It was a prospective study of diagnostic tests, that assessed the diagnostic capability of pupillometry, to discriminate pain by comparing the behaviour of the PDR with the BPS scale as a reference method. A study protocol was prepared, and the data from the study are available in a private repository of Osakidetza Basque Health System.
Patients older than 18 years who were admitted to the ICU of the Araba University Hospital, analgosedated and on mechanical ventilation, unable to communicate verbally and had a baseline BPS score of three and a RASS score between − 1 and − 4 were included. Those who presented limitations in the behavioural expression of pain (e.g., due to treatment with muscle relaxants, neuromuscular diseases with motor impairment or severe polyneuropathy) and those with ophthalmic pathologies with pupillary involvement, involvement of the third cranial nerve, Glasgow scores < 6, intracranial hypertension and the use of atropine, clonidine, dexmedetomidine, tramadol, ketamine, adrenaline, calcium antagonists and antiemetics were excluded.
Patients who met the eligibility criteria were selected consecutively during the first week of admission to the unit. Consent for the participation of relatives and/or legal representatives were obtained. A sufficient number of subjects were included until at least 50 measurements with pain response according to BPS were reached.
A single protocol was performed with each patient, during which pre-stimulus baseline measurements were taken and the maximum post-stimulus variations were recorded. The patient’s responses were recorded at different pain levels: non-painful (NP) stimulus, calibrated electrical stimulations with increasing intensities of 10 mA, 20 mA, 30 mA and 40 mA and endotracheal aspiration (ETA). As NP stimulus, we slided a gauze pad over an uninjured area of the patient's forearm. ETA is a commonly performed procedure in the ICU that is considered among the most painful daily interventions that a patient undergoes [1–3]. Prior to the protocol, the patients remained at rest and did not undergo potentially painful interventions for at least 1 hour. Alarms were silenced to avoid possible interference with the pupillary response. An interval of 5 minutes was maintained between stimulations until the initial pupil size was recovered. The analgesia and sedation regimen remained constant throughout the protocol. The patients showed a baseline pain level on the BPS equal to 3.
Pupillary measurements were performed with a Neurolight AlgiScan® portable video pupillometer (ID Company, Marseilles, France). The pupillometer has a video camera with infrared light, which allows measurements to be performed in the dark without interfering with the pupillary response and in response to both light and calibrated electrical stimuli. It has a measurement range between 0.1 and 10 mm (pupil size), with an accuracy of 0.1 mm and a resolution of 0.01 mm, maintaining an image acquisition and measurement frequency of 62 Hz. It measures the basal pupil diameter prior to stimulation and the maximum pupil diameter, and it calculates the percentage of pupil size variation with the formula (Var. = (Max-Min.)/Min.*100). The assessment of the variation in pupil size during NP and ETA stimulation was performed in DPR mode and was maintained for 20 seconds. Calibrated stimulations were delivered through an electrode placed on the ulnar nerve, on the inside of the wrist, and on clean skin free of any lesion. Pupil size was measured with the pupillometer in Tetanus mode at an optimal impedance level identified by the pupillometer. The measurements started 2–3 seconds after the placement of the pupillometer on the eye to allow the pupil to adapt to the dark environment generated by the silicone eye protector.
Pain was assessed using the validated BPS [17, 35–37]. Scores equal to or greater than 4 points were considered indicative of pain.
A team of two researchers took the measurements simultaneously and independently during the procedure: a researcher in charge of the PDR measurements and the researcher who collected the BPS. The nurse in charge of the patient performed the NP and the ETA (Fig. 1).
The sample size was calculated following the method proposed by Flahault et al. . With a predicted test sensitivity of 0.95 and a (95%) confidence limit not lower than 0.80, the need for approximately 50 cases with pain was identified.
A descriptive study of the baseline clinical and demographic characteristics of the participants was performed using means and deviations, medians and interquartile ranges for continuous variables, and numbers and percentages for qualitative variables. A study of diagnostic tests using PDR versus BPS as a reference test was performed. The receiver operating curve (ROC) was plotted, and the area under the curve (AUC) was calculated. We identified the cut-off points showing the highest sensitivity and specificity. Diagnostic performance was studied based on the Youden index, negative predictive value (NPV), positive predictive value (PPV), accuracy, positive likelihood ratio (PLR), and negative likelihood ratio (NPC) of each of them. They are presented with their 95% confidence intervals (CI). Finally, the reliability of the pupillometry was assessed by calculating the degree of agreement between the tools and the kappa index.
All complete data from both diagnostic tests (PDR and BPS) were analysed.
Statistical calculation was performed with the statistical programme IBM SPSS Statistics version 23.0. The level of significance was set at p < 0.05.