a progressive loss of reflexes and muscle tone. This stage is commonly referred to as “surgical anesthesia” and can further be subdivided into light, medium, and deep planes. Heart rate and respiratory rate are regular and progressively slow as depth increases. In a light plane of Stage III anesthesia the eyes are centrally positioned with medium to normal sized pupils, a sluggish palpebral reflex is present (tactile stimulation of the medial or lateral canthus results in blinking), and there is moderate jaw tone. As anesthetic depth increases from a light to medium plane, the eyes rotate to ventral and medial and the cornea should appear moist and glistening. The palpebral reflex will be absent but the corneal reflex remains (tactile stimulation of the cornea results in blinking) and some jaw tone is present. A medium plane of Stage III is the ideal depth of anesthesia for most surgical procedures. As the plane progresses to deep Stage III, the respiratory pattern will become slow and irregular with a pronounced diaphragmatic component, jaw tone will be absent, the corneal reflex will diminish, and eyes will progressively move back to a central position. The corneal reflex should always be present or the patient is too deep!
Stage IV is characterized by extreme CNS depression. Pulses may be weak or not palpable, respirations may cease, capillary refill time is prolonged, and the eyes will be at a central position with dilated pupils and a dry looking cornea. If the depth of anesthesia is not lightened then cardiovascular and/or respiratory arrest will ensue.
- Q. What information do I get from a pulse oximeter?
- A. A pulse oximeter provides continuous information regarding the respiratory (oxygen saturation of hemoglobin) and circulatory systems (pulse rate). Pulsatile blood flow in a tissue bed and differing light absorption characteristics of hemoglobin and oxyhemoglobin are used to determine the arterial hemoglobin saturation (SpO2). A patient’s SpO2 should be at least 95%. Patients are most likely to desaturate (SpO2 < 92%) on induction and/or recovery from anesthesia, thus these are critical times to monitor SpO2!
When SpO2 falls below 95% during induction, or shortly after, the endotracheal tube should be checked for proper placement. The combination of low SpO2 and an end-tidal CO2 (PE′CO2) of zero with spontaneous or assisted ventilation likely indicate esophageal intubation. The combination of a nonzero PE′CO2 and a low SpO2 suggests inadvertent bronchial intubation; the endotracheal tube should be checked for proper placement and backed out as needed.
On recovery from anesthesia patients can desaturate due to hypoventilation; they can have a normal or even increased respiratory rate but their tidal volume is inadequate. Pink mucous membrane color is not enough to ensure an SpO2 > 92%, thus SpO2 should be monitored until the patient can maintain a normal SpO2 while breathing room air. Any patient with a SpO2 less than 92% during recovery should receive oxygen supplementation. The human eye can only detect cyanosis when the SpO2 falls below 75%, therefore reliance on membrane color alone is not sufficient to ensure adequate oxygenation.
The ability of the pulse oximeter to detect problems with ventilation is limited during maintenance of anesthesia with inhalants delivered in 100% oxygen. This is due to the fact that a patient can have a less than ideal arterial partial pressure of oxygen (PaO2
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- Q. What should I monitor?
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