Laryngoscopy as part of endotracheal general anaesthesia can be associated with an acute increase in intraocular pressure (IOP) due to an increase in blood pressure (BP) and increase in ocular blood flow . The actual procedure of laryngoscopy, in a normal airway, is of brief duration of not more than 10-15 s in experienced hands. In the great majority of patients, endotracheal intubation is accomplished in the first attempt. However, in a small number of patients, repeat laryngoscopy may become necessary for various unanticipated reasons. Repeat laryngoscopy in such circumstances is usually performed within the next 15-20 s. Repeat manoeuvres may add to the pre-existing stress from previous laryngoscopy. Studies in the past have concentrated on the IOP changes during intubation . To our knowledge, no study has been performed to highlight the effect of repeat laryngoscopy on IOP. This prompted us to conduct this trial in patients without ocular disease.
After approval from our Hospital Ethics Committee and informed consent from patients, or parents in the case of children, 30 patients of either gender and ASA I-II, scheduled to undergo elective spine or nerve repair surgery, were enrolled in the study. Patients with suspected airway difficulty, hypertension, ischaemic heart disease, diabetes mellitus, chronic airway obstructive disease, history of any eye disease and those taking chronic medication, were excluded from the study. Two hours before surgery, the patients were premedicated with diazepam 5-10 mg orally. Routine monitoring (ECG, SPO2 and non-invasive BP) was attached (Datex Engström, Helsinki, Finland). Induction was with fentanyl 2 μg kg−1 and thiopental 4-5 mg kg−1; neuromuscular block was with rocuronium 1 mg kg−1, and the lungs were ventilated by mask with 50% oxygen in N2O for 60 s. After removal of the facemask, a baseline IOP was measured in both eyes with a Schiotz tonometer (accuracy ± 2 mmHg). Immediately afterwards, direct laryngoscopy with a Macintosh blade was performed. When the blade had been removed from the oral cavity, IOP was again measured in both eyes. Mask ventilation was resumed with the same gaseous mixture for 15-20 s and laryngoscopy and IOP measurement were repeated. Along with the IOP measurements, mean arterial pressure, heart rate and oxygen saturation were recorded. The study was terminated at this point.
Mean of IOP in both eyes was calculated for data analysis and results were analysed by repeat measures analysis of variance and P < 0.05 was taken as significant. Data are presented as mean ± SD. Patient's ages ranged between 10 and 55 yr (33 ± 14 yr). There was a preponderance of males. Baseline IOP was 6.75 ± 1.3 mmHg (range 4.90-10.00 mmHg). The first laryngoscopy significantly increased it to 13.0 ± 2.0 mmHg (range 9.4-17.3 mmHg) (P < 0.001). Repeat laryngoscopy 15-20 s following the first, increased IOP to 17.2 ± 3 mmHg (range 13.4-23.4 mmHg) (P < 0.001 compared to first laryngoscopy) (Table 1). There was a significant increase in mean arterial pressure from baseline following first as well as repeat laryngoscopy. However, there was no change in the heart rate following each laryngoscopy (Table 1). Oxygen saturation remained above 98% in all patients throughout the study period.
Repeat laryngoscopy is usually performed 15-20 s after a failed attempt. That was why we waited for 15-20 s between two laryngoscopic attempts. Both laryngoscopy and endotracheal intubation are likely to increase IOP significantly . We observed that the first laryngoscopy significantly increased IOP, and that there was superimposition of the stress of repeat laryngoscopy leading to further increase in IOP. Maintenance of normal or low IOP is frequently of critical importance to the successful performance of open eye surgery. Even small increases in IOP for a short time, particularly in patients with severe closed angle glaucoma can compromise optic disc perfusion, and may result in disc ischaemia and visual loss . Major factors, which can rapidly influence IOP, include central venous pressure (CVP), arterial pressure, choroidal blood flow and arterial carbon dioxide concentration. Various studies have concluded that arterial pressure plays some role in IOP control, but over a physiological range of arterial pressure this effect is minor . Increase in IOP is greater in glaucomatous eyes than in normal eyes whenever there is an increase in BP and conditions that impede the venous return . Direct adrenergic stimulation from laryngoscopy may cause vasoconstriction leading to increase in CVP, which has a closer relationship with IOP than arterial pressure. Also adrenergic stimulation increases the resistance to outflow of aqueous humour between anterior chamber and Schlemm's canal, thereby increasing IOP . Significant increase in BP and increase in resistance to outflow of aqueous humour, and possible increase in CVP resulting from adrenergic stimulation from laryngoscopy in combination were probably responsible for increase in IOP in our study. In conclusion, repeat laryngoscopy, 15-20 s following the first laryngoscopy, exaggerates the increase in IOP resulting from first laryngoscopy. Under such circumstances either some pharmacological intervention should be undertaken to forestall further increase in IOP, or laryngoscopy should be repeated only after the harmful effects of first laryngoscopy have abated. The choices of suitable drugs or proper timing for repeat laryngoscopy requires further investigation.
P. K. Bithal
T. S. Reddy
Department of Neuroanaesthesia; Neurosciences Centre; AIIMS; New Delhi, India
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