In Response.

Abstract

We thank Hrishi and colleagues1 for their comments on our recently published study.2 They give us the opportunity to clarify some points related to the use of quantitative pupillometry in critically ill patients. Two pupillary reflexes can be measured using portable infrared pupillometry: pupillary light reflex (PLR) in response to light stimulus and pupil reflex dilation (PRD) in response to noxious stimulus. By providing information on the brainstem integrity through the second and third cranial nerves, there is a growing interest in using quantitative PLR to estimate prognosis after cardiac arrest and traumatic brain injury. Contrary to the widely held opinion, PLR amplitude is relatively resistant to opioids, propofol, volatile agents, and neuromuscular blocking agents.3 We did not use PLR to assess nociception in our study. On the other way, PRD is an evoked reflex mediated through the ophthalmic nerve to detect painful reactions in nonverbal patients. It is largely admitted that opioids depress PRD in a dose-related fashion. Because γ-aminobutyric acid type A (GABAa) receptors might be involved in generating PRD,3 GABAa agonists such as propofol and midazolam could interfere with PRD measurements. Given that critically ill patients usually receive both opioids and sedatives, monitoring analgesia using PRD implies necessarily considering the level of sedation. In our study, all patients were kept sedated at Richmond Agitation Sedation Scale (RASS) −5 to −4, as shown in Table 2. The choice of noxious stimulus is critical to appropriately assess nociception in nonverbal patients. Laryngoscopy, tracheal intubation, and surgical incision are stimuli of very different uncontrolled noxious intensities. Tetanic stimulation is a more predictable source of painful stimulus than these procedures. Nursing, as used in the referenced poster presentation,4 is a multifaceted procedure that cannot be standardized to assess nociception. Nursing has been never used to validate any pain instrument. The pupillary pain index (PPI) is an elegant approach to estimate nociception while avoiding unnecessary pain and poor tolerance with the application of predefined intensity of electric stimulation. The maximal 13% dilation threshold from baseline was chosen to prevent systemic reactions such as tachycardia and arterial hypertension during tetanic stimulus. Accordingly, we found no changes in these systemic parameters or in Behavioral Pain Scale (BPS) scores during tetanic stimulation. Assessing nociception in brain-injured patients implies the integrity of PRD reflex pathway. Otherwise, PRD measurements can be used as an indicator of peripheral nerve block efficacy or to detect hemispheric brain lesion in a sedated patient. In the present study, there was no evidence of focal neurological lesion according to the computed tomography (CT) scan and PLR measurements. In the rare case of asymmetric PRD response, the highest PPI was taken for the analysis to detect patients with true nociception during endotracheal suctioning. Marc Vinclair, MDClotilde Schilte, MDGilles Francony, MDJean-Francois Payen, MD, PhDDepartment of Anesthesia and Critical CareGrenoble Alpes University HospitalGrenoble, France[email protected]