Effects of vagus nerve stimulation on respiration during sleep

To the Editor: Malow et al. recently reported that vagus nerve stimulation (VNS) worsened sleep apnea in four patients with epilepsy.1 Because VNS can induce tachypnea,2 we reviewed polysomnograms of six patients with epilepsy treated with VNS. Patients were referred for evaluation of sleepiness, witnessed apneas, or insomnia. Polysomnograms were reviewed with IRB approval. Most patients had abnormal EEG and sleep architecture, and all had moderate sleep disordered breathing. Two of our patients had frank apneas during VNS (30 and 25 Hz stimulation frequency, 3 mA current). This was not evident in four others (two at 25 Hz, two at 20 Hz). The two patients with VNS-induced apneas did not have more severe VNS-independent sleep apnea. Although Malow et al. suggested that VNS-induced airway narrowing or altered sleep architecture might cause these respiratory events, we doubt these are the main reasons, because VNS-triggered hypopneas were accompanied by immediate tachypnea, without any apparent change in behavioral state. We measured respiratory rate and heart rate in 30-second …