The alveolar to arterial oxygen partial pressure difference is associated with pulmonary diffusing capacity in heart failure patients

The Current Role of Cardiopulmonary Exercise Test in the Diagnosis and Management of Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease with a poor prognosis if left untreated. Despite remarkable achievements in understanding disease pathophysiology, specific treatments, and therapeutic strategies, we are still far from a definitive cure for the disease, and numerous evidences have underlined the importance of early diagnosis and treatment to improve the prognosis. Cardiopulmonary exercise testing (CPET) is the gold standard for assessing functional capacity and evaluating the pathophysiological mechanisms underlying exercise limitation. As effort dyspnea is the earliest and one of the main clinical manifestations of PAH, CPET has been shown to provide valid support in early detection, differential diagnosis, and prognostic stratification of PAH patients, being a useful tool in both the first approach to patients and follow-up. The purpose of this review is to present the current applications of CPET in pulmonary hypertension and to propose possible future utilization to be further investigated.

Brisk walking can be a maximal effort in heart failure patients: a comparison of cardiopulmonary exercise and 6 min walking test cardiorespiratory data

Aims

Cardiopulmonary exercise test (CPET) and 6 min walking test (6MWT) are frequently used in heart failure (HF). CPET is a maximal exercise, whereas 6MWT is a self‐selected constant load test usually considered a submaximal, and therefore safer, exercise, but this has not been tested previously. The aim of this study was to compare the cardiorespiratory parameters collected during CPET and 6MWT in a large group of healthy subjects and patients with HF of different severity.

Methods and results

Subjects performed a standard maximal CPET and a 6MWT wearing a portable device allowing breath‐by‐breath measurement of cardiorespiratory parameters. HF patients were grouped according to their CPET peak oxygen uptake (peakV̇O₂). One hundred and fifty‐five subjects were enrolled, of whom 40 were healthy (59 ± 8 years; male 67%) and 115 were HF patients (69 ± 10 years; male 80%; left ventricular ejection fraction 34.6 ± 12.0%). CPET peakV̇O₂ was 13.5 ± 3.5 mL/kg/min in HF patients and 28.1 ± 7.4 mL/kg/min in healthy subjects (P < 0.001). 6MWT‐V̇O₂ was 98 ± 20% of the CPET peakV̇O₂ values in HF patients, while 72 ± 20% in healthy subjects (P < 0.001). 6MWT‐V̇O₂ was >110% of CPET peakV̇O₂ in 42% of more severe HF patients (peakV̇O₂ < 12 mL/kg/min). Similar results have been found for ventilation and heart rate. Of note, the slope of the relationship between V̇O₂ at 6MWT, reported as a percentage of CPET peakV̇O₂ vs. 6MWT V̇O₂ reported as the absolute value, progressively increased as exercise limitation did.

Conclusions

In conclusion, the last minute of 6MWT must be perceived as a maximal or even supramaximal exercise activity in patients with more severe HF. Our findings should influence the safety procedures needed for the 6MWT in HF.

Exercise Dynamic of Patients with Chronic Heart Failure and Reduced Ejection Fraction

PURPOSE OF REVIEW

Exercise causes various dynamic changes in all body parts either in healthy subject or in heart failure (HF) patients. The present review of current knowledge about HF patients with reduced ejection fraction focuses on dynamic changes along a "metabo-hemodynamic" perspective.

RECENT FINDINGS

Studies on the dynamic changes occurring during exercise span many years. Thanks to the availability of advanced methods, it is nowadays possible to properly characterize respiratory, hemodynamic, and muscular function adjustments and their mismatch with the pulmonary and systemic circulations. Exercise is a dynamic event that involves several body functions. In HF patients, it is important to know at what level the limitation takes place in order to better manage these patients and to optimize therapeutic strategies.

Evaluating the Benefits of Exercise Training in HFrEF or COPD Patients: ISO-LEVEL COMPARISON CAN ADD VALUABLE INFORMATION TO V˙o2peak

BACKGROUND

Heart failure with reduced ejection fraction (HFrEF) and chronic obstructive pulmonary disease (COPD) are relatively common conditions with similar symptoms of exercise intolerance and dyspnea. The aim of this study was to compare exercise capacity, ventilatory response, and breathing pattern in patient groups with either advanced HFrEF or COPD before and after exercise training.

METHODS

An observational study was conducted with parallel groups of 25 HFrEF and 25 COPD patients who took part in 6 wk of inpatient rehabilitation with exercise training. All patients underwent cardiopulmonary exercise tests at the start and end of the training, with resting arterial blood gas measurements.

RESULTS

The average peak oxygen uptake (V˙o2) was low at the start of the study but increased significantly after training in both groups, or by 2.2 ± 2.1 mL/kg/min in HFrEF patients and 1.2 ± 2.2 mL/kg/min in COPD patients. At ISO-V˙o2 (ie, same level of V˙o2 in pre- and post-exercise tests), carbon dioxide production (V˙co2) decreased after exercise training in both groups. Similarly, at ISO-V˙E (ie, same level of ventilation), breathing frequency (f) decreased and tidal volume (VT) increased, resulting in an improved breathing pattern (lower f/VT ratio) after training.

CONCLUSION

The findings of this study show that exercise training in severely affected patient groups with HFrEF or COPD led to an increase in maximal exercise capacity, a more favorable breathing pattern, and a diminished V˙co2 during exercise. Therefore, comparisons of V˙co2 and breathing pattern at ISO-levels of V˙o2 or V˙E before and after training are valuable and underutilized outcome measures in treatment studies.

The Effects of Intraoperative Inspired Oxygen Fraction on Postoperative Pulmonary Parameters in Patients with General Anesthesia: A Systemic Review and Meta-Analysis

High intraoperative inspired oxygen concentration is applied to prevent desaturation during induction and recovery of anesthesia. However, high oxygen concentration may lead to postoperative pulmonary complications. The purpose of this study is to compare the postoperative pulmonary parameters according to intraoperative inspired oxygen fraction in patients undergoing general anesthesia. We identified all randomized controlled trials investigating postoperative differences in arterial gas exchange according to intraoperative fraction of inspired oxygen (FiO₂). A total of 10 randomized controlled trials were included, and 787 patients were analyzed. Postoperative PaO₂ was lower in the high FiO₂ group compared with the low FiO₂ group (mean difference (MD) −4.97 mmHg, 95% CI −8.21 to −1.72, p = 0.003). Postoperative alveolar-arterial oxygen gradient (AaDO₂) was higher (MD 3.42 mmHg, 95% CI 0.95 to 5.89, p = 0.007) and the extent of atelectasis was more severe (MD 2.04%, 95% CI 0.14 to 3.94, p = 0.04) in high intraoperative FiO₂ group compared with low FiO₂ group. However, postoperative SpO₂ was comparable between the two groups. The results of this meta-analysis suggest that high inspired oxygen fraction during anesthesia may impair postoperative pulmonary parameters. Cautious approach in intraoperative inspired oxygen fraction is required for patients susceptible to postoperative pulmonary complications.

Relevance of Oxygen Concentration in Stem Cell Culture for Regenerative Medicine

The key hallmark of stem cells is their ability to self-renew while keeping a differentiation potential. Intrinsic and extrinsic cell factors may contribute to a decline in these stem cell properties, and this is of the most importance when culturing them. One of these factors is oxygen concentration, which has been closely linked to the maintenance of stemness. The widely used environmental 21% O₂ concentration represents a hyperoxic non-physiological condition, which can impair stem cell behaviour by many mechanisms. The goal of this review is to understand these mechanisms underlying the oxygen signalling pathways and their negatively-associated consequences. This may provide a rationale for culturing stem cells under physiological oxygen concentration for stem cell therapy success, in the field of tissue engineering and regenerative medicine.

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Alveolar Air and O2 Uptake During Exercise in Patients With Heart Failure

BACKGROUND

Peak exercise pulmonary oxygen uptake (V̇O₂) is a primary marker of prognosis in heart failure (HF). The pathophysiology of impaired peak V̇O₂ is unclear in patients. To what extent alveolar airway function affects V̇O₂ during cardiopulmonary exercise testing (CPET) has not been fully elucidated. This study aimed to describe how changes in alveolar ventilation (V̇_A), volume (V_A), and related parameters couple with exercise V̇O₂ in HF.

METHODS AND RESULTS

A total of 35 patients with HF (left ventricular ejection fraction 20 ± 6%, age 53 ± 7 y) participated in CPET with breath-to-breath measurements of ventilation and gas exchange. At rest, 20 W, and peak exercise, arterial CO₂ tension was measured via radial arterial catheterization and used in alveolar equations to derive V̇_A and V_A. Resting lung diffusion capacity for carbon monoxide (DL_CO) was assessed and indexed to V_A for each time point. Resting R² between V̇O₂ and V̇_A, V_A, DL_CO, and DL_CO/V_A was 0.68, 0.18, 0.20, and 0.07, respectively (all P < .05 except DL_CO/V_A). 20 W R² between V̇O₂ and V̇_A, V_A, DL_CO, and DL_CO/V_A was 0.64, 0.32, 0.07, and 0.18 (all P < .05 except DL_CO). Peak exercise R² between V̇O₂ and V̇_A, V_A, DL_CO, and DL_CO/V_A was 0.55, 0.31, 0.34, and 0.06 (all P < .05 except DL_CO/V_A).

CONCLUSIONS

These data suggest that alveolar airway function that is not exclusively related to effects caused by localized lung diffusivity affects exercise V̇O₂ in moderate-to-severe HF.

Alveolar air and oxidative metabolic demand during exercise in healthy adults: the role of single-nucleotide polymorphisms of the β2AR gene

The predominating β‐adrenergic receptor subtype expressed on human alveolar tissue is the β₂AR. The homozygous arginine (Arg16Arg) single‐nucleotide polymorphism (SNP) at codon 16 of the β₂AR gene has been associated with abnormal β₂AR function accompanied by decreased resting alveolar‐capillary membrane gas‐transfer in certain healthy adults. Although not previously studied in the context of the β₂AR gene, pulmonary gas‐transfer is also influenced by alveolar volume (V_A) and with it the availability of alveolar surface area, particularly during exercise. Small V_A implies less alveolar surface area available for O₂ transport. We tested the following hypothesis in healthy adults during exercise: compared with Gly16Gly and Arg16Gly β2AR genotypes, Arg16Arg will demonstrate reduced V_A and ventilation (V̇_A) relative to V̇_E and oxidative metabolic demand. Age‐ BMI‐ and gender‐matched groups of Arg16Arg (N = 16), Gly16Gly (N = 31), and Arg16Gly (N = 17) performed consecutive low (9‐min, 40%‐peak workload) and moderate (9‐min, 75%‐peak workload) intensity exercise. We derived V_A and V̇_A using “ideal” alveolar equations via arterialized gases combined with breath‐by‐breath ventilation and gas‐exchange measurements; whereas steady‐state V̇O₂ was used in metabolic equations to derive exercise economy (EC = workload÷V̇O₂). Variables at rest did not differ across β₂AR genotype. Strongest β₂AR genotype effects occurred during moderate exercise. Accordingly, while V̇_E did not differ across genotype (P > 0.05), decreased in Arg16Arg versus Arg16Gly and Gly16Gly were V̇O₂ (1110 ± 263, 1269 ± 221, 1300 ± 319 mL/(min·m²), respectively, both P < 0.05), V̇_A (59 ± 21, 70 ± 16, 70 ± 21 L/min, respectively, both P < 0.05), and V_A (1.43 ± 0.37, 1.95 ± 0.61, 1.93 ± 0.65 L, respectively, both P < 0.05). Also reduced was EC in Arg16Arg versus Arg16Gly (P < 0.05) and Gly16Gly (P > 0.05) (1.81 ± 0.23, 1.99 ± 0.30, and 1.94 ± 0.26 kcal/(L·m²), respectively). Compared with Gly16Gly and Arg16Gly genotypes, these data suggest the Arg16Arg β₂AR genotype plays a role in the loss of oxidative metabolic efficiency coupled with an inadaptive V_A and, hence, smaller alveolar surface area available for O₂ transport during submaximal exercise in healthy adults.

A review of the experimental evidence on the toxicokinetics of carbon monoxide: the potential role of pathophysiology among susceptible groups

BACKGROUND

Acute high level carbon monoxide (CO) exposure can cause immediate cardio-respiratory arrest in anyone, but the effects of lower level exposures in susceptible persons are less well known. The percentage of CO-bound hemoglobin in blood (carboxyhemoglobin; COHb) is a marker of exposure and potential health outcomes. Indoor air quality guidelines developed by the World Health Organization and Health Canada, among others, are set so that CO exposure does not lead to COHb levels above 2.0%, a target based on experimental evidence on toxicodynamic relationships between COHb and cardiac performance among persons with cardiovascular disease (CVD). The guidelines do not consider the role of pathophysiological influences on toxicokinetic relationships. Physiological deficits that contribute to increased CO uptake, decreased CO elimination, and increased COHb formation can alter relationships between CO exposures and resulting COHb levels, and consequently, the severity of outcomes. Following three fatalities attributed to CO in a long-term care facility (LTCF), we queried whether pathologies other than CVD could alter CO-COHb relationships. Our primary objective was to inform susceptibility-specific modeling that accounts for physiological deficits that may alter CO-COHb relationships, ultimately to better inform CO management in LTCFs.

METHODS

We reviewed experimental studies investigating relationships between CO, COHb, and outcomes related to health or physiological outcomes among healthy persons, persons with CVD, and six additional physiologically susceptible groups considered relevant to LTCF residents: persons with chronic obstructive pulmonary disease (COPD), anemia, cerebrovascular disease (CBD), heart failure, multiple co-morbidities, and persons of older age (≥ 60 years).

RESULTS

We identified 54 studies published since 1946. Six studies investigated toxicokinetics among healthy persons, and the remaining investigated toxicodynamics, mainly among healthy persons and persons with CVD. We identified one study each of CO dynamics in persons with COPD, anemia and persons of older age, and no studies of persons with CBD, heart failure, or multiple co-morbidities. Considerable heterogeneity existed for exposure scenarios and outcomes investigated.

CONCLUSIONS

Limited experimental human evidence on the effects of physiological deficits relevant to CO kinetics exists to support indoor air CO guidelines. Both experimentation and modeling are needed to assess how physiological deficits influence the CO-COHb relationship, particularly at sub-acute exposures relevant to indoor environments. Such evidence would better inform indoor air quality guidelines and CO management in indoor settings where susceptible groups are housed.