Do Intraocular Pressure Measurements Under Anesthesia Reflect the Awake Condition?
We read this interesting article titled “Do Intraocular Pressure Measurements Under Anesthesia Reflect the Awake Condition?1 by Gofman et al published in Journal of Glaucoma in 2017. They reported intraocular pressure (IOP) changes under general anesthesia (GA) at 5 time points and compared them with baseline awake IOP in patients undergoing squint surgery. The authors have used target-controlled infusion (TCI) for inducing anesthesia. TCI is a technique of infusing IV drugs to achieve a user-defined predicted (target) drug concentration in a specific body compartment or tissue of interest. With TCI systems, the anesthetist enters a desired target concentration of IV anesthetic. The computer calculates the amount of drug to be delivered and directs an infusion pump to deliver the calculated bolus or infusion to achieve target drug levels. The computer also tailors the drug infusion based on the concentration of drug in the tissue.2 The point to be noted here is that TCI is not used in routine ophthalmic anesthesia in many centers as it is less-popular technique of administering GA. TCI models were developed on the basis of data from healthy volunteers and may not be uniformly applicable to all clinical situations, ultimately resulting in under or over dosing of the anesthetic. TCI infusion pumps may inaccurately predict target concentration in patients. It is dependent on continuity of functioning IV line and requires more equipment to deliver anesthetic drugs making it less cost effective. Elderly patients can have a cardiorespiratory compromise caused by increased concentration of the drug. It is contraindicated in children below 3 years of age.3 Hence, the results cannot be extrapolated to the population where TCI is not practiced.
The authors have compared 2 groups where end tidal sevoflurane concentration was increased to 0.5%, 2%, and 5% in 1 group, whereas it was decreased to 5%, 2%, and 0.5% in the other group, respectively. We should note that practically, 0.5% sevoflurane has no anesthetic effect, therefore comparing this concentration to higher concentration of sevoflurane may be inappropriate.
The authors have not mentioned if intubation was performed using muscle relaxants or performed under deeper planes of GA. This is critical, as muscle relaxants are commonly used before intubation for controlled ventilation during GA. Depolarizing muscle relaxants elevate the IOP, whereas nondepolarizing agents lower the IOP.4 Depth of anesthesia also affects IOP proportionately,5 hence this also needs to be clarified.
The authors have considered IOP measurements in both surgical and nonsurgical eyes. Although, squint is an extraocular procedure, it can cause congestion of extraocular tissues which could influence the IOP measurements. Considering this, IOP measurement in the nonsurgical eye alone would be more appropriate.