Previous studies have shown a high correlation between transthoracic impedance minute ventilation (IMV) determined by a pacemaker sensor and actual minute ventilation (VE) measured by standard methods. We hypothesized that several factors (e.g., posture, breathing pattern, and exercise type) could potentially affect the calibration between IMV and VE. In patients with Medtronic Kappa 400 pacemakers, VE (L/min) was monitored using a standard cardiopulmonary metabolic gas analysis system with simultaneous recording of IMV (ohms/min) using DR-180 extended telemetry monitors. Effects of posture and of breathing pattern at rest (19 patients; age 60 +/- 13 years) were evaluated by monitoring each patient under three conditions: (a) slow breathing, supine, (b) slow breathing, sitting, and (c) shallow breathing, supine. Calibration at rest was defined as the ratio of IMV to VE. Effect of type of exercise on calibration compared treadmill versus graded bicycle ergometer exercise (18 patients; age 62 +/- 14 years). Calibration during exercise was defined as: (a) "Begin" (the IMV to VE ratio at VE = 10 L/min, the typical VE value at beginning of exercise), and (b) slope of the IMV/VE regression line. Calibration of IMV/VE was significantly smaller for sitting versus supine position (0.7130.177, P < 0.001) and for shallow versus slow breathing (0.7210.373, P < 0.001), and larger for treadmill versus bicycle exercise (Begin: 1.240.43, P = 0.018; Slope: 1.260.42, P = 0.013). In conclusion, posture, breathing pattern, and type of exercise affect the IMV estimation of the actual VE, possibly by altering the static or dynamic geometry (thus, the impedance) of the intrathoracic viscera.