ORIGINAL ARTICLE Reizo Baba á Kazuo Tsuyuki á Yasuo Kimura Kenji Ninomiya á Masahiko Aihara á Kunio Ebine Nobuo Tauchi á Kenji Nishibata á Masami Nagashima Oxygen uptake ef®ciency slope as a useful measure of cardiorespiratory functional reserve in adult cardiac patients Accepted: 7 September 1998 Abstract In this study we aimed to elucidate the validity and usefulness of the oxygen uptake eciency slope (OUES) in the evaluation of adult cardiac patients. Cardiopulmonary exercise tests were performed on a treadmill by 50 adult patients with chronic heart failure. The OUES was calculated from data for the ®rst 75%, 90%, and 100% of exercise duration. The OUES is derived from the following equation: _ V O 2  a log _ V E  b, where _ V O 2 is oxygen uptake (ml/kg/min), _ V E is minute ventilation (l/kg/min), and the constant ``a'' represents OUES. We also determined the ventilatory anaerobic threshold (VAT). The correlation coecient of the logarithmic curve-®tting model was [mean (SD)] 0.986 (0.009). The OUES could be used to discriminate eectively between New York Heart Association func- tional classes (P < 0.001). OUES and maximum _ V O 2 were signi®cantly correlated (r  0:78, P < 0.01). Agreement between the OUES values for the ®rst 90%, 75%, and 100% of the exercise was excellent (intraclass correlation coecient = 0.99). Our results suggest that OUES is applicable to adult cardiac patients as an objective, eort-independent estimation of cardiorespi- ratory functional reserve. Key words Oxygen uptake eciency slope á Maximal oxygen uptake á Anaerobic threshold á Exercise testing Introduction Maximal oxygen uptake ( _ V O 2 max ) is considered to be the most reliable index of cardiorespiratory functional reserve of a patient with chronic heart failure (Taylor et al. 1955; Jones 1988) because it is an indirect estimate of maximal cardiac output (Franciosa et al. 1984; Fletcher et al. 1995). This index is de®ned as the point at which oxygen uptake ( _ V O 2 ) reaches a plateau despite further increases in the work rate. However, a true plateau in _ V O 2 is rarely observed in standard incre- mental exercise testing (Myers et al. 1989, 1990; Row- land and Cunningham 1992). Moreover, it is well known that bicycle exercise induces a smaller value for _ V O 2 max (Buller and Poole-Wilson 1990). Therefore, _ V O 2 max may not be the best clinical index of cardiorespiratory func- tional reserve. Peak _ V O 2 , measured at the end of a test, is widely used as a substitute. However, the end point of an exercise test is greatly in¯uenced by motivation on the part of the patients and the testing personnel. The ven- tilatory anaerobic threshold (VAT; Weber et al. 1982; Wasserman 1984; Ito et al. 1989; Ohuchi et al. 1996), a measurement that does not need maximal exercise test- ing, has been used to assess patients with heart failure. However, the usefulness of this variable has not gener- ally been accepted. The oxygen uptake eciency slope (OUES; Fig. 1) has been proposed as a measure of the cardiorespiratory functional reserve of pediatric patients with congenital heart disease (Baba et al. 1996). Since the OUES is derived from the slope of the relationship between _ V O 2 Eur J Appl Physiol (1999) 80: 397±401 Ó Springer-Verlag 1999 R. Baba Research Institute of Sports Medical Science Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-12, Japan K. Tsuyuki á Y. Kimura á K. Ninomiya á M. Aihara Department of Cardiology and Department of Cardiovascular Surgery, Odawara Cardiovascular Hospital, 296-1 Yahagi, Odawara, 250, Japan K. Ebine Department of Cardiovascular Surgery, Toho University School of Medicine, 2-17-6 Ohashi, Meguro-ku, Tokyo, 153, Japan N. Tauchi Department of Pediatric Cardiology, Ohgaki Municipal Hospital, 4-86 Minaminokawa-cho, Ohgaki, 503, Japan K. Nishibata á M. Nagashima Department of Pediatrics, Nagoya University School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya, 466, Japan R. Baba (&) Tokai University School of Physical Education, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-12, Japan e-mail: babar@keyaki.cc.u-tokai.ac.jp Tel.: +81-463-581211, Fax: +81-463-50205