Effect of operation table height on ease of mask ventilation, laryngeal view, and endotracheal intubation success

Abstract

Background and Aims:

Optimal patient positioning and operating table height are essential for an ergonomic posture of an anesthesiologist in which there is minimal or no strain on thewrist during mask ventilation. It also avoids flexion of the neck, lower back, and knee bending at the time of laryngoscopy and intubation.

Material and Methods:

One hundred eighty patients were randomly allocated to three groups based on different table heights. The height of the table is kept at the mid-sternum level of an anesthesiologist in group 1, at the xiphoid process in group 2, and at the level of umbilicus in group 3. Laryngoscopic view with or without postural changes (exertion at wrist joint, flexion of the neck, lower back, or knee bending) was graded as per Cormack Lehane’s (CL) grading. The degree of discomfort experienced by the anesthesiologist during mask ventilation or tracheal intubation was graded subjectively (1 = no discomfort, 2 = mild discomfort, 3 = moderate discomfort, and 4 = severe discomfort) at different table heights. Postural changes required to obtain the best glottic view and quality of endotracheal (ET) intubation (intubation time and attempts required) were also noted. For analysis, quantitative variables were expressed as mean ± SD and compared using unpaired t or analysis of variance test. Qualitative variables were expressed as frequencies/percentages and compared using the Chi-square test. Results with P value <0.05 were considered significant statistically.

Results:

Moderate discomfort (strain at wrist joint) during bag–mask ventilation was experienced by the anesthesiologist in a maximum number of patients in group 1 (81.7%). Significant improvement was seen in CL grade after the use of postural modifications in groups 1 and 2 (P value ≤0.05). Greater postural modifications were required during ET intubation at lower table heights (group 3).

Conclusions:

It is advisable to adopt higher table positioning in relation to anesthesiologist performing the laryngoscopy for smooth and single-attempt ET intubation since the best laryngoscopic view and intubation with minimal postural modifications was seen at higher table heights (at the mid-sternum level of an anesthesiologist).

Introduction

Optimal patient positioning and operating table height have a significant correlation with the physical work needed to perform bag–mask ventilation (BMV) and endotracheal (ET) intubation via direct laryngoscopy (DL) efficaciously.[1] During DL, the anesthesiologist observes the airway from outside. To see through the airway, light must travel from the glottic opening to the observer’s eye. As the light traverses in a straight line, the technique needs an uninterrupted linear path between the larynx and the observer’s eye. It requires proper positioning of the patient, which in turn is dependent on the various airway maneuvers and height of an operating table. Different maneuvers are done to achieve this position.[2] The airway usually has three axes. In the neutral position, these axes form acute and obtuse angles with each other. To align all three axes, McGill suggested the sniffing position. Sniffing position has two parts: cervical flexion and atlantooccipital extension. Cervical flexion helps in lining the pharyngeal and laryngeal axes while extension at the atlantooccipital joint brings the oral axis in alignment with the first two.[3] Multiple failed intubation attempts due to improperly visualized larynx is one of the consequences of inadequate positioning.[4] It increases patient morbidity and additionally increases anesthesiologists’ fatigue.[5]

A posture with a straight back, slight neck forward bending to have a view of the patient’s oral cavity, mild flexion at the shoulder, lower arm supported, and relaxed (60–100° at elbow joint) is an ergonomically correct posture for airway management in standing position.[6] However, to improve the laryngoscopic view and to facilitate single-attempt ET intubation, the anesthesiologist may require to modify their posture. Incorrect body posture has an adverse effect on the clinician’s health and may predispose them to back problems. So it is important to consider this occupational hazard and its consequences.[7] A fatigued operator might become inefficient in other work. Appropriate table height can help in reducing anesthesiologist discomfort by decreasing extra efforts required forflexion of the neck, lower back, and knee bending.[1,5]

Hence, to decrease anesthesiologist discomfort (strain at wrist joint) during mask ventilation, effortless (with minimal postural changes), and single-attempt laryngoscopy and intubation, it is important to study the influence of operating table height on laryngeal view and ease of BMV. Only a few studies specify the significance of operating table height in improving the laryngeal view. The present study is designed primarily to determine the influence of different operating table heights on the comfort of anesthesiologists during BMV and laryngeal view with and without postural modifications. Secondly, the changes in the posture required to achieve the best laryngeal view were also assessed.

Material and Methods

The present randomized single-blind study was done in a tertiary care hospital after obtaining approval from the institutional ethics committee and registering the trial. The purpose and the protocol of the study were explained and informed written consent was taken from all patients. Patients aged 18–65 years of either sex, ASA Grade 1 and 2, undergoing elective surgery under general anesthesia with ET intubation were included. Patients with body mass index (BMI)>30 kg m⁻², increased risk of aspiration, anticipated difficult airway, congenital or acquired airway anomalies, and loose teeth or edentulous patients were excluded from the study.

Sample size was calculated based on a previous study, where the difference in the mean intubation duration between different groups was 2 s.[1] A minimum of 48 patients were required per group in the study for the difference with a power of 80% and an alpha error of 5%and a beta error of 0.2. To compensate for a few dropouts, we enrolled 60 patients in each group. For analysis, quantitative variables were expressed as mean ± SD and compared using unpaired t or analysis of variance test. Qualitative variables were expressed as frequencies/percentages and compared using the Chi-square test. Results with P value <0.05 were considered significant statistically.

Using the sealed envelope method, patients were randomly allocated to one of the three groups based on the height of the operating table adjusted to place the patient’s forehead at one of the three levels, i.e., mid-sternum, xiphoid process, or umbilicus of the anesthesiologist performing the laryngoscopy. Group I (n = 60): at mid-sternum level, Group II (n = 60): at xiphoid process, and Group III (n = 60): at the level of umbilicus. One day before the surgery, detailed history was taken and the airway was assessed by an anesthesiologist who was not further involved in the study. The airway assessment included measurement of inter incisor distance, thyromental distance, sternomental distance, neck circumference, modified Mallampati grade, upper lip bite test, horizontal length and forward protrusion of the mandible, and patients with anticipated difficult airway were excluded from the study.

Patients were kept fasting 6 h before the operation. Patients were premedicated with tab. Alprazolam 0.25 mg and tab. Omeprazole 40 mg the night before and in the morning 2 h before surgery. Upon arrival in the operating room, all routine monitoring including heart rate (HR), electrocardiography (ECG), non-invasive blood pressure, and pulse oximetry (SpO₂) were established. The patient was kept in a supine position with a 10 cm pillow under the head and premedicated with an injection of glycopyrrolate 0.005 mg kg⁻¹ intravenously. The height of the operating table was adjusted to place the patient’s forehead at one of the three levels, i.e., mid-sternum, xiphoid process, or umbilicus of the anesthesiologist. Anesthesia was induced with an injection fentanyl 2 μg kg⁻¹ and propofol 2 mg kg⁻¹ intravenously. For muscle relaxation, atracurium 0.5 mg kg⁻¹ was given. Laryngoscopy and ET intubation was performed using Macintosh curved blade size 3or 4. During BMV, the anesthesiologist was directed to adopt an initial erect standing posture with a back straight, minimal neck bending that provides a view of the patient’ soral cavity, arm slightly flexed at the shoulder joint, and forearm in a relaxed and supported position (around 60–100° at elbow joint). In case of inadequate BMV, two hands ventilation and/or oropharyngeal airway were used. Patients whose BMV was not adequate even after the above measures were excluded from the study.

First, laryngoscopy was done in the initial erect posture in which the anesthesiologist was not allowed to do any movement of the neck, lower back, and knee. However, movements of the arm, hand, or both were allowed to visualize the patient’s laryngeal inlet. This laryngeal view is graded as per CL grading. Then the anesthesiologist was allowed to make changes in his/her posture by flexion or extension of the neck, lower back, and bending at the knees to get the best laryngeal view, and ET intubation was done in this position. Both laryngoscopic views (in initial posture and best view after postural modifications) were noted. Anesthesiologist postural changes (neck flexion, back flexion, and/or knee bending) were assessed and recorded objectively by an observer (an anesthesia resident not further involved in the study). After completion of induction, the anesthesiologist was asked to subjectively grade the discomfort experienced (exertion at wrist joint) by him/her during BMV and during tracheal intubation (exertion at wrist or arm, flexion of the neck, lower back, or knee). The degree of discomfort during mask ventilation or tracheal intubation was graded subjectively (1 = no discomfort, 2 = mild discomfort, 3 = moderate discomfort, and 4 = severe discomfort). The intubation time was recorded after obtaining the best laryngeal view from the time of picking up the ET tube for insertion till the capnograph trace appeared on the monitor. Heart rate and blood pressure were recorded at 1, 3, 5, and 15 min after tracheal intubation. Two intubation attempts were allowed and patients in whom there was a failure to intubate or desaturation occurs (SpO₂ <90%) were excluded from the study and were managed according to the preference of the performing anesthesiologist.

Results

One hundred eighty four patients were assessed for inclusion in study and out of which four patients were not included in the study in view of anticipated difficult airway [Figure 1]. These 180 patients were then randomally allocated to three groups of 60 patients each. The mean age, gender, weight, height, BMI, ASA, Mallampati class, and various airway parameters were comparable with no statistical difference among the three groups (P value ≥0.05) [Tables 1 and 2]. Moderate discomfort (strain at wrist joint) during BMV was experienced by anesthesiologists in a maximum number of patients in group 1 (81.7%). However, in 93.30% of patients of group 3, no discomfort while doing BMV was experienced by the anesthesiologist. In groups 1 and 2, anesthesiologists experienced only mild discomfort during laryngoscopy and intubation in most cases, while in group 3 mild, moderate, and severe discomfort was noticed in 10, 60, and 30% of cases, respectively. Intubation discomfort was found to be statistically significant among the groups with maximum discomfort present at lower table heights [Table 3]. CL grades before and after the use of posture changes were compared and found that the CL grade improved significantly in groups 1 and 2 (P value <0.05). Although some improvement was seen in group 3 after postural modifications, the difference was not statistically significant [Table 4]. A greater number of patients belonging to group 1 (91.7%) and group 2 (83.3%) were intubated on the first attempt compared to group 3 (80%) [Table 5]. Time taken for intubation was minimal in group 1 (9.9 seconds) and maximum in group 3 (11.9 seconds). Intubation duration among the three groups was found to be statistically significant with minimal duration seen at higher table height [Table 5]. Postural modifications such as neck flexion, back flexion, and knee bending were minimally required in group 1 (16.7%) as compared to group 2 (33.3%) and group 3 (41.6%). Postural modifications were maximum at lower table heights and the difference was statistically significant [Table 5]. Hemodynamic parameters were comparable with no statistically significant difference (P value ≥0.05) among the three groups.

Figure 1.

Consort

Table 1.

Demographic profile of the study participants

Parameter	Group 1 (n=60)	Group 2 (n=60)	Group 3(n=60)	P
Age (yr)	36.2±11.87	38.4±14.83	39.2±12.37	0.58#
Gender M/F(%)	33/27 (55/45)	28/32 (46.7/53.3)	33/27 (55/45)	0.57#
Height (m)	1.61±0.06	1.67±0.07	1.65±0.09	0.71#
Weight (kg)	56.48±9.51	57.76±7.55	56.48±9.51	0.66#
BMI (kgm−2)	21.88±4.28	22.23±3.86	21.88±4.28	0.86#
ASAgrades I/II(%)	19/41 (31.7/68.3)	32/28 (53.3/46.7)	18/42 (30/70)	0.11#

Values are in mean±SD, number, percentage, ^# Statistically insignificant

Table 2.

Airway parameters of study participants

Parameter	Group 1 (n=60)	Group 2(n=60)	Group 3(n=60)	P
MPG I/II(%)	8/52 (13.3/86.7)	15/45 (25/75)	8/52 (13.3/86.7)	0.63#
Thyromental distance (cm)	8.23±0.67	8.06±0.57	8.23±0.67	0.26#
Neck circumference (cm)	34.9±1.24	35.5±1.81	34.9±1.24	0.54#
Interincisor distance (cm)	5.81±0.52	5.72±0.88	5.81±0.52	0.47#
Sternomental distance (cm)	17.10±0.69	17.20±0.76	17.10±0.69	0.09#
Horizontal length mandible (cm)	17.84±1.36	17.75±0.67	17.84±1.36	0.88#
Upper lip bite test Grade I/II (%)	46/14 (76.7/23.3)	50/10 (83.40/16.60)	46/14 (76.70/23.30)	0.08#

Values are in mean±SD, number, percentage, ^#Statistically insignificant, MPG-Mallampati grade

Table 3.

Comparison of discomfort during mask ventilation and laryngoscopy

Discomfort No/mild/moderate/severe	Group 1 (n=60)	Group 2 (n=60)	Group 3 (n=60)	P
				Gp 1vs2	Gp 1vs3	Gp 2vs3
Bag and mask(%)	0/11/49/0 (0/18.3/81.7/0)	0/58/2/0 (0/96.7/3.3/0)	56/3/1/0 (93.3/5/1.7/0)	0.001*	0.001*	0.001*
Intubation discomfort (%)	1/59/0/0 (1.7/98.3/0/0)	0/59/1/0 (0/98.3/1.7/0)	0/6/36/18 (0/10/60/30)	0.87#	0.001*	0.001*

*Statistically significant, ^#Statistically insignificant, Gp-Group

Table 4.

CL before and after postural changes

CL grade	Group 1 (n=60)	Group 2 (n=60)	Group 3 (n=60)	P			P Gp 1 vs 2 vs 3
				Gp 1vs2	Gp 1vs3	Gp 2vs3
Before posture changes Grade1/2a/2b (%)	50/8/2 (83.3/13.3/3.33)	40/9/11 (66.7/15/18.3)	35/15/10 (58.3/25/16.6)	0.05#	0.04*	0.02*	0.001*
After posture changes Grade1/2a/2b (%)	59/1/0 (98.3/1.67/0)	50/5/5 (83.3/8.33/8.33)	42/13/5 (70/21.7/8.33)	0.01	0.04*	0.03*	0.03*
P-value (before vs after posture changes)	0.03*	0.001*	0.41#

*Statistically significant, ^#Statistically insignificant, Gp-Group, test-Chi-square

Table 5.

Comparison of intubation attempts, intubation time, and postural modifications among study groups

Parameter	Group 1 (n=60)	Group 2 (n=60)	Group 3 (n=60)	P
				Gp 1vs2	Gp 1vs3	Gp 2vs3
Attempts 1st/2nd (%)	55/5 (91.7/8.3)	50/10 (83.3/16.7)	48/12 (80/20)	0.09#	0.02*	0.73#
Intubation time (s)	9.97±0.54	11.38±0.12	11.92±0.13	<0.0001*	<0.0001*	<0.0001*
Neck flexion %	10 (16.7)	20 (33.3)	25 (41.6)	0.001*	0.001*	0.53#
Back flexion(%)	10 (16.7)	20 (33.3)	25 (41.6)	0.001*	0.001*	0.53#
Knee bending(%)	10 (16.7)	20 (33.3)	25 (41.6)	0.001*	0.001*	0.53#

Values are in mean±SD, number, percentage, *Statistically significant ^#Statistically insignificant

Discussion

The height of the operating table can be manipulated to reduce anesthesiologist discomfort by reducing extra efforts of back bending, knee bending, and neck flexion.[1] It has been seen that increasing age, male gender, BMI more than 30 kg m⁻², and increase in ASA grade are associated with difficult laryngoscopy and intubation.[8,9,10,11] As the study groups were comparable with respect to age, gender, anthropometric parameters, and ASA grading, none of the confounding factors affected our results [Table 1]. On the contrary, Lee et al.[1] also included ASA grade three patients although the number was very less. No patient with an anticipated difficult airway was included in the study like Grade 3 upper lip bite test, horizontal length of mandible <9 cm, sternomental distance of <12.5 cm, reduced interincisor distance, neck circumference of more than 42 cm, Mallampati Grade 3 and 4, and reduced thyromental distance for ethical reasons and to prevent anesthesia-related morbidity and mortality.[8,9,12,13,14,15] In other authors who included Mallampati classes 3 and 4 patients, equal distribution among all study groups was observed.[1,16] In the present study, all groups had comparable airway parameters [Table 2]. So, this provided us with a uniform platform to evenly compare the observed results. Discomfort in mask ventilation was maximum at the mid-sternum level [Table 3]. Similarly, Puri et al.[7] also observed maximum mask ventilation discomfort when the table height was 5 cm above the xiphisternum. Lee et al.[1] also noticed more discomfort on BMV when table height was set at higher positions (i.e., at the nipple and xiphoid level) than at the lower positions (umbilicus or lowest rib margin). Lee et al. also recorded the use of wrist exertion and joint strains by the anesthesiologist while ventilating at higher table positions. In the present study, the anesthesiologist was allowed to use any postural modifications in the form of wrist movements, and mask ventilation discomfort was evaluated only subjectively. During laryngoscopy, the quality of the laryngeal view before the use of any maneuvers was found to be better in group 1 (mid-sternum level) as compared to groups 2 and 3 [Table 4]. Similarly, Lee et al.[1] stated that the quality of the laryngeal view before postural changes was better at higher table heights. Although Jayakumar et al.[5] observed that there was no change in laryngeal view on changing trolley height, but time to tracheal introducer placement was highest at the lower trolley height position (at the level of anterior superior iliac spine of an operator). After the use of maneuvers, significant improvement in CL grade was seen only in groups 1 and 2 [Table 4]. By this, we can hypothesize that postural changes can compensate for the table height only when table height is at mid-levels but complete compensation for the table height cannot be obtained by postural adjustments at lower table heights as seen in group 3. Similarly, Lee et al.[1] showed that the difference in CL grades which was seen at different table heights before postural changes was not present afterward. They attributed this change in CL grades to the compensation by postural adjustment for the table heights.

It is also found that intubating in the sniffing position allows the operator to perform intubation in a better posture as compared to the simple head extension of the patient.[16] Hence, sniffing position was made in all patients for intubation. In the present study, discomfort experienced by the anesthesiologist during intubation was graded subjectively by him/her and also assessed objectively by recording the joint movement (neck flexion, knee bending, and/or back flexion) required to obtain the best glottic view. On lowering the table height from mid-sternum to umbilicus, the level of discomfort during intubation increased significantly [Table 3]. The present study is in agreement with Lee et al.[1] who stated that the anesthesiologist’s discomfort scores during intubation were higher in the lower table height position. Similarly, Jayakumar et al.[5] showed that the anesthesiologist adopted poor body posture at lower trolley heights. Puri et al.[7] also demonstrated that intubation with the table at 5 cm below the xiphisternum had the maximum discomfort grading. The maximum first-attempt success rate was recorded in group 1 (at mid-sternum level) compared to groups 2 and 3 [Table 5]. Similarly, Jayakumar et al.[5] found a high intubation failure rate at lower trolley heights. Lee et al.[1] observed that although the difference in the intubation attempts among all groups was not significant but minimal postural changes were required at higher table heights signifying that intubation conditions were better at that level. Intubation time was minimal with table height at mid-sternum level and maximum at umbilicus level [Table 5]. However, the mean intubation time difference among the three groups was minimal, but this minimal delay can significantly affect the outcome in some emergency conditions like in children, morbidly obese patients, and patients who are at risk of rapid desaturation. Jayakumar et al.[5] also observed that the height of the trolley had a significant effect on time to tracheal introducer placement. Similarly, Puri et al.[7] also showed that the laryngoscopy was fastest at higher table height. Contrarily, Lee et al.[1] found that the duration of tracheal intubation was similar in all the groups.

Postural modifications such as neck flexion, back flexion, and knee bending were maximum at lower table heights [Table 5]. Lee et al.[1] also observed that degree of neck and back flexion was greater when the operation table is at the umbilical level. Similarly, Puri et al.[7] also showed that maximum joint movements were seen when the table is at 5 cm below the xiphisternum while minimal to absent movements were seen when the table is at 5 cm above the xiphisternum. Jayakumar et al.[5] also found that poor posture was exaggerated at the lower trolley position (i.e., at the level of anterior–superior iliac spine).

We acknowledge that our study has a few limitations. Patients with anticipated difficult airways were not included. Laryngoscopic view was graded using CL grade which is a standard and less cumbersome method of laryngoscopic grade evaluation although the use of the percentage of glottis opening score would have provided a better measure of the laryngoscopic view. Only subjective assessment of wrist exertion was assessed, and the degree of joint movements was not measured. Objective measurement of muscle strains (electromyography) in intubating conditions such as neck flexion, back flexion, and knee bending was not done whichcould be a better evaluation of muscle strain, due to non-availability of enough resources for the large sample size.

Conclusions

To conclude, the present study shows that the operator’s discomfort during BMV was minimal when the table height was adjusted to lower levels (umbilical level of an operator), while the best laryngoscopic view and intubation with minimal postural modifications were seen at higher table heights (mid-sternum level of an operator). So far, it is best advisable to adopt higher table positioning for smooth and single-attempt endotracheal intubation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.