Pre-warming the Streamlined Liner of the Pharynx Airway (SLIPAâ„¢) improves fitting to the laryngeal structure: a randomized, double-blind study
Study protocol and patients
The protocol of this study was approved by the Institutional Review Board of Chung-96
Ang University Hospital [C2014020(1216)] and registered at Clinical Research Information
Service [KCT0001059]. The study was conducted according to the principles of the 2000
revision of the Declaration of Helsinki, and written informed consent was obtained
from all patients.
This study was intended for American Society of Anesthesiologists (ASA) class I-II
patients aged 20 to 65 years who received general anesthesia for elective gynecologic,
orthopedic, or abdominal surgery performed in the supine position and lasting for
2 h. The exclusion criteria were as follows: body weight of 40 kg or 100 kg, body
mass index??30 kg/m
2
, pregnancy or lactation, a history of upper abdominal surgery, diabetes mellitus,
hiatus hernia and gastroesophageal reflux disease, peptic ulcer, risk of aspiration
or regurgitation, and concomitant serious cardiovascular, pulmonary, renal, hepatic,
hematologic, or cervical disease. The decision to enroll or exclude patients was made
by the principle investigator, who did not otherwise participate in the study and
data collection.
Study design and randomization
This was a randomized, double-blind study. Randomization into one of two groups was
based on a table generated using PASS 11â„¢ software (NCSS, Kaysville, Utah, USA). Details
of the series, which was generated by a statistician who did not otherwise participate
in the study, were unknown to both the investigators and patients, and the numbers
were contained in a set of sealed envelopes. Fifteen minutes before patients were
admitted to the operating room, the appropriate numbered envelope was opened, and
the card inside determined whether the patient was assigned to Group W (warming group)
or Group R (room temperature group). The investigator who read the card prepared the
SLIPA at either body or room temperature. For Group R, the SLIPA was immersed in water
at room temperature (22 ° C) 15 min before insertion, whereas for Group W, it was
immersed in warmed (37 ° C) water, which was kept in a heating cabinet set to 37 °
C.
To blind the anesthesiologist who inserted the SLIPA to its temperature, he wore both
cotton and PVC gloves.
Anaesthesia
The patients fasted from midnight on the day of surgery, and no premedication was
administered before anesthesia. After the placement of standard monitoring systems
(electrocardiograph, non-invasive arterial blood pressure sensor, pulse oximeter),
each patient was denitrogenated with 100 % oxygen. Anesthesia was induced with intravenous
fentanyl (2 ?g/kg), lidocaine (0.5 mg/kg), and propofol (2 mg/kg). After confirming
that the patient had become unconscious and corneal reflex had disappeared, rocuronium
(0.6 mg/kg) was administered, and manual ventilation with sevoflurane (3 vol. %) in
100 % O
2
(5 L/min) was simultaneously performed. After the peripheral nerve stimulator (NMT
MechanoSensor, GE Health Care Filand Oy, Helsinki, Finland) with electrodes placed
over the ulnar nerve revealed that the count of twitches of the train-of-four stimulation
reached zero, the appropriately prepared SLIPA was inserted. The size of the SLIPA
was chosen by matching the width the thyroid cartilage with that of the bridge of
the SLIPA 5]. First, a sniffing position was made by placing a 5-cm pillow under the patient’s
head, and the bridge area of the SLIPA was collapsed. One hand lifted the patient’s
jaw, whereas the other hand pushed the SLIPA into position so that the heel portion
of the SLIPA was in the nasopharynx over the base of the tongue. The insertion was
performed by a single anesthesiologist with more than 3 years of experience in SLIPA
insertion.
When insertions were attempted more than twice without success, tracheal intubation
was performed, and these patients were excluded from the study.
The time taken to complete the insertion, assessed from the touching of the SLIPA
to the teeth to its fixation to the laryngeal structure, was measured. The anesthesiologist
who inserted the SLIPA subjectively evaluated the difficulty of the insertion (easy,
normal, or hard).
Successful insertion was confirmed by uniform movement of both lungs, a normal capnogram,
and a peak inspiratory airway pressure of 30 cmH
2
O 6]. After insertion, OLP was immediately measured, and this measurement was repeated
at 1-min intervals for 5 min. After converting to the manual ventilation mode and
setting the adjustable pressure limiting valve to maximum, OLP was measured when the
leakage sound began to be heard through a stethoscope placed over the patient’s mouth.
If the sound was not heard until 40 cmH
2
O, OLP was recorded as 40 cmH
2
O.
The mechanical ventilation settings were as follows: tidal volume of 6–8 mL/kg of
the patient’s ideal body weight, an inspiratory-to-expiratory ratio of 1:2, fixed
using a volume-controlled ventilator (Datex-Ohmeda Aestiva/5â„¢, GE Health Care, Madison,
WI, USA), and a respiratory rate of 10–12/min.
Anesthesia was maintained with desflurane (6–10 vol. %) and 60 % N
2
O (3 L/min). All surgeries were performed with patients in the supine position.
At the end of surgery, following the return of spontaneous breathing and if the patient
could obey the commands, the SLIPA was removed. We examined the device for the presence
of blood staining or evidence of gastric reflux. The severity of postoperative sore
throat was determined using the visual analog scale (VAS) in the recovery room after
at least 30 min. A second anesthesiologist who did not insert the SLIPA and who had
been blinded to the patients’ assignment to groups recorded postoperative data as
an independent observer.
Statistical analysis
The primary outcome measurement of the study was OLP, measured immediately and 1,
2, 3, 4, and 5 min after insertion. To estimate the necessary group size for the study,
a pilot study was conducted to measure OLP in 10 patients in whom the SLIPA was inserted
at room temperature. The average OLPs immediately after insertion and 1, 2, 3, 4,
and 5 min after insertion were 22.0, 24.1, 24.7, 26.3, 27.0, and 28.9 mmHg, respectively.
The standard deviations of OLP ranged from 4.1 to 6.5, and an autocorrelation between
adjacent measurements on the same individual of 0.7 was found. For our power calculation,
we assumed that first-order autocorrelation adequately represented the autocorrelation
pattern. Thus, it was necessary to detect a 10 % higher OLP in Group W than in Group
R. With an ? value of 0.05 and a power of 80 %, 42 patients were required for each
group. Considering a likely insertion failure or dropout rate of 5 %, 90 patients
were required for the study. PASS 11â„¢ software (NCSS, Kaysville, UT, USA) was used
to calculate the necessary sample size.
For continuous variables, the normal distribution of the collected data was first
evaluated using the Shapiro-Wilk test. Normally distributed data were presented as
the mean?±?standard deviation, and groups were compared using the unpaired t-test.
Non-normally distributed data were expressed as medians (25th percentile–75th percentile)
and were analyzed using the Mann–Whitney U test.
As OLP did not pass the Shapiro-Wilk test, we additionally checked the q-q plot, which
did not show marked deviation from linearity. Therefore, we decided to apply the normal
assumptions for the repeated-measured analysis of variance (ANOVA) in the analysis
of OLP. Because the sphericity assumptions failed, we used multivariate analysis of
variance (MANOVA), followed by a t-test with Bonferroni’s correction. The descriptive variables were analyzed by either
Chi-squared analysis or Fisher’s exact test, as appropriate. P??0.05 was considered statistically significant. Statistical analysis was performed
using SPSS version 18.0 (IBM Corp., Armonk, NY, USA).