A minute ventilation sensing rate responsive pacemaker was implanted in 11 patients with hradycardias. Their mean age was 59 ± 4 years (mean ± SEM). The pacemaker measures minute ventilation by sensing intravascular impedance using a standard bipolar electrode. The rate responsive programming was simple: apart from ascribing an upper and lower rate, the only programmable parameter was the slope of rate response. This could be derived approximately by assessing the suggested slope value during an exercise test in the ‘adaptive VVI’ mode. Compared with exercise in the VVI mode, symptom limited treadmill tests in the rate responsive mode showed a 33% improvement of exercise capacity and a 44% improvement of cardiac output as determined noninvasively by continuous wave Doppler measurements of the ascending aortic blood flow. The pacing rate was appropriately increased during a variety of daily activities such as walking at different speeds and gradients, and ascending and descending stairs. Voluntary interference of the respiratory pattern such as during coughing and hyperventilation increased the pacing rate from a resting rate of 70 bpm to 111 ± 10 and 86 ± 4 bpm respectively. Continuous talking during exercise attenuated the expected rate response. The pacemaker can sense activity induced by arm swinging.
In conclusion, the Meta pacemaker improved cardiac output and exercise capacity in patients with bradycardias. Its rate response was related to workload. Although voluntary interference affected the pacing rate, excessive rate acceleration was not encountered.