## 1 Introduction  
Cardiopulmonary exercise testing (CPET) has gained recognition as a valuable tool for assessing functional capacity based on various factors and on athletic performance. Various studies have shown that the utilization of CPET underscores the importance of identifying non-invasive prognostic markers to enhance the detection of cardiovascular risk (CVR) [1]. Previous studies have specifically highlighted the significance of key markers, including maximal oxygen consumption (\(V^{\prime}\)O\({}_{\text{2 max}}\)) [2; 3; 4], the \(V^{\prime}\)E-\(V^{\prime}\)CO\({}_{\text{2}}\) slope ratio (ventilatory efficiency index) [5; 6], periodic breathing [7; 8], \(\Delta V\)O\({}_{\text{2}}\)/\(\Delta\)WR (work rate) [9], kinetics of oxygen consumption [10; 11], circulatory power [12; 13], and oxygen-uptake-efficiency slope (OUES) [14; 15]. These markers provided valuable insights into patients' physiological status and overall cardiovascular health (Table 1).  
Recent developments in this field have included the introduction of a novel marker known as _the cardiorespiratory optimal point_ (COP) (first described by [PERSON]) [16], which can be easily identified during submaximal tests (without the need to predispose patients to strenuous physical activity) [17]. The COP represents the exercise intensity that maximizes cardiorespiratory efficiency and is characterized by the lowest value of the ventilatory equivalent of oxygen (\(V^{\prime}\)E/\(V^{\prime}\)O\({}_{\text{2}}\)) within a specific minute before reaching the first ventilatory threshold [17; 18]. This marker is interpreted as the minimum number of liters of air required to extract one liter of oxygen through the bloodstream, thereby reflecting the efficiency of the respiratory response to oxygen consumption during exercise [17; 18; 19; 20].  
A lower COP implies better cardiovascular conditioning, and the subject exerts less effort to transport the same amount of oxygen, demonstrating a more efficient interaction between the heart and the lungs [17; 19; 20]. It is important to note that further research and scientific inquiry are necessary to better understand the implications and potential applications of the COP in CPET.  
The COP is influenced by factors, such as fitness level, sex, and age, that are important in clinical practice. For instance, research has shown variations in the COP between men and women, and it may also vary across age brackets [17; 18]. Understanding and identifying this marker is crucial because of its potential prognostic value [21]. Nevertheless, we acknowledge the existence of several other non-invasive prognostic markers, some of which have already been consolidated in the literature due to their predictive capacity for mortality, such as \(V^{\prime}\)O\({}_{\text{2}}\), \(V^{\prime}\)E-\(V^{\prime}\)CO\({}_{\text{2}}\) slope, OUES, and others. Recently, [PERSON] et al. [22] demonstrated that the COP proved to be an excellent predictor of adverse outcomes in patients with heart failure (HF), similar to the relative peak of \(V\)O\({}_{\text{2 peak}}\) and superior to other classic maximal, submaximal, and multicomponent variables, and can be used as an alternative to \(V^{\prime}\)O\({}_{\text{2 peak}}\).  
Despite these findings, the extent to which the COP may be superior to these markers remains unknown. Additionally, there is a gap in knowledge regarding the behavior of the COP in patients with inspiratory muscle weakness, such as patients with HF. Therefore, our objective was to review studies with the highest quality of evidence regarding these topics and that evaluated and/or compared the COP with other variables acquired during CPET. Furthermore, we explore this new gap in the population with HF, in order to encourage new research on this relevant marker.