When patients are anesthetized, the veterinarian needs to take responsibility for appropriate monitoring and supportive measures to avoid mortality and/or irreversible damage to important organs. Anesthetic crises are unpredictable, and tend to be rapid in onset and devastating in nature. Close attention to vital signs allows us to judge the depth of anesthesia, avoiding overdoses and ensuring a positive outcome maximizing the safety of the anesthetic drugs and allowing us to treat any observed complication as soon as possible.

Monitoring the Central Nervous System

During anesthesia, monitoring the CNS is a simple and reliable method used to determine the stage of anesthesia. This includes monitoring muscle tone, reflexes activity, and eye position. Basically, the swallowing reflex should be absent after induction of anesthesia and return during the recovery of anesthesia. It is usually considered safe to remove the endotracheal tube of a patient after the observation of the second swallowing reflex. The palpebral reflexes are also very useful. The palpebral reflexes can guide you during all phases of anesthesia. They are subdivided into lateral and medial palpebral reflexes. In dogs and cats, the lateral palpebral reflex disappears with light anesthesia and heavy sedation. The medial palpebral reflex disappears during the induction of anesthesia and is usually associated with good muscle relaxation. Usually in dogs and cats, it is considered safe to intubate when medial palpebral reflex disappears. Palpebral reflexes should be absent when patient is well anesthetized. However, the swallowing and palpebral reflexes can be present when ketamine is used for induction. The corneal reflex should always be present unless your patient is too deep or dead. This reflex should not be tested regularly due to high risk of corneal damage. Keep this test only for real emergencies, when you are testing if the patient is alive or not. The anal reflex is unpredictable. Jaw tone is an indicator of muscle relaxation and should be relaxed when patients are ready for surgery.

The eye positions in combination with the reflexes are very useful to grade the level of anesthesia in dogs and cats. Central eye position with strong palpebral reflex and strong jaw tone equals to light anesthesia. Ventromedial rotation of the eye with no palpebral reflexes and relaxed jaw tone equals to surgical anesthesia level. Central eye position with absence of palpebral reflex, relaxed jaw tone equals to too deep anesthesia level.

Monitoring the Cardiovascular System

Ideally, we should monitor cardiac output of patients under anesthesia; however, there is no cost-effective, noninvasive monitor currently available for veterinary patients that could be used during the day-to-day anesthesia. Since cardiac output is not an option, the veterinarian should focus the cardiovascular monitoring blood pressure (BP), heart rate (HR) and rhythm.

Peripheral Pulse Palpation

Peripheral pulse palpation is unreliable for monitoring the BP but can help you to calculate the HR and identify arrhythmias. Pulse deficits occur when the pulse rate is higher than the auscultable HR, or some pulses feel weaker than others. If a pulse deficit is detected, then an ECG is recommended. Peripheral pulse is only the difference between systolic and diastolic BPs. The presence of a palpable pulse can give you an inaccurate indication of BP. Some authors believe that a systolic BP higher than 50 mm Hg is necessary to ensure palpation of pulse. However, pulse palpation cannot substitute the real BP monitoring with noninvasive or invasive techniques.

Mucous Membranes Color

Mucous membranes color may change with room light. To be precise, check both the gum and tongue color.

• Pink=good perfusion & oxygenation.
• Pale or gray=vasoconstriction, significant hypotension or cardiac arrest.
• Bright red=endotoxic or septic shock.
• Bright pink=hypercarbia.

Capillary Refill Time (CRT)

Capillary refill time (CRT) should be <2 seconds. When prolonged, a possible hypovolemia, low BP and/or low cardiac output may be present. Also peripheral vasoconstriction due to use of alpha2 agonists or hypothermia can cause prolonged CRT.

Arterial Blood Pressure

The normal range that is associated with adequate tissue perfusion is: systolic arterial pressure (SAP)>90 mm Hg; mean arterial pressure (MAP)>60 mm Hg (small animals) and diastolic BP (DAP)>40 mm Hg. When values measured are less than these minimum values, we attempt to correct the hypotension with fluids, anticholinergics and sympathomimetic agents.

There are several methods of BP measurement:

Doppler Ultrasound BP Method

Usually considered to be low cost. The Doppler sounds indicate HR and rhythm. Dysrhythmias will often sound distinctly “different” on a Doppler. Then, you can confirm the type of dysrhythmia with an ECG. To measure BP, a cuff is placed proximal to the probe on the leg (the cuff width must be 40% circumference of the area to which it is being applied) after that, the cuff is inflated with a sphygmomanometer until Doppler sound disappears, then pressure is slowly released. In theory, the pressure at which sound is again heard is the systolic BP. However, research shows that Doppler is not a reliable method for measurement in cats.

Oscillometric BP Equipment

This technology is designed to detect oscillations in the cuff pressure that occur with arterial pulsations when the cuff is placed around an area. The monitor is able to electronically measure the pressures and display HR, systolic, mean, and diastolic pressures. Oscillometric BP monitors do not always give accurate pressures, particularly in patients with extremely slow or fast HRs, dysrhythmias and when they are very small animals, such as cats and dogs <10 kg. Again: Cuff width should be 30–40% of limb circumference. Only buy BP monitor if research is available to prove the accuracy of that particular brand and model. Not all monitors are the same.

Direct (Intra-arterial or Invasive) BP Monitoring Technique

This is the most accurate method available. It is considered the “gold standard” for BP measurement and is the preferred method when exact measurements are important. For this technique, a catheter is inserted into a peripheral artery (most commonly, the dorsal-pedal artery or the femoral artery). Then the catheter is connected to an electrical transducer via a fluid-filled extension set and the transducer is then connected to the monitor. The transducer then converts the mechanical energy of a pressure wave into an electrical signal that is displayed on the monitor screen as a waveform and as a numerical display of systolic, diastolic, and mean BP, and HR. The resultant waveform on the oscilloscope screen gives an indication of contractility, vasoconstriction or vasodilation, and volume status of the patient. For instance, myocardial contractility is indicated by the rate of upstroke of pulse pressure wave. Stroke volume is indicated by the area under the systolic ejection phase. Vasodilation is associated with steep downstroke and low dichroitic notch. Vasoconstriction is often described by gradual downstroke and high dichroitic notch. Hypovolemic patients can show exaggerated variations in size of the waveform with respiration (generally just see this with IPPV) and arrhythmias can also be detected as an absent or altered pressure wave form.

Electrocardiography (ECG)

Cardiac impulse generation and conduction are assessed electrocardiographically. The sinus node is the site of normal impulse generation, and the typical “P,” “QRS,” and “T”waves are the result of propagation of sinus impulses in the normal, orderly sequence of atrial and ventricular depolarization and repolarization. The appearance of the normal cardiac waveform varies with the choice of lead orientation, but the morphology of electrocardiograms is directly related to the site of impulse generation, and/or the route of impulse conduction.

Remember: ECG provides only HR and rhythm. A normal ECG does not indicate adequate cardiac output or tissue perfusion. Patients with significant arrhythmias identified on ECG (i.e., ventricular premature contractions or ventricular tachycardia) should have BP measurement performed to determine if tissue perfusion is adequate.

Monitoring the Respiratory System

The respiratory system is monitored to ensure adequate oxygenation and ventilation. Oxygenation is also dependent on adequate cardiac output, lung function, inspired oxygen levels (FiO₂), ventilation and hemoglobin concentration.

Capnography (ETCO₂): The capnograph or end-tidal carbon dioxide (ETCO₂) will help you understand the ventilator status of your patient. The normal range is between 35–45 mm Hg. If ETCO₂ is higher than 45 mm Hg, the patient is hypoventilating (most common reason under anesthesia). This can also be caused by equipment failure such as stuck one-way valve or exhausted soda lime (when patient is re-breathing CO₂) in a rebreathing circuit or the fresh gas flow is too low in non-rebreathing circuit. If the patient is hypoventilating, you can correct the ETCO₂ with intermittent positive pressure ventilation (IPPV); if the equipment is malfunctioning, repair the equipment.

When the ETCO₂ is less than 30 mm Hg, your patient is hyperventilating. This can be caused by: iatrogenic hyperventilation, V/Q mismatch, sampling of dead space, there is a leak in the breathing system, the patient’s endotracheal tube is disconnected from the anesthesia machine, the endotracheal tube is in the esophagus or perhaps your patient has low cardiac output.

References

References are available upon request.