Effect of Cricoid Pressure on the Glottic View and Intubation with King Vision® Video Laryngoscope.

CONTEXT: To establish the usefulness of King Vision® video laryngoscope (KVL) in patients with rapid sequence anesthesia. AIMS: This study aims to compare the role of KVL on glottic visualization, intubation time and associated sympathetic response in routine intubations to those intubations done with cricoid pressure (CP). SETTINGS AND
DESIGN: Randomized controlled study in a tertiary care hospital.
METHODOLOGY: Seventy-six patients intubated with KVL were randomized to two groups - Group C (who did not receive any CP) and Group CP - who received CP. The percentage of glottic opening (POGO), intubation time, subjective assessment, and number of attempts taken to introduce KVL and endotracheal tube (ETT) were noted. The saturation, end-tidal carbon dioxide concentration and hemodynamic response (heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure, and rate pressure product) in the peri-intubation period were also recorded.
RESULTS: The demographics, airway, and technical characteristics of insertion of KVL and ETT were comparable between the groups (P > 0.05). POGO score was 100% in both groups. The significant time in insertion of KVL (Group C 29.87 ± 11.64 s and Group CP 40.68 ± 18.93 s, P = 0.004) and ETT (Group C 17.53 ± 8.71 s and Group CP 22.42 ± 10.77 s, P = 0.033) contributed to prolonged overall intubation time in CP (Group C 41.11 ± 11.65 s and Group CP 51.05 ± 17.31 s, P = 0.005). The intergroup and intragroup hemodynamic variables did not show any statistical significance (P > 0.05) over time.
CONCLUSION: Although overall intubation time with KVL is prolonged in patients with CP, it provides excellent glottic view, eases intubation, and causes insignificant hemodynamic variation.

INTRODUCTION

The national and international guidelines recommend video laryngoscope (VLS) for routine and difficult airway management.[123] The applicability of VLS has widened because of the success of intubation in various suites such as operation theatre (OT), intensive care units, emergency department, and prehospital environment.[4] Although a variety of VLSs are available in the market, their efficacy is yet to be established.[56]

During rapid sequence induction (RSI) of anesthesia, cricoid pressure (CP) is applied in patients who are at risk of aspiration. Since CP physically compresses the pharyngeal structures, the technique inevitably affects the VLS insertion, impedes tracheal intubation, and prolongs the overall intubation time.[789] Intubation with CP in conventional laryngoscopy by itself causes sympathetic response which maybe detrimental in moribund patients. There is a paucity in the literature regarding the usefulness of VLS in RSI. Our aim was to compare the role of King Vision® video laryngoscope (KVL) on glottic visualization, intubation time, and associated sympathetic response in routine intubation to those intubations done with CP. To the best of our knowledge, this is the first study of its kind. We hypothesized that the CP may prolong intubation time with KVL and result in hemodynamic variations.

METHODOLOGY

This prospective, unblinded randomized controlled study was conducted from September 2017 to September 2018 in a tertiary care hospital. The study protocol was approved by the Institutional Review Board and was registered at ctri.gov.nic.in (CTRI/2017/12/010960).

The required sample size was derived based on a pilot study that estimated mean difference in intubation time between two desired groups with KVL (mean ± standard deviation [SD] of 52 ± 18.28 and 42 ± 12.16 respectively) as 10 s. With 80% power and 5% level of significance, the sample size was estimated to be 38 patients in each group.

Seventy-six American Society of Anesthesiologists (ASA) physical Status I and II patients, aged 18–60 years belonging to either sex scheduled for elective surgery under general anesthesia (GA) were included in the study. Those patients with anticipated difficult airway, body mass index (BMI) >30 kg/m2, pregnant, full stomach, and history of reflux were excluded. Preanesthetic evaluation was done and informed consent obtained from the recruited patients. They were randomized into two groups: Group C (control group)-who did not receive any CP and Group CP – who received CP. Randomization was done using computer generated table, and opaque sealed envelope was used for allocation [Figure 1].

Figure 1

CONSORT diagram

Once in OT, the patients were connected to standard monitors (electrocardiography, noninvasive blood pressure, capnography, and pulse oximetry) and intravenous (i.v.) access was secured. After optimal preoxygenation, they were premedicated with i.v. midazolam 1 mg, glycopyrrolate 0.2 mg and fentanyl 2 μg/kg.

Standard GA was given to all patients in both groups. An experienced anesthetist (who has used KVL at least 30 times prior) did the laryngoscopy and intubation with KVL. An experienced assistant provided gentle pressure over the cricoid cartilage in Group CP. The patient was induced with titrated doses of propofol until loss of verbal contact. In Group CP, once the patient was asleep, the assistant increased the force of CP. Atracurium 0.5 mg/kg was given, and the patient was ventilated with volume control mode with a tidal volume of 8 mL/kg, respiratory rate of 12 bpm, inspired oxygen of 100%, PEEP of 5 cm H2O. After 3 min, KVL (King Systems, Noblesville, IN, USA) channelled blade was introduced, and the endotracheal tube (ETT) - 7.5 mm for females and 8.0 mm for males was placed and secured.

The percentage of glottic opening (POGO), the intubation time (time of introduction of KVL till trace of capnograph for confirmation of the tube in trachea), subjective assessment and number of attempts taken to introduce KVL and advance ETT was noted. The saturation (SpO2), end-tidal carbon dioxide (EtCO2) concentration and hemodynamic response-heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), and rate pressure product (RPP), in the peri-intubation period (baseline, after induction, immediate postintubation at 1 min, 3 min, and 5 min) were recorded. Thereafter, standard anesthesia was continued till the end of surgery. The data collected was compiled in an excel sheet, tabulated, and statistically analyzed using R statistical software version 3.5.0 (R Core team 2018-https://www.R-project.org/).[10]

Statistics

Descriptive statistics was summarized for continuous (mean and standard deviation) and categorical (counts with percentages) variables. Chi-square test was used to test the association between the categorical variables. Student's t-test was used to compare the intubation time taken between the two groups. Repeated measures ANOVA were used to compare the hemodynamic parameters between the two groups over time. P < 0.05 was considered statistically significant.

RESULTS

The demographic variables - age, sex, ASA, and BMI - were comparable between the two groups [Table 1]. The airway assessment - Mallampati grading, mouth opening, thyromental distance, neck extension between the two groups were also comparable [Table 2]. POGO score was 100% in both groups. Table 3 shows the technical characteristics for the insertion of KVL and the passage of ETT into glottis. The results were comparable and did not show any statistical significance between the groups. The intubation time recorded was T1 – time of insertion of KVL till glottic view, T2 – time of insertion of KVL till the passage of ETT into glottis, and T3 – overall intubation time from KVL insertion till trace of EtCO2. Statistically significant difference of T1, T2, and T3 were noted between the groups [Table 4]. The overall intubation time was prolonged in Group CP (P = 0.005). The intergroup hemodynamic variables did not show any statistical significance (P > 0.05). When compared to postinduction, the RPP, SBP, DBP, MAP showed significant decrease at 1st min but remained similar at 3rd and 5th min [Figure 2]. However, the decrease was the same in both groups and was not statistically significant (P > 0.05).

Table 1

Demographics

Parameters	Group C	Group CP	P
Age (years) - mean±SD	37.63±12.28	33.5±12.63	0.152#
Sex (M/F) - number	28/10	22/16	0.227##
ASA (I/II)- number	20/18	23/15	0.487##
BMI (kg m-2)	24.53±2.42	24.07±2.26	0.386#

#Student t-test ##Chi square test. CP=Cricoid pressure, SD=Standard deviation, ASA=American Society of Anesthesiologists, BMI=Body mass index

Table 2

Airway characteristics

Parameters	Group C number (%)	Group CP number (%)	P##
Mallampati grade
1	17 (44.7)	26 (68.4)	0.059
2	19 (50)	9 (23.7)
3	2 (5.3)	3 (7.9)
Thyromental distance
>6.5 cm	28 (73.7)	33 (86.8)	0.249
6-6.5 cm	10 (26.3)	5 (13.2)
<6 cm	0 (0)	0 (0)
Mouth opening
<3 cm	1 (2.6)	0 (0)	1.0
>3 cm	37 (97.4)	38 (100)
Neck extension
No reduction	34 (89.5)	38 (100)	0.123
1/3rd reduction	4 (10.5)	0 (0)
Percentage of glottis opening (POGO)	38 (100)	38 (100)	-

##Chi square test. CP=Cricoid pressure, POGO=Percentage of glottic opening

Table 3

Technical characteristics

Parameters	Levels	Group C	Group CP	P (Chi-square)
Insertion of KVL	Excellent	15 (39.5)	13 (34.2)	0.888
	Fair	3 (7.9)	3 (7.9)
	Good	20 (52.6)	22 (57.9)
Number of attempts for KVL	1	27 (71.1)	22 (57.9)	0.238
	2	11 (28.9)	14 (36.8)
	3	0 (0)	2 (5.3)
Ease of ETT insertion	Easy	16 (42.1)	13 (34.2)	0.191
	Modified	22 (57.9)	22 (57.9)
	Unachievable	0 (0)	3 (7.9)
Number of attempts for ETT pass	1	16 (42.1)	13 (34.2)	0.067
	2	22 (57.9)	20 (52.6)
	3	0 (0)	5 (13.2)

CP=Cricoid pressure, KVL=Kingvision videolaryngoscope, ETT=Endotracheal tube

Table 4

Intubation time

Parameters	Group C mean±SD(95% CI)	Group CP mean±SD(95% CI)	P$
T1	17.53±8.71(14.66-20.39)	22.42±10.77(18.88-25.96)	0.033*
T2	29.87±11.64(26.04-33.69)	40.68±18.93(34.46-46.91)	0.004*
T3	41.11±11.65(37.28-44.93)	51.05±17.31(45.36-56.74)	0.005*

*Significant, $ANOVA test. CP=Cricoid pressure, SD=Standard deviation, CI=Confidence interval

Figure 2

Hemodynamic parameters (heart rate, rate pressure product, systolic blood pressure, diastolic blood pressure and mean arterial pressure) SaO2 and end tidal carbon dioxide

DISCUSSION

Laryngoscopy and endotracheal intubation forms an integral part of GA. Successful tracheal intubation requires a “line of sight” to the larynx attained by positioning the head and neck and then using a laryngoscope to retract the tongue/soft tissues of floor of the mouth. Unlike conventional laryngoscopy, the VLS does not require alignment of oral, pharyngeal and laryngeal axes to view the glottis. The other advantages over conventional laryngoscopy include easier laryngeal exposure with less force, ability to achieve laryngeal view despite unfavorable anatomy, opportunity for both the operator, and assistants to observe the procedure.[1112] VLS uses video technology - a blade with a camera in its distal tip that transmits a lighted video image to a screen. KVL is a mobile VLS with an organic light emitting diode screen (2.4 inch) and an angulated channeled blade. The blade's tube guiding channel does not require use of a stylet that is commonly used in conventional laryngoscopy and intubation The viewing angle of KVL (OX axis 51.8° and OY axis 40.8°) is better than Mc Grath (OX axis 28.1° and OY axis 40.3°) and lesser than CMAC (OX axis 63.1° and OY axis 47.8°) The distance between the camera and distal blade tip in KVL is comparable to CMAC (34 mm in KVL and 35 mm in CMAC).[13]

In the recent times, CP application has been a topic of debate.[14] This is because the effective use of CP requires that the applied force is sufficient to occlude the esophageal entrance while avoiding airway obstruction leading to interference with manual ventilation, laryngeal visualization, and tracheal intubation.[15] In this context, the use of VLS during RSI of anesthesia is gaining importance, and thus, independent evaluation of each VLS with CP requires to be established.

Glottic view

The “line of sight” in direct laryngoscopy shows “direct view” of the glottis. Unlike this, the ease of intubation is decoupled in VLS as it is an “indirect view.” Cormack and Lehane (CL) and POGO are used to estimate the view of glottis. We used POGO scoring system in our study which showed 100% glottic view in both groups. This was similar to study conducted by Komasawa et al. on Penatax-AWS airway scope which did not show any difference in CL grading (P = 0.49) and POGO (P = 0.60) with and without CP.[16] Kleine-Brueggeney et al. showed the median percentile of POGO was 100 in GlideScope®; 90 in C MAC-D blade, Mc Grath, Airtraq, and Kingvision; 60 in A.P. Advance.[17] On applying CP, the CL grade did not change or improve in Macintosh, Mc Grath and C MAC X blade, but worsened with GlideScope (P < 0.001).[18] In contrast, Corda et al. showed that the laryngeal view grade did not change with GlideScope but significantly decreased the glottic area by forcing vocal cord opposition. The use of jaw thrust as a first-line maneuver to aid in glottic visualization and tracheal intubation during GlideScope video laryngoscopy was thus recommended.[19] The CL and POGO scoring thus maybe significantly different in many devices and is attributed to the difference in the design and angulation of the blade. In view of a variety of VLS devices, recently Freemantle scoring - a three element scoring describing view (full, partial or none), ease of intubation (1 - easy, 2 - modified and 3 - unachievable) and the device (name of the device and blade) used has been considered.[20] Higher accuracy and inter-rater reliability of Freemantle and POGO over CL scoring suggests its use in documenting VLS.[20]

The position of the head may have a role in glottic visualization. Increasing head elevation and laryngoscopy angle (neck flexion) significantly improves POGO scores during laryngoscopy which is an advantage in obese individuals, obstetric or anticipated difficult airway scenarios.[2122] Mendonca et al. compared the sniffing and neutral position using King vision (channeled) and C-MAC (nonchanneled) VLS. The POGO was lower with C-MAC use in neutral position (median [interquartile range (IQR)] 100 [40-100]) compared to the C-MAC in sniffing position (100 [100-100]) and King vision in sniffing (100 [70-100]) and neutral position (100 [75-100]) with statistical significance (P = 0.010).[23]

Risks (airway obstruction) versus benefits (prevention of aspiration) have to be considered when appropriate CP is deemed necessary.[24] The standard application of 30N force has been recently questioned. Different cricoid forces may have to be used depending on the scenario as in children, obese, use of head up position, preanesthetic nasogastric tube in situ and with use of VLS.[1425] Zeidan et al. showed the median cricoid force for female is 18.7 N (95% confidence interval 17.1–20.3) and 30.8 N (95% confidence interval 28.15–33.5) for males suggesting that median force necessary to occlude esophageal entrance to prevent regurgitation is less in women.[26] Oh et al. showed that CP with increasing force resulted in a worse laryngeal view with Pentax AWS.[27] Goldmann et al., in their study concluded that the laryngeal view with C-MAC VLS improved with CP, but the time required for intubation did not decrease in video laryngoscopy-guided CP.[28] Loughnan et al. found 41% improvement, 45% unchanged, and 14% worsening in the laryngeal view with CMAC.[11] Such discrepancy may be prevented if the CP is applied by a trained staff who should have access to the video image and facilitate adjustment accordingly.[1112] The widest viewing angle (OX axis 63.1 and OY axis 47.8) and largest diagonal size of the display in CMAC, allows the operator to see relevant finer details.[13] Glottic view of 100% in our study groups may have contributed by various factors such as an experienced and trained personnel for the use of KVL and CP application, head up position and laryngoscopy and intubation on ASA I and II patients who had insignificant airway parameters.

Intubation

Tracheal intubation involves three components - laryngeal exposure, delivery of the tube to glottic opening, and advancing it into the trachea. In a systematic review of 64 studies on VLS versus direct laryngoscopy for adult patients requiring tracheal intubation, Lewis et al. found that VLS improved the glottic view, reduced laryngeal/airway trauma but did not reduce the number of intubation attempts or affect the time required for it.[29] In channeled devices, however getting around the tongue is straightforward but there maybe difficulty in passing the ETT through glottic opening. There is a difference in angles with camera showing from base of tongue towards larynx and tracheal axis descending away from it.[30] The increase in angle as the trachea is pushed downward during CP may exacerbate this potential problem of tube placement through glottis.[16] “Neck antiflexion” (head elevation) after laryngoscopy has been proposed to alleviate this deviation and facilitate the smooth passage of the tube.[31] At the same time, Mendonca et al. did not find any significant difference in laryngoscopy time (P = 0.020), intubation time (P = 0.272), and success rate (P = 0.968) with use of KVL or C MAC with the patients in neutral and sniffing position.[23] VL scopy is a dynamic process and changes in the position of the patient's head and neck to suit the need has to be considered during difficult airway situations.

Without cricoid pressure

The technical characteristics of KVL and ETT were comparable in both the groups in our study. The time taken for insertion of the device, passage of tube and the overall time taken was lesser in Group C. Although glottic view was 100% in Group C, we had to modify our technique in 22 patients with manipulation in the KVL blade. The blade was either tilted to right or left, lifted upward or withdrawn slightly to facilitate the tube passage.[2430] Kleine-Brueggeney et al. evaluated channeled (Airtraq, A.P. Advance, and Kingvision) and unchanneled VLS (GlideScope, C MAC-D blade, Mc Grath) on 720 patients with a simulated difficult airway (cervical collar decreasing mouth opening from 46 ± 7 mm to 23 ± 3).[17] First attempt success rate with various blades was 98% (Mc Grath), 95% (C MAC-D blade), 87% (King vision) and 85% (GlideScope and Airtraq) and 37% in A.P. Advance. They found excellent (21%) to good (46%) grading in the insertion of KVL and excellent (34%), good (39%), and fair (20%) for ease of ETT insertion with P < 0.01. Time to view the vocal cords in KVL (median [25th, 75th percentile] [range]) was 26 s (16, 32) (7–117) and to advance tube was 31 s (24, 46) (4–140) giving overall intubation time of 59 s (46, 78) (31–180). Other channeled blades showed overall median intubation time of 47 s (36, 60) (18–175) in Airtraq and 93 s (54, 144) (33–180) in A.P. Advance compared to unchanneled blades which showed median time of 53 s (42, 77) (20–179) in Mc Grath, 56 s (45, 85) (20–177) in CMAC-D blade and 60 s (48, 98) (17–180) in GlideScope blades. Thus, both channeled and nonchanneled blades have variation in overall intubation time. In our study, the use of KVL in normal airways showed overall intubation time of 41.11 ± 11.65s unlike in the simulated difficult airway study by Brueggeney et al. which showed 59 s. This re-emphasizes that KVL can be used in both normal and difficult airway.

The study by Abdelgawad et al. showed intubation time (mean ± SD) of 26.1 ± 3.2 s in Macintosh, 19.1 ± 2.4 s in UE VLS and 25. 6 ± 3.1 in UE intubation stylet with statistical significance amongst them (P < 0.001). They had no failed intubations, and the number of attempts of intubation was also insignificant.[32] Komasawa et al. found 96% success with the first attempt of insertion of airway scope and 80% first attempt for passage of tube and a median intubation time of 32 s (IQR-28–45).[16] The above studies indicate that different designs of the blade in VLS (shape, curvature, and position of the video camera) influence the performance of the device.

With cricoid pressure

In our study, the technical characteristics of KVL with number of attempts for device insertion and ease of intubation did not show any statistical difference between the two groups. The significant time in inserting the device (Group C 29.87 ± 11.64 s and Group CP 40.68 ± 18.93 s, P = 0.004) and ETT (Group C 17.53 ± 8.71 s and Group CP 22.42 ± 10.77s, P = 0.033) contributed to prolonged overall intubation time in CP (Group C 41.11 ± 11.65 s and Group CP 51.05 ± 17.31 s, P = 0.005) in our study. Komasawa et al. found a prolonged intubation time with Pentax AWS and CP (median [IQR]) (45 [40-59]) compared to control (32 [28-45]) with 95% confidence interval for the median difference of 5–24 s (P = 0.003).[16] The number of attempts for insertion of device (P = 0.08) was insignificant, but had a significant difference in number of attempts for ETT insertion (P = 0.002). Arslan and Solak found median (25–75 percentile) insertion time of 12 s (9–15.8) in Macintosh and 13 s (11–17) in Mc Grath, but significantly prolonged (P < 0.001) in GlideScope 18 s (14–22) in all patients with CP. The intubation time (insertion of the device until visualization of entry of ETT) was similar in Macintosh and Mc Grath but longer in GlideScope than the other two (P = 0.001 and P = 0.003 respectively). They attributed the midline approach and angle of blade with GlideScope as the reason for this significance.[18] Similarly, in our study, the insertion of KVL was good (57.9%) but the number of attempts for first time insertion was less in Group CP (71.1% in Group C and 57.9% in Group CP). Optimization maneuvers of KVL blade (right/left tilt and/or withdrawal or lift upwards) were also required in 57.9% patients in Group CP. The CP was released in three patients as it was impossible to pass the ETT. Unlike control group (42.1%), only 34.2% had first attempt tube passage through glottic opening. In KVL, the rigid fixation of the liquid crystal display screen and a relatively large cross section of the device causes the operator to focus on the insertion of the blade in the oral cavity first and later look onto the screen for an effective evaluation.[13] All the above factors have contributed to prolonged overall mean intubation time with CP in our study.

Hemodynamics

The technique of CP by itself, or it being used during laryngoscopy and intubation, increases the incidence of hypertensive or tachycardic episodes.[33] However, the sympathetic stimulation with KVL is lesser than a conventional laryngoscopy. The stretch of tissues in epipharynx and laryngopharynx is minimized to view the line of sight.[21] The optical stylet and Bonfil retromolar intubation devices are advantageous as they do not require manipulation of base of tongue or epiglottis. There may not be improvement in glottic view, but head up position of 25° shortened intubation time by 14% in conventional laryngoscopy and reduced the need for ancillary maneuvers (backward-upward-rightward-push, optimal external laryngeal manipulation) or use of ancillary equipment such as Mc Coy, bougie or stylet (P = 0.004).[21] Apart from improving oxygenation and increasing safe apnea period, it brings “line of sight” more easily and with less effort. Koyama et al. found that in comparison to conventional laryngoscopy, the Airway scope attenuated hemodynamic response (SBP, DBP, and HR) to tracheal intubation over time in normotensive patients (P < 0.003) but no difference in changes was observed in hypertensive patients (P > 0.05).[34] Abdelgawad et al. also found similar results with Macintosh, UE VLS and UE video intubation stylet. There was no difference in cardiac output variables (cardiac index [CI] and stroke volume index [SVI]) and hemodynamic variables (HR, SBP, and DBP) in normotensive patients.[32] However, in hypertensive patients the SBP, DBP was significantly higher in Macintosh (P < 0.05 and P < 0.01) although the HR, CI, SVI remained the same when compared to the UE VLS or UE intubation stylet. Similar results were also seen by Nishikawa et al. when lightwand and Pentax AWS were used.[3536] Even though the stimulation by VLS was lesser when used in these studies, greater sensitivity in hypertensives might have resulted in such BP variations.[34]

Very few studies have investigated the hemodynamic response with CP and VLS. Our study on normotensive patients demonstrated that the intragroup comparison of hemodynamic variables with postinduction findings showed a significant decrease at 1st min but remained similar at 3rd and 5th min with respect to RPP, SBP, DBP, MAP. There was no statistical or clinical significance between the groups. In contrast, Arslan and Solak showed CP caused a significant increase in MAP in Macintosh and GlideScope (P = 0.004 and P = 0.001 respectively) and increase in HR with Macintosh, GlideScope and Mc Grath MAC X – Blade when compared with postinduction values. The increase in these variables was not statistically significant. The lesser dose of Fentany 1 μg/kg in their study may have resulted in increase in HR, and MAP. The head-up position, better glottic view and lesser force used by an experienced operator on the KVL might have contributed to the less hemodynamic response in our study. Further studies with individual VLS and amongst various devices, along with continuous invasive arterial BP monitoring and serum catecholamine level measurement are warranted to conclude on the hemodynamic effects of VLS.

The concern drawn from our study is that if the CP prolonged intubation time in KVL in elective cases with ASA 1 and 11 patients, will it be safe to use in high risk, emergencies and where RSI seems mandatory? Though there was no statistical or clinical hemodynamic variations, the prolonged apnea time may cause hypoxia in patients with reduced oxygen reserve.[25] The use of a single design, broad channeled blade of KVL was also a limitation. The results would probably vary if non-channeled KVL blade was used as it is a thin blade that is easier to insert when the midpharyngeal space gets narrowed with the application of CP. Objective quantification of CP and use of KVL in RSI has to be affirmed. The physical characteristics of KVL with their mechanical and optical parameters may be different in other VLS, and more studies are warranted for comparison of KVL with different VLS and CP.

CONCLUSION

There is a definite role for a KVL during RSI since it provides excellent glottic view, eases the ETT insertion, and causes minimal hemodynamic variations. However, overall intubation time is prolonged with CP.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.