## 5 Is the COP Concept Comparable to That of the Oxygen-Uptake-Efficiency Slope?  
The concept of the COP is not similar to that of an OUES. These are distinct concepts in the field of cardiopulmonary physiology. On the one hand, the COP refers to the ideal balance between the cardiovascular and pulmonary systems during exercise, representing the optimal level of oxygen uptake, ventilation, and other physiological variables that maximize exercise efficiency.  
In contrast, the OUES is a measurement of functional cardiopulmonary reserve derived from the submaximal values of oxygen uptake obtained during CPX [37; 38]. It involves plotting oxygen uptake (\(V^{\prime}\)O\({}_{2}\)) against the logarithm of minute ventilation (\(V^{\prime}\)E) at submaximal work rates and calculating the slope of the line using the following equation:  
\[[V^{\prime}\mathrm{O}_{2}=\mathrm{a}\times\log V^{\prime}\mathrm{E}+\mathrm{b}]\]  
where \"a\" denotes the OUES value. A higher \"a\" value indicates a steeper slope and superior oxygen-uptake efficiency during exercise [39]. The OUES value represents the steepness of the slope, which reflects oxygen-uptake efficiency during exercise.  
A general interpretation is that a higher OUES is associated with greater efficiency in oxygen uptake during exercise. This means that an individual can achieve a higher level of \(V^{\prime}\)O\({}_{2}\) relative to ventilation, indicating better cardiopulmonary performance. Furthermore, the OUES has been used as an indicator of cardiopulmonary function in diverse popula  
Figure 4: The figure presents ROC (receiver operating characteristic) curves comparing the prognostic capacity of different variables obtained from cardiopulmonary tests. The area under the curve (AUC) indicates the predictive accuracy of each parameter. The OUES (oxygen-uptake-efficiency slope) demonstrated the highest accuracy (AUC = 0.976), followed by PImax (maximum inspiratory pressure) with an AUC of 0.769. In contrast, COP (cardiorespiratory optimal point) exhibited lower performance (AUC = 0.371), indicating reduced sensitivity and specificity in predicting adverse clinical outcomes. The reference line represents a random prediction model (AUC = 0.5). This comparison highlights the potential of an OUES as a robust marker for clinical decision-making in HF patients.  
tions [40; 41; 42], such as patients with HF [38; 43; 44] and coronary artery disease [45; 46]. A lower OUES value has been associated with worse clinical outcomes and prognoses for patients with these conditions [47; 48], whereas a higher OUES value is associated with a better prognosis and response to therapeutic interventions.  
However, it is important to note that the specific interpretation of the OUES may vary depending on the clinical context, study population, and other variables assessed. In this context, it is crucial to comprehensively consider the clinical information and results of other cardiopulmonary function tests in conjunction with the OUES to understand an individual's functional capacity. We evaluated 256 patients with HF and categorized them into two groups: COP < 36 (n = 105) and COP \(\geq\) 36 (n = 151). We assessed the OUES (2775 \(\pm\) 574 vs. 1760 \(\pm\) 664 and \(p\) < 0.001), \(V\)'O\({}_{2}\) (19.70 \(\pm\) 5.6 vs. 16.93 \(\pm\) 6.0 mL/kg.min\({}^{-1}\) and \(p\) < 0.001), and PImax (68 \(\pm\) 16 vs. 53 \(\pm\) 11 cmH\({}_{2}\)O and \(p\) < 0.001) [36]. These data suggest that low OUES values result in higher COP values, further supporting the notion that the COP may have a significant clinical utility as an independent prognostic marker.