Proper depth of anesthesia: how do you tell?

During anesthesia, we frequently find ourselves working hard to keep the patient in a delicate balance between a plane of anesthesia deep enough to prevent movement or perception of pain but yet not so deep as to obliterate the homeostatic mechanisms keeping the patient perfused, ventilated, and oxygenated. What signs in the patient tell us that they are in a plane of anesthesia that is insufficient for a procedure, i.e. “too light,” or a plane that is too deep to be compatible with life, i.e.“too deep”?  See Table 1.

The visual and tactile inquiries that the anesthetist can easily make in most procedures are to check the jaw tone, eye position, and palpebral/corneal reflexes. The exception is when a pet is purposefully paralyzed with neuromuscular blocking agents. Otherwise, an anesthetist can monitor the strength required to gently open the jaw. A tight jaw generally means a pet is in a lighter plane of anesthesia than a loose jaw but there is no “absolute value” for this measurement that ensures the correct plane of anesthesia. An anesthetist should open the lids and examine the eye position.  In brief, the eye position begins with the iris centered, and then moves, ventromedially, towards the medial canthus as the plane of anesthesia deepens, and then moves back outward with a dilated pupil if the animal is too deep. If the animal is emerging from anesthesia, the iris returns to its normal centered position and is constricted or normal. Other useful information is gained from the palpebral reflex, is diminished or absent (not always, especially when using injectable anesthetics such as ketamine) and the corneal reflex should always be present.[1],[2] Monitoring jaw tone and eye position at set intervals and at the initiation of a painful stimulus is easy can give the anesthetist useful information, in addition to blood pressure, heart rate, respiratory rate, and end-tidal carbon dioxide (CO2). This can aid the decision-making process.  
Next, we consider a case example. Patient: 6 month old intact female yellow Labrador retriever in good health. Presents for OVH, Premedication: hydromorphone 0.1 mg/kg IM, dexmedetomidine 0.005 mg/kg IM, glycopyrrolate 0.01 mg/kg (not given). Induction and intubation: Propofol 4 mg/kg to effect. Maintenance: Isoflurane 2% and LRS @10 ml/kg/hr

At about 45 minutes post-premedication, the anesthetist notes that the patient’s heart rate, respiratory rate, and blood pressure have risen.  The anesthetist considers that the medetomidine may have been eliminated, or the surgical stimulation has increased, and the patient is now “too light” and may need a higher concentration of inspired isoflurane to maintain a surgical level of anesthsia. Before turning up the vaporizer, she checks the jaw tone and finds it to feel tight. She notes the iris of the eye appears to be returning the center of the orbit, as previously the iris was only partially visible at the medial canthus. She checks the end-tidal carbon dioxide monitor and notes the dog has hyperventilated herself to a 30 mm Hg of carbon dioxide. This confirms to her that the patient is “too light” so she increases the patient from 2% to 3% of inspired isoflurane.

For a while, her patient seems perfect:

Blood pressure: mean = 70 mmHg, Jaw tone: loose (but not too loose), Respiratory rate: 15 per minute, End tidal CO2: 45 mm Hg, Heart rate: 80 bpm, Eye position: ventromedial, Palpebral: decreased,Corneal: intact

The anesthetist is asked to retrieve a surgical pack. When she looks at the monitor again, she sees that the heart rate is now at a new peak and the patient is taking many frequent, shallow breaths.  Her first instinct is to think “the patient is still too light” but before she increases the isoflurane concentration again, she checks the jaw tone and finds that it is looser than before and that the iris is again only partially visible at the medial canthus of the eye. 

Blood pressure: mean = 40 mm Hg, Jaw tone: very loose, Respiratory rate: 60 per minute, End-tidal CO2= 60 mm Hg, Heart rate: 120 bpm, Eye position: central with dilated pupil, Palpebral absent, corneal intact

The anesthetist decides the patient is instead, “too deep” and she reduces the concentration of inspired isoflurane and assists ventilation.  The heart rate returns to normal and the respirations become less frequent and deeper.  The parameters return to baseline

Summary: In this case, the rapid heart rate could be attributed to the body’s compensatory mechanism for a falling arterial blood pressure or an elevated level of carbon dioxide, or both.  The rapid respiratory rate was a response to the high end-tidal CO2 measurement, which reflects a high arterial blood content of CO2 in the healthy dog. In this case, the respiratoand heart rate by themselves could have misled the anesthetist to increase the % of isoflurane delivered to the patient and caused harm to the health of the patient. By considering all the available information, from both electronic monitors and hands-on examination of the patient, the correct next-step was taken. By integrating jaw tone and eye position assessments into your anesthesia practice, one can improve the safety of anesthesia and prevent a common mistake of assuming that a rapid heart and coupled with a rapid respiratory rate is always a sign of a “too light” plane of anesthesia.

Assessing Anesthetic Depth in Patients Anesthetized with Isoflurane:

Isoflurane anesthesia
Heart rate
Respiratory rate
Blood pressure
jaw Tone
Palpebral reflex
Eye Position
Corneal reflex (if very very deep)
End-tidal Carbon Dioxide (spontaneous ventilation)

Assessment: Lighter plane with tachycardia, tachypnea, High blood pressure, jaw tone present or increased normocapnea or hypocapnea;Palpebral reflex Normal with eye rolled central and nystagmus Intact corneal reflex 

What do you do? Increase isoflurane or add analgesic medication(s).

Deeper plane can have: tachycardia or bradycardia, Tachypnea or bradypnea or apnea, Low blood pressure, Jaw tone decreased or absent, Central, dilated pupil. Lost palpebral, eye position staring straight at you, ETCO2 is >50 mm Hg. Tachycardia can follow hypotension, eventually this leads to bradycardia as plane deepens

Assessment: deeper plane
What do you do: Reduce isoflurane, assist ventilation, increase IVF rate,  ±-pressor agents (hypotension), ±anticholinergics (bradycardia)

Acceptable plane is defined as follows:
Normal heart rate
Near normal respiratory rate
Blood pressure Mean >60 mm Hg or Systolic >90 mm Hg
Jaw tone neither loose nor tight
Slow palpebral or none
Central or ventral, normal pupil size
Intact corneal reflex, but don't check it unless one has to because of damage to corneal. 
Normal or slightly elevated; Supplement ventilation as required.



[1] Chapter 2, Lumb and Jones’ Veterinary Anesthesia and Analgesia, 4th ed.,

[2] Chapter: 2 & 6 (Box 2-1, 2-2, 2-6, 6-4), William W. Muir III. Anesthesia and analgesia: a guide to canine, feline, and exotic animal practice, 2nd ed. For VCA Animal Hospitals.