Physiological sensitivity of respiratory-dependent cardiac pacing: four-year follow-up.

Clinical and physiological data on long-term follow-up of 143 patients with respiratory-dependent pacemakers (RDP3) are reported; 121 patients received ventricular (VVI-RD) and 22 patients atrial (AAI-RD) respiratory-dependent stimulation. Functional evaluation was based on the exercise testing (130 pts) with oxygen uptake VO2, ventilation, ECG and arterial pressure monitoring and the dynamic Holter electrocardiogram (95 pts). In each patient, the stimulation rate curve selected was that which produced the best work tolerance and moved the anaerobic threshold to the right. Respiratory levels were assessed by telemetry verifying proper sensing of tidal volume variations and absence of interference and artefacts. In patients with VVIR or AAIR stimulation, exercise tolerance, oxygen uptake and anaerobic threshold increased significantly in comparison with VVI or AAI pacing respectively. The physiological sensitivity of the stimulation system (i.e., a linear relationship of the pacing rate with metabolic requirements) was excellent (up to exhaustion) in 70%, very good (up to anaerobic threshold) in 20% and erratic (no relationship between pacing rate and VE/VO2) in 10% of patients. In dynamic electrocardiographic monitoring, the automatic pacing rate was always predominant during the night and during rest periods; the pacing rate increased properly with daily activity; myopotential inhibition (none longer than 3,500 ms) was observed in 38 patients, but without subjective complaints. The incidence of the RDP3 malfunction was less than 8%; it may have stemmed from the pacing system itself, or from other clinical conditions. Oversensing of impedance system pulses has not been recorded in the last 3 years. Partial respiration undersensing results from incorrect accessory lead position, pulmonary emphysema, marked obesity or other causes. Respiratory sensing becomes erratic at the anaerobic threshold point in such patients, but functions well at submaximum exercise levels. In patients with left ventricular failure, exercise tolerance was improved by setting a lower ratio between the pacing rate and respiration, which prevented the occurrence of excessive pacing rates.