The laryngeal chemoreflex: An evaluation of the normoxic response

Objectives: The laryngeal chemoreflex is a reflexive central apnea, bradycardia, and cardiovascular collapse that occurs in young, maturing mammals in response to exposure of the laryngeal mucosa to acidic and/or organic stimuli. The severity of the laryngeal chemoreflex varies within a species from one animal to another, and in some animals, the response can be fatal. This study seeks to identify those factors that contribute to fatal laryngeal chemoreflex responses when the larynx is stimulated under normoxic conditions, and to define how the normoxic response differs from the hypoxic laryngeal chemoreflex response. Study Design: This is a prospective study evaluating the response to laryngeal stimulation of 80 newborn piglets. Methods: The laryngeal chemoreflex response was elicited from 67 piglets under normoxic conditions. The data were collected from a combination of three separate experimental protocols, each of which included inducing the laryngeal chemoreflex under normoxic conditions as the first step. The physiologic response was recorded with a combination of arterial blood gas, pulse oximetry, blood pressure, and continuous cardiac monitoring. Results: Resumption of respirations occurred when the pCO₂ rose by a mean of 8.34 (Standard Deviation [SD]=4.8) mmHg, regardless of response severity (ANOVA, P >.05). Moderate (requiring supplemental O₂ for recovery) and profound (fatal) responders had a significantly higher prestimulation pCO₂ (95% confidence interval [CI] 39.8-44.8 mmHg and 40.5-46.4 mmHg, respectively) than did mild (recovery without assistance) responders (95% CI 36.8-40.8 mmHg, ANOVA, P <.05). Baseline pH was statistically significantly different as a function of response severity (95% CI profound: 7.29-7.37, moderate: 7.33-7.38, and mild 7.36-7.39, P <.05). Conclusions: Accumulation of arterial CO₂ is associated with resumption of respirations during the normoxic laryngeal chemoreflex. The combination of an elevated prestimulation pCO₂ and a low prestimulation pH predicts a profound laryngeal chemoreflex response under normoxic conditions.