An observational study to determine the accuracy of Oxygen Reserve Index (Ori) to predict impending desaturation in patient’s undergoing procedures under apnoea.

INTRODUCTION AND BACKGROUND-

Pulse oximetry provides continuous,non-invasive assessment of arterial oxygen saturation and is a sensitivedetector of hypoxemia and major hypoxic events. With a pulse oximeter wemonitor the peripheral arterial oxygen saturation (SpO2). The shape of theoxygen dissociation curve is sigmoidal showing that the relationship betweenarterial partial pressure of oxygen (PaO2) and arterial oxygen saturation(SaO2) is not linear. Once the decline of PaO2 passes the inflection point ofthe oxygen-haemoglobin dissociation curve, the SpO2 decreases rapidly with thedecline of PaO2. This means that there is a rapid SaO2 and SpO2 decline oncePaO2 decreases to <80 mm Hg. A SpO2 of 95% corresponds to a PaO2 of 80 mm ofHg and a SpO2 of 90% corresponds to a PO2 of 60 mm of Hg. Above a PaO2 80 mm ofHg the SpO2 will always read as 98-100%. Thus, SpO2 may not provide advancewarning of falling arterial oxygenation until PaO2 approaches thislevel.  It is difficult to predict when desaturation will start inapnoeic patients.

The Oxygen Reserve Index (ORI) is arelatively new technology by Masimo InternationalTM. (1-3)Itis a non-invasive and continuous parameter intended to provide insight into thepatient’s oxygen reserve in the moderate hyperoxic range i.e. between PaO2 of100 and 200 mm of Hg. Itis a nondimensional index that ranges from 0 to 1. Itis measured by optically detecting changes in venous oxygen saturation (SvO2)after arterial oxygen saturation (SaO2) saturates to the maximum. Applegate etal(1) showed a positive linear relationship for PaO2 up to 240mm Hg and ORI using linear regression analysis (r2 = 0.536). When ORI was over0.24, all measured PaO2 were ≥100 mm Hg and SaO2was 100%. WhenORI 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 toapproximately 100 mm Hg when SpO2 is>98%.ORI could indicate PaO2 decreasesbefore SpO2 falls providing advance warning of impending desaturation events.ORI is an index that is intended to supplement, not replace SaO2, SpO2 and PaO2measurements.

In a study done in children by Szmuk etal 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) Thisrepresents a clinically important warning time, which might give clinicianstime for corrective actions.

Head and neck malignancies requiredirect laryngoscopic evaluation for disease mapping. Some patients may requiremicrolaryngoscopic evaluation and laser surgery. Similarly, some thoracicprocedures like tracheal dilatation or stenting are done in the interventionalradiology suite. These procedures are done under general anaesthesia withmuscle relaxant and apnoeic oxygenation to have immobile vocal cords forevaluation, biopsy and laser surgery.

During this evaluation a lower limit of95% SpO2 is taken as a warning as patients start rapidly desaturating belowthat. Reoxygenation or mask ventilation is done once the patient SpO2 dropsbelow 90%on pulse oximetry. The purpose of using ORI in these cases in additionto a pulse oximeter is to reliably predict the impending hypoxia once the ORIstarts dropping before the patient starts desaturating on the pulse oximeter.

HYPOTHESIS AND AIMS-

We hypothesise that ORI may help us inpredicting impending desaturation on pulse oximetry and hypoxia during patientsundergoing procedures under apnoeic oxygenation.

The aim of our study is to estimate theaccuracy of ORI in predicting impending desaturation and hypoxia duringpatients undergoing procedures under apnoeic oxygenation.

STUDY DESIGN AND METHODOLOGY-

Study design- Prospective observationalstudy

Site- TMH

Population:

Inclusion Criteria:

1.      Adults18 - 80 years’ old

2.      AmericanSociety of Anesthesiologists (ASA) physical status classification system:1 or 2

3.      Anypatient undergoing any procedure under general anaesthesia with apnoea andapnoeic oxygenation. e.g. direct laryngoscopic/ microlaryngoscopic evaluationor laser surgery for head and neck cancer, tracheal dilatation or stenting doneunder apnoeic oxygenation.

Exclusion Criteria:

1.      Anypatients with respiratory ailments (e.g. chronic obstructive pulmonary disease,asthma) or poor pulmonary function- Objective parameter to identify pooreffort tolerance is ability to climb less than 1 flight of stairs.

2.      Patientswith baseline saturation less than 95% on room air.

3.      Expecteddifficulty in tracheal intubation due to insufficient mouth opening and trismuswhere an awake fibreoptic intubation is planned.

4.      Patientswith stridor or an obstructed airway.

5.      Patientswith nail polish or Henna on fingers and nails- in whom the pulse oximeterreadings may not be accurate.

6.      Noinformed consent

7.      Inabilityto wear the sensor due to deformity or hypoperfusion of the fingers

 This study will be initiated afterapproval from the institutional ethics committee and registration with CTRI.Adult patients between 18-80 years of age posted for elective surgery otherthan those meeting exclusion criteria and requiring general anaesthesia withapnoea will be included in this prospective observational study. Consent forthe 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-invasiveblood pressure and ORI probe from MasimoTM monitor. After doingthe surgical safety checklist, IV access will be secured. The pulse oximeterprobe and the ORI monitor probe of the MasimoTM monitor will beput on separate fingers on the same hand. The non-invasive blood pressure cuffwill be attached on the opposite arm so that the measurements do not lead toloss of the pulse oximeter signal and plethysmograph. The patient will bepreoxygenated with 100% oxygen for 3 minutes. The corresponding SpO2 and ORIvalues will be noted. Induction of general anaesthesia will be done as per thediscretion of the attending anaesthesia consultant. After the patient isapnoeic following administration of a muscle relaxant apnoeic oxygenation withnasal prongs and 15 litres of oxygen will be started. The corresponding time ofapnoea will be noted. The surgeon will be allowed to proceed with the procedurewhich may include direct laryngoscopic or microlaryngoscopic mapping of thedisease or laser surgery. The corresponding SpO2 and ORI values will berecorded every 5 seconds for 3 minutes. In case the ORI and SpO2 does not dropin 3 minutes the time at which ORI drops to 0.24 and Spo2 drops to 95% will benoted from the time of apnoea. The time interval between the ORI alarm and thefall in SpO2 to 95% will also be recorded. Oxygenation with anaesthesiabreathing circuit will be resumed once the patient starts desaturating to lessthan 95%. The time taken for the ORI to rise back to 0.24 and SpO2 to rise to100% after reoxygenation will also be noted in seconds from the time ofresuming of ventilation of the patient.

The current standard of care forpatients undergoing procedures with apnoeic oxygenation technique is recognitionof desaturation using a pulse oximeter. When the SpO2 falls to 95% the surgeonis informed the need to reoxygenate the patient. If the procedure is likely totake more time the reoxygenation is started once the SpO2 drops below 90%.

The additional intervention in ourstudy is only recording ORI values using an ORI probe. Since ORI is still beingevaluated, ORI readings will not be used to decide patient management. Themanagement of these patients will continue to be done based on oxygen saturationwhich is the current standard of care.

Objectives of the study-

Primary objective:-

To find out the time lag between theORI alarm at 0.24 and Sp02 to drop to 95%.

Secondary Objective-

Time taken, for the ORI to riseup to 0.24 and the SpO2 to rise to 100% after resuming ventilation and the timedifference between the two.

Sample size and statistical analysis-

Sample size justification-

The primary objective is to calculate mean timebetween ORI 0.24 and SaO2 95%. Assuming that this is average 35 seconds(3) and5 sec standard deviation, a sample size of 25 is required to produce atwo-sided 95% confidence interval with a distance from the mean to the limitswhich is equal to 2 sec.

The primary objective is tocalculate mean time difference between ORI decreasing to 0.24 and SaO2decreasing to 95%. Assuming a 5 sec standard deviation, a sample size of 28 isrequired to find the true mean difference with a margin of error of 2 secondswith 95% confidence. This means that if we study 28 patients and find that themean time difference between ORI 0.24 and SaO2 of 95% is x seconds, we can be95% sure that in the population, the mean time difference between ORI 0.24 andSaO2 of 95% will lie between x-2 and x+2 seconds.

Since the duration ofsurgical procedures and physiological status of patients is variable, somepatients may not desaturate to 95% during the procedure (procedure may becompleted before the end-point is reached) - such patients will not beanalyzable for the primary outcome. Therefore, we plan to accrue 50 patients tohave analyzable data in 28 patients.

Since this is a thesis project, we willanalyse whatever cases we accrue at the end of Nov 2021. The study will howevercontinue till we complete accrual of 28 analyzable patients.

Statistical Analysis Plan: -

Data will be descriptively analysedusing mean and standard deviation for continuous variables and frequency andpercentage for categorical variables.

The time that elapses betweenactivation of the oxygen Reserve alarm until saturation reached 95% withoutventilation, that is, the warning time the index provides of impendingdesaturation will be calculated as the mean time in seconds and the standarddeviation will be reported depending on the normality of the data. We willreport 95% confidence intervals for this estimate

We will also report the means of thetime (with 95% CI) for ORI to recover to 0.24, for SaO2 to recover to 100% andthe difference between the two.