Can yawning be used as an endpoint of induction of anesthesia?

Dae Woo Kim, MD, PhD,* Ho Yeong Kil, MD, PhD,^† Paul F. White, PhD, MD, FANZCA^‡

* Department of Anesthesiology, The Catholic University of Korea; ^† Hallym University, Seoul, Korea; ^‡ University of Texas Southwestern Medical Center at Dallas

Background and Goal of Study: We can usually see the yawning at induction of anesthesia, however, it has not been studied as such an endpoint of anesthesia. The yawning is one means of changing arousal level, and a sign or marker that such a change is occurring, although its functions are not well understood. The purpose of the present study was to evaluate the yawning whether it could be used as an endpoint of induction of anesthesia, using its property as a marker of changed arousal level.

Materials and Methods: In sixty healthy adult patients (aged 20-50 years), after obtaining baseline values, patients were induced with either propofol (after lidocaine 0.5 mg/kg iv) or thiopental using the Stelpump TCI software (J. F. Coetzee, University of Stellenbosch, South Africa). A target propofol concentration (C_T) of 6 µg/ml with the flash mode (up to 1500 ml/h) was selected using Marsh's pharmacokinetic model. With thiopental, the infusion was started at a C_T of 30 mg/ml with the flash mode using the pharmacokinetic model of Stanski and Maitre. Clinical endpoints of induction of anesthesia were measured as follows, (1) loss of verbal responsiveness, (2) loss of eyelash reflex, (3) the yawning. In addition, (4) the occurrence of apnea were measured, too. We assessed the hypnosis levels of endpoints of induction of anesthesia including the yawning and demonstrate their effect site concentrations and elapsed time. Furthermore, we compared the incidences of yawning and apnea between both groups.

Results: Clinical endpoints of induction occurred in the order of LOV > LOE > the yawning >> OOA in both groups. With respect to BIS, the yawning showed the lowest BIS and the highest effect site concentrations except OOA in both groups. The incidence of the yawning in thiopental was higher than in propofol group (83 vs 63%). On the contrary, the incidence of apnea in propofol group was higher than in thiopental group (77 vs 53%).

	Onset (sec)	BIS	CE (mg/ml)	Dosage (mg)
Loss of verbal responsiveness (LOV)
Propofol	50 ± 9	80 ± 15	0.8 ± 0.3	132 ± 21
Thiopental	45 ± 8	88 ± 5	6.5 ± 1.6	190 ± 15
Loss of eyelash reflex (LOE)
Propofol	61 ± 11*	70 ± 16*	1.1 ± 0.3*	137 ± 22
Thiopental	52 ± 10	80 ± 11*	7.7 ± 2.0	205 ± 24
Yawn response
Propofol	66 ± 14*	68 ± 14*	1.2 ± 0.3*	139 ± 27
Thiopental	57 ± 14*	73 ± 14*	9.3 ± 2.1*	212 ± 22*
Occurrence of apnea (OOA)
Propofol	111 ± 14^=	40 ± 14^=	2.3 ± 0.5^=	155 ± 24
Thiopental	113 ± 24^=	58 ± 13^=	16.3 ± 2.6^=	299 ± 51^=

Values are means ± SD, *Significantly different from value for loss of verbal responsiveness, p<0.05, ⁼Significantly different from values for loss of verbal responsiveness, eyelash reflex and “yawn” response, p<0.05.

	Propofol (n=30)	Thiopental (n=30)
Loss of verbal responsiveness (n, %)	28, 93	29, 97
Loss of eyelash reflex (n, %)	25, 83	27, 90
Yawn at induction (n, %)	19, 63	25, 83*
Apnea at induction (n, %)	23, 77	16, 53*

Number (n) or percentage (%), * Significantly different from propofol group, p<0.05.

Conclusions: As far as the yawning could be shown, it was a more easily detectable and reliable endpoint than loss of eyelash reflex for determining the onset of unconsciousness. Yawning also demonstrated a good correlation with “hypnotic” BIS values during induction of anesthesia with thiopental and propofol.