The Clinical Significance of Isocapnic Buffering Phase During Exercise Testing: An Overview

Abstract

DescriptionDuring an incremental cardiopulmonary exercise test (CPET), lactic acid begins to accumulate after anaerobic threshold (AT) [1]. Circulating bicarbonate compensates for the lactic acidosis along with increased hyperpnea [2]. Beyond a certain point reaching higher exercise intensity, lactic acid production can no longer be compensated by circulating bicarbonate and thus hyperventilation begins. This point is called the respiratory compensation point (RCP) [3]. The period from AT to RCP is known as isocapnic buffering (IB) phase [2]. In this article, we will review current concepts about the clinical significance of the IB phase.Most studies regarding the IB phase put emphasis on athletes and healthy individuals. Oshima, Y et al. reported a positive and significant correlation between the duration of IB phase and maximal oxygen consumption (VO 2 max) in young athletes [4]. A correlation between the increase in VO 2 max and the increase in IB phase after 6 months of training has also been reported [5]. Mauro Lenti et al. further demonstrated that the duration of IB phase reduces with aging and is higher in trained individuals with better endurance independent of age in cyclists [6]. Some studies, however, showed different results. Chicharro et al. [7] defined IB as the range of VO 2 and power output from AT to RCP. They found no significant increase in the range of IB throughout the course of a training season in professional cyclists. Another study [8] revealed that in male endurance athletes, short (20-min) but not a longer (90-min) cycling time trial performance had correlation (r=0.58, p<0.05) with the range of IB, whereas the correlation was weak. They suggested that IB is not representative of endurance performance in time trial in endurance athletes. Nevertheless, according to the above findings, longer IB phase indicates better endurance performance, and after endurance training, IB phase is increased despite aging in athletes.The studies about IB phase in patients with heart diseases are scarce. Masaaki Tanehata et al. [9] reported that in chronic heart failure patients, the period of IB phase is closely related to the slope of VO 2 as a function of work rate (∆VO 2 /∆WR), but there is no correlation between the RCP-AT time and the anaerobic threshold. They suggested that the RCP-AT time is an indicator of aerobic metabolism after AT. However, it is still not clear whether the IB phase could indicate cardiopulmonary function, endurance training effects or prognosis in patients with cardiac diseases.Numerous studies have demonstrated a beneficial effect of exercise training in chronic heart failure (CHF) patients, which is revealed by increased peak O 2 consumption (VO 2 ) in CPET after exercise training [10][11][12][13][14], despite limited improvement in left ventricular ejection fraction (LVEF) [15]. On the other hand, ventilatory efficiency (VE/VCO 2 slope) [16,17], partial pressure of end tidal CO2 (PetCO 2 ) at AT [18][19][20] and peak VO 2 [21,22] all serve as significant prognostic facto...