Cardiorespiratory Optimal Point: A New Kid in the Block or an Established Star?

between oxygen transport (circulation: heart and arteries) and oxygen uptake (lungs and cellular respiration), during an incremental exercise test.

As written before, COP is a submaximal parameter, and it does not depend on the operator or the patient's performance.It is easy to determine by the operator; it is practically free of observer error, and it does not require the performance of a true maximal CPET.
In the first paper by Ramos et al. about this topic, 2 published in 2012, moderate inverse correlations were found with maximum oxygen consumption (VO 2 max) (r = −0.47;p < 0.001), oxygen consumption (VO 2 ) at the anaerobic threshold (r = −0.42;p < 0.001), and oxygen uptake efficiency slope (OUES) (r = −0.34;p < 0.001), which was confirmed by Charitonidis et al. 3 and Silva et al., 4 who also did not find a correlation between COP and VO 2 max in the assessment of 11 male (15.18 ± 0.75 years old) and 13 female (14.77 ± 0.44 years old) adolescent volleyball players or in 198 soccer players, respectively.The authors' scientific hypothesis was that COP, as it is related to cardiorespiratory fitness, obtained at a submaximal level of exercise, easy to identify, and almost free of interobserver variability, could become an alternative parameter or obtain an added value to oxygen consumption at peak exercise (pVO 2 ), which is the gold DOI: https://doi.org/10.36660/ijcs.20230124standard for cardiorespiratory fitness and is inversely related to cardiovascular mortality in many populations, for example, apparently healthy subjects or in the setting of several diseases like heart failure (HF).
After these preliminary papers, several groups around the world started to assess the prognostic value of COP in different populations, some mixed (healthy subjects and patients with diseases) or involving specific groups, such as HF or congenital heart disease, with different endpoints, including cardiovascular death, all-cause death, or sudden cardiac death.Peterman et al. 5 found that COP was related to all-cause mortality in apparently healthy males but not in females, which was unexpected considering other previously published papers.Female patients seem to have naturally higher levels of COP than men.
Table 1 summarizes the published articles where the prognostic value of COP was studied according to the different endpoints mentioned above.
In all of these studies, it is clear that COP proved to be useful prognostic assessment in several middleaged populations, mostly male, for example, a mixed population of community-dwelling adults, including mainly people with unspecified chronic disease (n = 3331), 6 healthy adults (n = 2190; n = 2205; n = 3160), 5,7,8 HF (n = 442; n = 277) 9,10 and congenital heart disease (n = 30), 11 in terms of identifying subjects or patients with higher risk of cardiovascular death, 9 all-cause mortality, 5,6,8 sudden cardiac death, 7 or clinical severity. 11nsidering the seven published papers, only Reis 9 compared the relative value of COP regarding the most recognized CPET parameters, for example, pVO 2 , VO 2 max, VE/VCO 2 slope, or OUES, for prognostic assessment of the different populations.They found that, at the submaximal level, COP demonstrated to have a higher prognostic value than pVO 2 , VE/VCO 2 slope, and OUES, but at maximal exercise, it was overtaken by these variables showing an area under the curve (AUC) of 0.632, while pVO 2 and VE/VCO 2 slope showed an AUC of 0.749 and 0.750, respectively, for the primary combined endpoint (cardiac death or urgent heart transplant) during 12 months follow-up.These authors' work pointed out what eventually needs to be done to consecrate COP, a submaximal CPET parameter, as a major variable for prognostic assessment of several populations, including apparently healthy subjects and patients with several types of cardiovascular diseases, cardiovascular risk factors, coronary artery disease, different HF phenotypes, and other chronic diseases, such as COPD , in addition to frail individuals.
It will be necessary to identify the value of COP in more populations, with more robust samples, in patients who can perform a maximal CPET, to identify which is the net added value of COP versus pVO 2 , VE/VCO 2 slope, OUES, or other CPET parameters at maximal exercise level.In other words, is it enough to perform a submaximal test, or is it necessary to perform a maximal CPET in order to obtain the highest risk prognostic assessment in every clinical setting?Future research must include patients with ethnic diversity, more female patients, and patients from different socioeconomic and educational strata.
In terms of different cardiovascular entities, it is advisable to identify New York Heart Association and Canadian Cardiovascular Society functional classes, NT-proBNP or BNP level, left ventricular ejection, ventricle volumes, presence of valvular heart disease, HF, HF phenotype, clinical stability, and type of medication.
It also seems necessary to define better the cutoff values for risk stratification for COP in specific populations, in terms of age, sex, and type of disease.

Summary
COP researchers have already conducted very important research that must be expanded, including larger and more diverse and better defined populations, with patients around the world, different cardiovascular and non-cardiovascular diseases, tests using a treadmill (more experience with treadmill CPET is needed), and evaluation of the prognostic values of sub-maximal parameters, such as COP and eventually VO 2 at VT1, versus the maximal parameters, such as pVO 2 , VE/VCO 2 slope, and OUES.
A clear answer is needed: Is the information obtained from a submaximal CPET enough, or should it be considered only as an alternative when it is not possible to perform a maximal CPET or when peak VO 2 was not reached?
The new research providing a response to those questions will make a great difference concerning how COP is considered among the scientific community involved in research with cardiorespiratory fitness: the difference between an interesting promise, "a new kid in the block," or a fundamental parameter, "an established star."

Table 1 -Summary of the published papers correlating COP and cardiovascular mortality, all-cause death, or sudden cardiac death Published paper Study population Population type and size and age Main results Value of COP to predict mortality versus other CPET parameters
AUC: