INTRODUCTION AND BACKGROUND- Pulse oximetry provides continuous,
non-invasive assessment of arterial oxygen saturation and is a sensitive
detector of hypoxemia and major hypoxic events. With a pulse oximeter we
monitor the peripheral arterial oxygen saturation (SpO2). The shape of the
oxygen dissociation curve is sigmoidal showing that the relationship between
arterial partial pressure of oxygen (PaO2) and arterial oxygen saturation
(SaO2) is not linear. Once the decline of PaO2 passes the inflection point of
the oxygen-haemoglobin dissociation curve, the SpO2 decreases rapidly with the
decline of PaO2. This means that there is a rapid SaO2 and SpO2 decline once
PaO2 decreases to <80 mm Hg. A SpO2 of 95% corresponds to a PaO2 of 80 mm of
Hg and a SpO2 of 90% corresponds to a PO2 of 60 mm of Hg. Above a PaO2 80 mm of
Hg the SpO2 will always read as 98-100%. Thus, SpO2 may not provide advance
warning of falling arterial oxygenation until PaO2 approaches this
level. It is difficult to predict when desaturation will start in
apnoeic patients. The Oxygen Reserve Index (ORI) is a
relatively new technology by Masimo InternationalTM . (1-3) It
is a non-invasive and continuous parameter intended to provide insight into the
patient’s oxygen reserve in the moderate hyperoxic range i.e. between PaO2 of
100 and 200 mm of Hg. Itis a nondimensional index that ranges from 0 to 1. It
is measured by optically detecting changes in venous oxygen saturation (SvO2)
after arterial oxygen saturation (SaO2) saturates to the maximum. Applegate et
al(1) showed a positive linear relationship for PaO2 up to 240
mm Hg and ORI using linear regression analysis (r2 = 0.536). When ORI was over
0.24, all measured PaO2 were ≥100 mm Hg and SaO2 was 100%. When
ORI was over 0.55, they found that 96.6% of PaO2 measurements were ≥150 mm Hg.
ORI decrease to 0.24 provides advance warning of PaO2 declining to
approximately 100 mm Hg when SpO2 is>98%.ORI could indicate PaO2 decreases
before SpO2 falls providing advance warning of impending desaturation events.
ORI is an index that is intended to supplement, not replace SaO2, SpO2 and PaO2
measurements. In a study done in children by Szmuk et
al they found that ORI detected impending desaturation in median of 31.5 s
(interquartile range, 19–34.3 s) before noticeable changes in SpO2 occurred.(3) This
represents a clinically important warning time, which might give clinicians
time for corrective actions. Head and neck malignancies require
direct laryngoscopic evaluation for disease mapping. Some patients may require
microlaryngoscopic evaluation and laser surgery. Similarly, some thoracic
procedures like tracheal dilatation or stenting are done in the interventional
radiology suite. These procedures are done under general anaesthesia with
muscle relaxant and apnoeic oxygenation to have immobile vocal cords for
evaluation, biopsy and laser surgery. During this evaluation a lower limit of
95% SpO2 is taken as a warning as patients start rapidly desaturating below
that. Reoxygenation or mask ventilation is done once the patient SpO2 drops
below 90%on pulse oximetry. The purpose of using ORI in these cases in addition
to a pulse oximeter is to reliably predict the impending hypoxia once the ORI
starts dropping before the patient starts desaturating on the pulse oximeter. HYPOTHESIS AND AIMS- We hypothesise that ORI may help us in
predicting impending desaturation on pulse oximetry and hypoxia during patients
undergoing procedures under apnoeic oxygenation. The aim of our study is to estimate the
accuracy of ORI in predicting impending desaturation and hypoxia during
patients undergoing procedures under apnoeic oxygenation. STUDY DESIGN AND METHODOLOGY- Study design- Prospective observational
study Site- TMH Population: Inclusion Criteria: 1. Adults
18 - 80 years’ old 2. American
Society of Anesthesiologists (ASA) physical status classification system:1 or 2 3. Any
patient undergoing any procedure under general anaesthesia with apnoea and
apnoeic oxygenation. e.g. direct laryngoscopic/ microlaryngoscopic evaluation
or laser surgery for head and neck cancer, tracheal dilatation or stenting done
under apnoeic oxygenation. Exclusion Criteria: 1. Any
patients with respiratory ailments (e.g. chronic obstructive pulmonary disease,
asthma) or poor pulmonary function- Objective parameter to identify poor
effort tolerance is ability to climb less than 1 flight of stairs. 2. Patients
with baseline saturation less than 95% on room air. 3. Expected
difficulty in tracheal intubation due to insufficient mouth opening and trismus
where an awake fibreoptic intubation is planned. 4. Patients
with stridor or an obstructed airway. 5. Patients
with nail polish or Henna on fingers and nails- in whom the pulse oximeter
readings may not be accurate. 6. No
informed consent 7. Inability
to wear the sensor due to deformity or hypoperfusion of the fingers This study will be initiated after
approval from the institutional ethics committee and registration with CTRI.
Adult patients between 18-80 years of age posted for elective surgery other
than those meeting exclusion criteria and requiring general anaesthesia with
apnoea will be included in this prospective observational study. Consent for
the participation in the study will be taken from all eligible patients.
Patients will be taken to the operating room and the monitors will be attached.
The monitoring will include electrocardiography, pulse oximeter, non-invasive
blood pressure and ORI probe from MasimoTM monitor. After doing
the surgical safety checklist, IV access will be secured. The pulse oximeter
probe and the ORI monitor probe of the MasimoTM monitor will be
put on separate fingers on the same hand. The non-invasive blood pressure cuff
will be attached on the opposite arm so that the measurements do not lead to
loss of the pulse oximeter signal and plethysmograph. The patient will be
preoxygenated with 100% oxygen for 3 minutes. The corresponding SpO2 and ORI
values will be noted. Induction of general anaesthesia will be done as per the
discretion of the attending anaesthesia consultant. After the patient is
apnoeic following administration of a muscle relaxant apnoeic oxygenation with
nasal prongs and 15 litres of oxygen will be started. The corresponding time of
apnoea will be noted. The surgeon will be allowed to proceed with the procedure
which may include direct laryngoscopic or microlaryngoscopic mapping of the
disease or laser surgery. The corresponding SpO2 and ORI values will be
recorded every 5 seconds for 3 minutes. In case the ORI and SpO2 does not drop
in 3 minutes the time at which ORI drops to 0.24 and Spo2 drops to 95% will be
noted from the time of apnoea. The time interval between the ORI alarm and the
fall in SpO2 to 95% will also be recorded. Oxygenation with anaesthesia
breathing circuit will be resumed once the patient starts desaturating to less
than 95%. The time taken for the ORI to rise back to 0.24 and SpO2 to rise to
100% after reoxygenation will also be noted in seconds from the time of
resuming of ventilation of the patient. The current standard of care for
patients undergoing procedures with apnoeic oxygenation technique is recognition
of desaturation using a pulse oximeter. When the SpO2 falls to 95% the surgeon
is informed the need to reoxygenate the patient. If the procedure is likely to
take more time the reoxygenation is started once the SpO2 drops below 90%. The additional intervention in our
study is only recording ORI values using an ORI probe. Since ORI is still being
evaluated, ORI readings will not be used to decide patient management. The
management of these patients will continue to be done based on oxygen saturation
which is the current standard of care. Objectives of the study- Primary objective: - To find out the time lag between the
ORI alarm at 0.24 and Sp02 to drop to 95%. Secondary Objective- Time taken, for the ORI to rise
up to 0.24 and the SpO2 to rise to 100% after resuming ventilation and the time
difference between the two. Sample size and statistical analysis- Sample size justification- The primary objective is to calculate mean time
between ORI 0.24 and SaO2 95%. Assuming that this is average 35 seconds(3) and
5 sec standard deviation, a sample size of 25 is required to produce a
two-sided 95% confidence interval with a distance from the mean to the limits
which is equal to 2 sec. The primary objective is to
calculate mean time difference between ORI decreasing to 0.24 and SaO2
decreasing to 95%. Assuming a 5 sec standard deviation, a sample size of 28 is
required to find the true mean difference with a margin of error of 2 seconds
with 95% confidence. This means that if we study 28 patients and find that the
mean time difference between ORI 0.24 and SaO2 of 95% is x seconds, we can be
95% sure that in the population, the mean time difference between ORI 0.24 and
SaO2 of 95% will lie between x-2 and x+2 seconds. Since the duration of
surgical procedures and physiological status of patients is variable, some
patients may not desaturate to 95% during the procedure (procedure may be
completed before the end-point is reached) - such patients will not be
analyzable for the primary outcome. Therefore, we plan to accrue 50 patients to
have analyzable data in 28 patients. Since this is a thesis project, we will
analyse whatever cases we accrue at the end of Nov 2021. The study will however
continue till we complete accrual of 28 analyzable patients. Statistical Analysis Plan: - Data will be descriptively analysed
using mean and standard deviation for continuous variables and frequency and
percentage for categorical variables. The time that elapses between
activation of the oxygen Reserve alarm until saturation reached 95% without
ventilation, that is, the warning time the index provides of impending
desaturation will be calculated as the mean time in seconds and the standard
deviation will be reported depending on the normality of the data. We will
report 95% confidence intervals for this estimate We will also report the means of the
time (with 95% CI) for ORI to recover to 0.24, for SaO2 to recover to 100% and
the difference between the two.
|