Cataract removal under MAC(Monitored Anesthetic Care)

Cataract removal under MAC

The following case will describe the use of monitored anesthetic care (MAC).

Learning objectives:

·           pre-operative management of the elderly patient

·           methohexital

·           oculo-cardiac reflex.

An 85-year old woman comes for removal of a cataract.

History. She has suffered significant visual loss from the cataract. Her past medical history reveals that she has given birth to four children and that she had an uncomplicated chole-cystectomy under general anesthesia 40 years ago. She has no family history of anesthetic complications. She takes no medications, has no allergies and does not smoke. She drinks wine socially.

Physical examination reveals:

African American woman in no acute distress; weight 65 kg; height 5 2  (155cm)

BP 150/90 mmHg; HR 90 beats/min; respiratory rate 16 breaths/min

Airway: Edentulous, Mallampati II, 4 fb mouth opening, 4 fb thyromental distance, full neck extension

CV: S1,S2 no murmur

Resp: Lungs clear to auscultation bilaterally

No additional preoperative laboratory or other studies are required in this ASA I patient.

Such minor eye procedures are usually performed under local anesthesia (peribulbar or retrobulbar) administered by the ophthalmologist. Injection of the local is not without pain, however transient, so we usually anesthetize the patient briefly (minutes).

Anesthetic preparation: We discuss the risks/benefits of the anesthetic plan, giving the patient an idea of what to expect: “You will be asleep for about 2 minutes while the surgeon places numbing medicine around your eye. After you wake up you will not be able to see as there will be a drape over your face. We will blow air under the drape and we will be monitoring your heart and lungs. You should feel no pain but let us know should you be uncomfortable or if you need to cough or move.”

Induction of anesthesia: The eye block may be placed in the operating room, or in the preoperative holding area, allowing more time for it to take effect. Either way, we apply standard monitors, give the patient some supplemental oxygen, and then administer a short-acting induction agent such as methohexital (Brevital®) or thiopental. When the patient loses consciousness, the ophthalmologist places the block and tests it as soon as the patient awakens. Some patients become transiently apneic following the induction agent, and we need to support their airway (chin lift) or their ventilation with a mask until they resume spontaneous breathing.

Once the eye is anesthetized, the patient must remain still for some time. Because many patients will move as they doze off, following their brief anesthesia-induced respite, we administer no additional sedatives and the patient remains awake for the remainder of the procedure.

Maintenance of anesthesia: No additional sedatives are administered.

Complication: Suddenly the patient’s heart rate falls to 30 beats/min and she complains of not feeling well.

The most likely culprit of sudden onset bradycardia in this setting is the oculo-cardiac reflex. Traction on the eye and ocular muscles can result in a slowing of the heart via a trigeminovagal pathway.¹ The bradycardia usually resolves imme-diately upon removal of the stimulus. The reflex response fatigues over time, but if it prevents progress of the operation, an anti-cholinergic may be required.

The ophthalmologist releases pressure on the eye, with immediate recovery of the heart rate to 70 beats/min. When the surgeon attempts to resume the operation, the heart rate again falls. After several attempts, we give glycopyrrolate 0.4 mg i.v. The patient’s heart rate rapidly increases to 90 beats/min and the surgery proceeds.

For its lack of central effects we choose glycopyrrolate over atropine.

When the surgery is complete we take the patient to the PACU where she is monitored for surgical complications for a brief time, then discharged home with a companion.

N o t e

The full pathway is ciliary ganglion → ophthalmic division of trigeminal nerve → gas-serian ganglion → main trigeminal sensory nucleus in the fourth ventricle → vagus nerve.