Validation of the 4-second exercise test in the orthostatic position.

OBJECTIVE To test the operational viability of and validate the 4-second exercise test (4sET) protocol in the orthostatic position (ORTHO). METHODS The ORTHO protocol, an alternative to the conventional protocol (CYCLO), was used. The ORTHO protocol consists of performing sudden exercise in the orthostatic position -- accelerated stationary walking (alternate upward flexion of the thighs) -- from the fourth to the eighth second of a 12-second maximum inspiratory apnea, instead of rapid cycling without load. The adimensional cardiac vagal index (CVI) was calculated using the ratio between the longest RR interval (RRB) -- the one immediately before, or the first during exercise -- and the shortest RR interval during exercise -- usually the last (RRC) -- measured on electrocardiographic tracings at a 10-ms resolution. Forty-seven individuals (40+/-17 years, 169+/-9 cm, 72+/-14 kg) of both sexes, healthy or unhealthy, randomly underwent 3 consecutive repetitions of the 2 protocols, the first being performed only to acquaint patients with the procedure. RESULTS Although differences in the CVI were found in both protocols (1.48+/-0.04 vs 1.42+/-0.04; P<0.001), no physiological relevance was observed. In 5 (11%) cases, a clinically significant difference between the ORTHO and CYCLO protocols was observed for CVI. The results of RRB, RRC, and CVI in the 2 protocols were strongly correlated, being 0.84, 0.85, and 0.93, respectively (P<0.001). CONCLUSION The 4sET performed in the orthostatic position proved to be a valid option for assessing the vagal cardiac tonus in laboratories lacking a cycloergometer, without jeopardizing clinical interpretation. In addition to simplicity and applicability, the procedure also provides low operational costs.