RESUMO
Among the different sensors available to date for rate responsive pacing, the minute ventilation sensor is claimed to be the most physiological. This study aims to verify the reliability, physiological sensitivity and rate modulating ability of minute volume as a sensor for rate responsive pacing, together with the utility of cardiopulmonary stress test [CPX] to optimize pacing rate. In addition, comparison between the cardiac performance during exercise in fixed-rate VVI versus that in rate responsive modes of pacing was studied. We implanted the META MV in 5 patients [3 males and 2 females, mean age 36. 4 +/- 1.6 yrs, NYHA class 1] affected by CHB. These patients were followed for 3-12 months. Treadmill symptoms-limited exercise test was performed for each patient in both adaptive and rate responsive modes using Bruce protocol. Cardiopulmonary parameters were measured on 1-minute interval. During rate responsive mode exercise capacity improved significantly in comparison to UVI adaptive mode: anaerobic threshold [AT] increased from 1.15 +/- 0.13 to 1.34 +/- 0.16 1/min [16.7%; P 0.005]; time of AT increased from 7 +/- 1.9 to 9.8 + 2.14 minutes [40%; P 0.0025]; VO2 max increased from 1.15 +/- 0.17 to 1.7 +/- 0.18 1/min [12.5%; P]. Sensitivity of the sensor determined from the correlation between VO2 and pacing rate was very strong during the maximal incremental Bruce test [mean r value 0.9650 +/- 0.02]. This correlation was also maintained during our laboratory, brief, low and discontinously graded test [mean r value: 0.9338 +/- 0.01]; however, the response was weaker when compared with that of 15 normal controls. In order to identify the optimal pacing rate which is the lowest rate able to attain the best cardiac performance with preservation of liner correlation between VO2 and heart rate - we repeated for each patient the CPX several times with different slope rates. In 3 patients the optimal pacing rate was 1 or 2 slopes less than the suggested one. In one patient, the slope needed to be reduced after 3 months of implantation; this was due 10 improvement of physical fitness. Follow-up confirmed a correct pacing status. Rate responsive pacing improves cardiac performance belter than fixed-rate VVI pacing. The MV sensor is reliable and highly sensitive, but has poor rate modulating ability. The optimal rate response is not always that suggested by Adaptive mode; moreover, it may change with time. The CPX is mandatory to program and properly follow-up the patients