RESPIRATORY-RELATED HEART-RATE-VARIABILITY PERSISTS DURING CENTRAL APNEA IN DOGS - MECHANISMS AND IMPLICATIONS

The aim of this study was to determine the mechanism(s) responsible fo r the persistence of respiratory sinus arrhythmia (RSA) during central apnea. In five awake dogs, heart rate (HR) was recorded during consta nt mechanical ventilation (MV) and during central apneas produced by c essation of MV. For each of 10 control ventilator cycles before MV was stopped, instantaneous HR was plotted against the time from the onset of lung inflation; the fundamental and first harmonic of a sine wave (at the ventilator frequency) was then fitted to the HR data. For the control cycles, the mean r(2) from the curve fits was 0.57 +/- 0.07, s howing that a significant component of the HR variability was linked t o the ventilator cycle. After MV was stopped, RSA persisted and only b y the third ''phantom'' ventilator cycle during apnea had the degree o f fit consistently decreased compared with control dogs (P < 0.02). Th e persistence of ventilator-linked RSA at the onset of central apnea s upports the concept of a ''memory'' in the respiratory system. Toward the end of central apnea, HR variability reappeared and had the period icity and rhythmic pro file of RSA on 81% of occasions. The presence o f RSA-like activity toward the end of central apnea suggests that subt hreshold rhythmic respiratory-related activity may be present even bef ore the onset of detectable lung volume changes.