Introduction

Since the first description of circulatory response to laryngoscopy and intubation , there have been numerous studies concerning both responses and maneuvers by which it may be attenuated. Multiple methods for attenuation have been studied and more work has been carried out with different pharmacological agents such as local anaesthetics, vasodilators, beta adrenoceptor blocking agents, calcium entry blockers, opioids, gabapentin, volatile anaesthetics and neuromuscular blocking agents.[1,2,3,4,5,6,7]
Laryngoscopy is known to have profound cardiovascular  effects which include a pressor response and tachycardia along with an increase in catecholamine concentrations.[8,9]
The major part of the sympathoadrenal response is believed to arise from supraglottic region by the laryngoscope blade. Tracheal tube insertion and inflation contributes only little additional stimulation.[10]
During laryngoscopy and intubation , nociceptor signals are generated by elevation of epiglottis with the laryngoscope and by insertion of the tube into  trachea. The signals are conducted to the brain through the glossopharyngeal nerve and the vagus. Pressor response to laryngoscopy and intubation is mediated via sympathetic nerves and the efferent pathway is also composed of sympathetic nerves. [10,11,12]
The McCoy laryngoscope blade is a modification of Macintosh blade to facilitate tracheal tube placement in a difficult intubation. It is a ‘hinged tip’ blade controlled by a lever on the handle of laryngoscope, allows elevation of epiglottis with less overall laryngoscopic elevation and resulting in exertion of lesser force. [ 13, 14]  Consequently it is known to impart less stress response as compared to the Macintosh blade. Thus we conducted a study comparing HR and MAP changes during laryngoscopy and intubation using these two blades.

Methods

After the approval of institutional ethical committee and written informed consent the study was carried out. It included patients of both sexes belonging to ASA grade I and II and Mallampatti classification(MPC) of I and II between the age group of 18 to 60 years, scheduled for elective surgery requiring general anaesthesia with endotracheal intubation. Patients with acid peptic disease, obesity , anticipated difficult intubation , coronary artery disease (CAD) , hypertension , intestinal obstruction, diagnosed case of pheochromocytoma, and chronic smokers were excluded. The selected patients were randomly divided into 2 groups of 100 each, Macintosh group (MK) and McCoy group (MY) i.e. laryngoscopy and intubation carried out with Macintosh or McCoy laryngoscope. Laryngoscopy was performed by a person with a reasonable expertise to avoid interference in observations and results.
All patients received Tab. Alprazolam 0.5 mg at night prior to the day of surgery and Inj. Glycopyrrolate 0.01 mg/kg intramuscularly, 45 minutes prior to surgery.
In the operating room after intravenous access and application of monitors,  baseline readings of HR and MAP were noted. Preinduction - Patients received  Inj. Ondansetron 4 mg, Inj. Midazolam 0.03mg/kg and Inj. Fentanyl 1.5mcg/kg intravenously. After 3 minutes of preoxygenation, anaesthesia was induced  with Inj. Thiopentone sodium 6-7mg/kg and Inj. Vecuronium 0.1 mg/kg as muscle relaxant .All patients were ventilated with 100% O2 with bag and mask for 3 minutes. The HR and MAP was recorded 1minute prior to laryngoscopy .After adequate relaxation direct laryngoscopy with either Macintosh or McCoy blade was performed enabling the view of larynx and vocal cords [Cormack and Lehane (C&L) Classification grade I and II ]. The duration of laryngoscopy and intubation was restricted to less than 15 seconds.  The tip of either laryngoscope blade was gently passed towards the glosso-epiglottic fold. The larynx with Macintosh blade was visualized  by forward , upward movement of the handle of Macintosh laryngoscope, while with McCoy blade by just pressing the lever tip of the blade to lift the epiglottis. The trachea was then intubated with appropriate sized cuffed endotracheal tube & cuff was inflated.
The HR and MAP were monitored and recorded every 1 minute interval for 5 minutes since the laryngoscopy &intubation. Intraoperative vital parameters were monitored and maintained. No other drug was used for reducing the pressor response. The complications of laryngoscopy & intubations were anoted.The observations were analysed statistically by ‘unpaired, paired and chi-squared t test’. Statistical significance was assumed if p<0.05.

Observations and Results

All  patients  representing  both  sexes  and  age  ranging  from  18  to  60  years  belonging  to  ASA  Grade  I  and  II  were included in study. Patients  were  scheduled  to  undergo  various  elective  surgical  procedures under  general  anaesthesia and were  divided  in  2  groups  of  100  each :
Group  MK :  Laryngoscopy  and  Intubation  carried  out  using  Macintosh  blade.
Group  MY :  Laryngoscopy  and  Intubation  carried  out  using  McCoy  blade.
All  patients  had  a  comparable  demographic  data  with  respect  to  age, sex  and  weight .  Patients  from  both  groups  were  assessed  for Mallampatti  classification,  Cormack - Lehane  grade,   time  taken  for  intubation  and  data was comparable.

Table 1

Comparison of Demographic and Air-way assessment data in Group MK and Group MY

HAEMODYNAMIC RESPONSES

Table 2

Comparison of percent change in Heart rate and MAP in Group MK and Group MY

Percentage change  of   heart  rate  and MAP from baseline was  more  in  Group MK  as  compared  to Group MY  during laryngoscopy and intubation and 1 minute following that.

Table 3

Comparison of Heart Rate changes in Group MK  and Group MY

In both the groups, heart rate before and after induction was comparable (P > 0.05).
Following  laryngoscopy  and intubation, the mean rise in heart rate from  pre-induction value of  80.60 ± 7.67  to 106.92 ± 12.07 Beats Per Minute (bpm) was noted in group MK, while in group MY the mean rise in heart rate  was observed from 82.99 ± 10.87 to 96.84 ± 12.50 bpm. Mean heart rate in both groups at laryngoscopy and intubation was compared and difference was highly significant (P < 0.001).
After 1 minute, the heart rate in group MK was 97.00 ± 11.88 bpm while in group MY it was 92.68 ±10.03 bpm. The difference was statistically significant (P < 0.01). 
Following 2 minutes the difference was statistically not significant and return to baseline was observed at the end of 5 minutes.

Figure 1

Comparison of Heart Rate changes in Group MK and Group MY

Table 4

Comparison of Mean Arterial Pressure (MAP) changes in Group MK and Group MY

Following  laryngoscopy and  intubation, the mean rise in MAP from  pre-induction value  from  90.18 ± 9.52  to 115.13 ± 12.30 mm of Hg  was  noted  in group MK,   while  in  group MY  the  mean  rise  in  MAP  was  observed  from 91.91 ± 8.52 to 103.21 ± 11.17 mm of  Hg. This mean rise in MAP at laryngoscopy and intubation was compared  and the difference was  highly significant (P < 0.001).
After 1 minute, the  MAP in group MK was 101.88 ± 12.63 mm of Hg while in group MY it was 98.04 ± 11.35 mm of Hg. The difference in rise in MAP was statistically significant (P < 0.02). 
Return to baseline value was observed at 3 minutes.

Figure 2

Comparison of MAP changes in Group MK and Group MY

DISCUSSION

The pressor response is observed during laryngoscopy and intubation due to sympathetic stimulation of laryngo-pharyngeal region. Efforts of attenuating this response have been concentrated locally and systemically with pharmacological agents. Reduced sympathetic stimulation with the McCoy blade was an accidental finding when used for difficult airway access. Further evaluation of this property and comparison with other blades was studied. The adverse circulatory responses to laryngoscopy and intubation can manifest in the form of increase in heart rate, blood pressure, intracranial tension, intraocular tension, increased catecholamine surge and appearance of arrhythmias and cardiac asystole leading to morbidity or mortality in cardiac compromised patients.
In our study, we investigated if use of McCoy blade is associated with additional benefit of alleviating the pressor response to laryngoscopy and endotracheal intubation by studying two parameters HR and MAP.
The percent rise in HR and MAP from baseline during laryngoscopy and intubation and at 1 minute was  observed to be higher in MK group as compared to MY group. We also studied comparison of HR and MAP changes in  MK and MY group and the values were compared and statistically analyzed using unpaired t test and observed to be significant during laryngoscopy and intubation and 1 minute following that and return to baseline parameters was observed at 3 to5 minutes.
Similar to our study, E. P. McCoy and R.K. Mirakhur, [15] in 1995 compared the changes in cardiovascular response and catecholamine concentration in 20 patients before and after laryngoscopy with either the Macintosh or the McCoy laryngoscope blade. Significant increase in the HR (33%) and arterial blood pressure (27%) and increase in catecholamine (nor-adrenaline) levels were noted with Macintosh blade as against McCoy blade where no significant change in arterial blood pressure was found. The results of our study are comparable with the study except we have not included the measurements of plasma noradrenaline and adrenaline concentration.

Conclusion

The reduced pressor response with the McCoy blade observed in the present study is most probably related to minimal pharyngolaryngeal stimulation due to decrease in overall laryngoscopic movement and the force used. The  blade, primarily  devised  for difficult  intubation, alleviated  pressor  response  to  laryngoscopy & intubation. Laryngoscopy using McCoy blade, although requires adequate practice, can serve as an additional tool  with different pharmacological agents to avoid exaggerated pressor response in indicated high risk patients.