Cardiac output determination during progressive exercise in cystic fibrosis

Exercise intolerance in cystic fibrosis-the role of CFTR modulator therapies

Exercise intolerance is common in people with CF (pwCF), but not universal among all individuals. While associated with disease prognosis, exercise intolerance is not simply a reflection of the degree of lung disease. In people with severe CF, respiratory limitations may contribute more significantly to impaired exercise capacity than in those with mild-moderate CF. At all levels of disease severity, there are peripheral factors e.g., abnormal macro- and micro-vascular function that impair blood flow and reduce oxygen extraction, and mitochondrial defects that diminish metabolic efficiency. We discuss advances in understanding the central and peripheral mechanisms underlying exercise intolerance in pwCF. Exploring both the central and peripheral factors that contribute to exercise intolerance in CF can help inform the development of new therapeutic targets, as well as help define prognostic criteria.

Exercise Physiology Across the Lifespan in Cystic Fibrosis

Cystic fibrosis (CF), a severe life-limiting disease, is associated with multi-organ pathologies that contribute to a reduced exercise capacity. At present, the impact of, and interaction between, disease progression and other age-related physiological changes in CF on exercise capacity from child- to adult-hood is poorly understood. Indeed, the influences of disease progression and aging are inherently linked, leading to increasingly complex interactions. Thus, when interpreting age-related differences in exercise tolerance and devising exercise-based therapies for those with CF, it is critical to consider age-specific factors. Specifically, changes in lung function, chronic airway colonization by increasingly pathogenic and drug-resistant bacteria, the frequency and severity of pulmonary exacerbations, endocrine comorbidities, nutrition-related factors, and CFTR (cystic fibrosis transmembrane conductance regulator protein) modulator therapy, duration, and age of onset are important to consider. Accounting for how these factors ultimately influence the ability to exercise is central to understanding exercise impairments in individuals with CF, especially as the expected lifespan with CF continues to increase with advancements in therapies. Further studies are required that account for these factors and the changing landscape of CF in order to better understand how the evolution of CF disease impacts exercise (in)tolerance across the lifespan and thereby identify appropriate intervention targets and strategies.

Exercise Stroke Volume in Adult Cystic Fibrosis: A Comparison of Acetylene Pulmonary Uptake and Oxygen Pulse

Cardiac hemodynamic assessment during cardiopulmonary exercise testing (CPET) is proposed to play an important role in the clinical evaluation of individuals with cystic fibrosis (CF). Cardiac catheterization is not practical for routine clinical CPET. Use of oxygen pulse (O₂pulse) as a noninvasive estimate of stroke volume (SV) has not been validated in CF. This study tested the hypothesis that peak exercise O₂pulse is a valid estimate of SV in CF. Measurements of SV via the acetylene rebreathe technique were acquired at baseline and peak exercise in 17 mild-to-moderate severity adult CF and 25 age-matched healthy adults. We calculated O2pulse=V.O2HR. Baseline relationships between SV and O₂pulse were significant in CF (r = .80) and controls (r = .40), persisting to peak exercise in CF (r = .63) and controls (r = .73). The standard error of estimate for O₂pulse-predicted SV with respect to measured SV was similar at baseline (14.1 vs 20.1 mL) and peak exercise (18.2 vs 13.9 mL) for CF and controls, respectively. These data suggest that peak exercise O₂pulse is a valid estimate of SV in CF. The ability to noninvasively estimate SV via O₂pulse during routine clinical CPET can be used to improve test interpretation and advance our understanding of the impact cardiac dysfunction has on exercise intolerance in CF.

The relationship between cardiac hemodynamics and exercise tolerance in cystic fibrosis

BACKGROUND

Individuals with cystic fibrosis (CF) have reduced pulmonary function and exercise tolerance. Additionally, these individuals may develop abnormal cardiac function. The implications of abnormal cardiac function on exercise tolerance are unclear in CF.

OBJECTIVE

Study relationships between exercise cardiac hemodynamics and exercise tolerance in CF.

METHODS

17 CF and 25 controls participated in cardiopulmonary exercise testing to measure exercise duration and peak workload (PW). Cardiac index (QI) was measured using acetylene rebreathe and oxygen uptake (VO2) breath-by-breath. Forced expiratory volume in 1-second (FEV1) was performed at rest.

RESULTS

Peak QI was 6.7 ± 0.5 vs. 9.1 ± 0.3 mL/min/m(2), CF vs. controls, respectively (P < 0.05). Linear regressions between QI (R(2) = 0.63 and 0.51) and exercise duration or PW were stronger than VO2 (R(2) = 0.35 and 0.37) or FEV1 (R(2) = 0.34 and 0.36) in CF, respectively (P < 0.05).

CONCLUSION

These data are clinically relevant suggesting attenuated cardiac function in addition to low airway function relate to exercise tolerance in CF.

Impaired cardiac and peripheral hemodynamic responses to inhaled β₂-agonist in cystic fibrosis

Background

Pulmonary system dysfunction is a hallmark of cystic fibrosis (CF) disease. In addition to impaired cystic fibrosis transmembrane conductance regulator protein, dysfunctional β₂-adrenergic receptors (β₂AR) contribute to low airway function in CF. Recent observations suggest CF may also be associated with impaired cardiac function that is demonstrated by attenuated cardiac output (Q), stroke volume (SV), and cardiac power (CP) at both rest and during exercise. However, β₂AR regulation of cardiac and peripheral vascular tissue, in-vivo, is unknown in CF. We have previously demonstrated that the administration of an inhaled β-agonist increases SV and Q while also decreasing SVR in healthy individuals. Therefore, we aimed to assess cardiac and peripheral hemodynamic responses to the selective β₂AR agonist albuterol in individuals with CF.

Methods

18 CF and 30 control (CTL) subjects participated (ages 22 ± 2 versus 27 ± 2 and BSA = 1.7 ± 0.1 versus 1.8 ± 0.0 m², both p < 0.05). We assessed the following at baseline and at 30- and 60-minutes following nebulized albuterol (2.5mg diluted in 3.0mL of normal saline) inhalation: 12-lead ECG for HR, manual sphygmomanometry for systolic and diastolic blood pressure (SBP and DBP, respectively), acetylene rebreathe for Q and SV. We calculated MAP = DBP + 1/3(SBP–DBP); systemic vascular resistance (SVR) = (MAP/Q)•80; CP = Q•MAP; stroke work (SW) = SV•MAP; reserve (%change baseline to 30- or 60-minutes). Hemodynamics were indexed to BSA (Q_I, SV_I, SW_I, CP_I, SVR_I).

Results

At baseline, CF demonstrated lower SV, SV_I, SW, and SW_I but higher HR than CTL (p < 0.05); other measures did not differ. At 30-minutes, CF demonstrated higher HR and SVR_I, but lower Q, SV, SV_I, CP, CP_I, SW, and SW_I versus CTL (p < 0.05). At 60-minutes, CF demonstrated higher HR, SVR, and SVR_I, whereas all cardiac hemodynamics were lower than CTL (p < 0.05). Reserves of CP, SW, and SVR were lower in CF versus CTL at both 30 and 60-minutes (p < 0.05).

Conclusions

Cardiac and peripheral hemodynamic responsiveness to acute β₂AR stimulation via albuterol is attenuated in individuals with CF, suggesting β₂AR located in cardiac and peripheral vascular tissue may be dysfunctional in this population.

Ventilatory pattern and energy expenditure are altered in cystic fibrosis mice

BACKGROUND

Altered ventilatory pattern and increased energy expenditure are facets of the complex cystic fibrosis (CF) phenotype. It is not known whether these are inherent attributes of CF, secondary consequences of lung infection or other disease complications.

METHODS

Studies were performed in congenic C57BL/6J, F508del (Cftr((tm1kth))) and CF gut-corrected (F508del) mice. Ventilatory patterns were measured using whole-body plethysmography. Indirect calorimetry was used to determine oxygen consumption, carbon dioxide production and resting energy expenditure.

RESULTS

CF mice (F508del and F508del gut-corrected) have a significantly faster respiratory rate and increased ventilatory pattern variability as compared to non-CF mice. F508del but not CF gut-corrected mice had significantly increased energy expenditure per gram body weight.

CONCLUSIONS

CF mice exhibit a faster, more variable ventilatory pattern. These changes were present in the absence of detectable infection or illness due to gastrointestinal obstruction. Increased resting energy expenditure does not completely account for these differences.

The evolution of exercise capacity and its limiting factors in cystic fibrosis

The ability to perform exercise is an important determinant of both longevity and quality of life for patients with Cystic Fibrosis. There are a variety of physical and behavioural factors that contribute to exercise limitation. These, such as lung function or habitual physical activity, change over time. However, these factors can also be modified by treatments and interventions. This review discusses the various factors that contribute to exercise limitation in Cystic Fibrosis, and how these change with age.

Resting and exercise cardiorespiratory function in survivors of congenital diaphragmatic hernia

Our objective was to study exercise capacity and cardiorespiratory response to exertion in survivors of congenital diaphragmatic hernia (CDH). This was a cross-sectional cohort study of 23 CDH survivors, aged 10-16 years, and 23 gender- and age-matched controls. Exercise testing was performed on a cycle ergometer, with cardiac output measurements made using exponential CO2 rebreathing. Pretest cardiorespiratory assessment was done by echocardiography and pulmonary function testing. Statistical analysis was performed using Student's t-test, regression analysis, and longitudinal model computing with spatial covariance structure. No echocardiographic evidence for pulmonary hypertension was found at rest (right ventricular systolic pressures, 27 +/- 6 mmHg). Mean pulmonary artery diameter on the side of the CDH was significantly smaller than contralaterally, but was within normal range (z-score, 0 +/- 1.1 vs. 1.2 +/- 1.6, P < 0.01). Exercise capacity was mildly reduced in CDH compared to controls and predictive data (maximum workload, 77% +/- 12% vs. 91% +/- 16% of predicted, P < 0.01). Cardiorespiratory response to exertion was not significantly different between groups. In conclusion, most adolescent CDH survivors have nearly normal exercise capacity and cardiorespiratory response to exertion. This study may prove useful in comparisons with future cohorts comprising more severely affected individuals now surviving due to improved neonatal care.

Carbon dioxide pressure-concentration relationship in arterial and mixed venous blood during exercise

To calculate cardiac output by the indirect Fick principle, CO(2) concentrations (CCO(2)) of mixed venous (Cv(CO(2))) and arterial blood are commonly estimated from PCO(2), based on the assumption that the CO(2) pressure-concentration relationship (PCO(2)-CCO(2)) is influenced more by changes in Hb concentration and blood oxyhemoglobin saturation than by changes in pH. The purpose of the study was to measure and assess the relative importance of these variables, both in arterial and mixed venous blood, during rest and increasing levels of exercise to maximum (Max) in five healthy men. Although the mean mixed venous PCO(2) rose from 47 Torr at rest to 59 Torr at the lactic acidosis threshold (LAT) and further to 78 Torr at Max, the Cv(CO(2)) rose from 22.8 mM at rest to 25.5 mM at LAT but then fell to 23.9 mM at Max. Meanwhile, the mixed venous pH fell from 7.36 at rest to 7.30 at LAT and to 7.13 at Max. Thus, as work rate increases above the LAT, changes in pH, reflecting changes in buffer base, account for the major changes in the PCO(2)-CCO(2) relationship, causing Cv(CO(2)) to decrease, despite increasing mixed venous PCO(2). Furthermore, whereas the increase in the arteriovenous CCO(2) difference of 2.2 mM below LAT is mainly due to the increase in Cv(CO(2)), the further increase in the arteriovenous CCO(2) difference of 4.6 mM above LAT is due to a striking fall in arterial CCO(2) from 21.4 to 15.2 mM. We conclude that changes in buffer base and pH dominate the PCO(2)-CCO(2) relationship during exercise, with changes in Hb and blood oxyhemoglobin saturation exerting much less influence.

Energy metabolism during anaerobic exercise in children with cystic fibrosis and asthma

PURPOSE

The nature of a child's daily physical activity requires both aerobic and anaerobic energy metabolism. Aerobic exercise becomes compromised with advancing airway obstruction in children with cystic fibrosis (CF) and asthma (AS). Whether children with CF will have altered metabolic responses to supramaximal exercise when compared with asthmatics or healthy controls is still undetermined.

METHODS

Twenty-five children with CF, 22 with AS, and 23 healthy controls (CN) performed an incremental graded aerobic and Wingate anaerobic test (WAnT) on a cycle ergometer. Analysis of gas exchange and ventilatory data was collected and averaged every 5 s to estimate ventilatory kinetics and energy system contributions during both tests.

RESULTS

The CF and AS groups had mild lower airway obstruction (FEF25-75% < 80%) as compared with the CN. All three groups demonstrated similar anaerobic (mean and peak power during the WAnT) and aerobic exercise performance (peak oxygen consumption). In contrast to the AS or CN groups, children with CF used a lower percentage of their peak VO2 and V(E) during each phase of the WAnT, suggesting a preferential use of ATP/phosphocreatine and glycolytic energy stores compared with aerobic pathways. Greater reliance on anaerobic pathways during the WAnT in children with CF could be due to the physiologic sequelae underlying chronic obstructive lung disease.

CONCLUSIONS

Oxygen uptake kinetics appeared similar for all three groups. Although the energy needed to perform the WAnT can be met by subjects with CF, abnormalities in energy metabolism may exist for this group during exercise.

Maximal exercise capacity and peripheral skeletal muscle function following lung transplantation

BACKGROUND

There have been many suggestions that diminished exercise capacity in patients that have undergone lung transplantation is due, in part, to peripheral muscle dysfunction, brought on by either detraining or immunosuppressive therapy. There is limited data quantifying skeletal muscle function in this population, especially in those more than 18 months post-procedure. The present study sought to quantitate skeletal muscle function and cardiopulmonary responses to graded exercise in 19 lung transplant recipients, 15 of which were mostly more than 18 months post-procedure.

METHODS

Ten single- (SLT) and 9 double-lung transplantation (DLT) underwent anthropometric measures and performed expiratory spirometry, whole body plethysmography to assess lung volumes, static maximal mouth pressures to assess respiratory muscle strength, progressive exercise testing on a cycle ergometer (with cardiac output measurements being performed every second workload) and isokinetic cycling to assess peripheral muscle power and work capacity.

RESULTS

The DLT group was younger than the SLT group (23.0 [21.0-32.0] vs 47.5 [43.0-55.0] median [interquartile range], p < .05) with no differences in height, weight, or BMI. Despite the DLT group having significantly better spirometric values (FEV1: 86% vs 56.5% median) and less airtrapping (RV/TLC: 30% vs 53.5%), both groups were equally limited in exercise capacity (Wmax)(38.0 percent predicted [30.0-65.0] vs 37.5 percent predicted [30.0-44.0], SLT vs DLT), leg power (76.1 percent predicted [53.8-81.4] vs 69.0 percent predicted [58.3-76.0]) and leg work capacity (63.3 percent predicted [34.7-66.8] vs 38.4 percent predicted [27.5-57.3]). This lack of difference in performance persisted when the analysis was limited to those more than 18 months post-procedure. Respiratory muscle strength was also not different for the two groups, and was within normal limits. Wmax was best correlated with leg work capacity (r = .84), but also with leg power, RV/TLC, FEV1 (r = .49, -.52, .58). When normalized for age, height, and sex, percent predicted Wmax only correlated with percent predicted leg work capacity (r = .58). Cardiac output was appropriate for the work performed.

CONCLUSIONS

We conclude that peripheral skeletal muscle work capacity is reduced following lung transplantation and mostly responsible for the limitation of exercise performance. While the causes of muscular dysfunction have yet to be clarified, the preservation of respiratory muscle strength with the concomitant reduction in leg power and work capacity suggests that most of the muscular dysfunction post-transplantation is attributable to detraining.

Impedance cardiography accurately measures cardiac output during exercise in children with cystic fibrosis

OBJECTIVES

After validation of impedance cardiography (ICG) in healthy children, this same device was tested in children with cystic fibrosis (CF) to validate its capability of measuring cardiac output (Q) in this population.

DESIGN

Comparative study of ICG vs the indirect Fick (CO2) method.

SETTING

Tertiary care children's teaching hospital.

PATIENTS

Twenty-one CF children with mean FEV1 of 77 +/- 21% predicted.

MEASUREMENTS

ICG results were compared with CO2 rebreathing (RB) measurements of Q with sampling of capillary blood gases at two levels of exercise (0.5 and 1.5 W/kg). ICG measurements were made each minute, and duplicate RB measurements from 6 to 8 min at each workload. Q was regressed against oxygen uptake and results by each method were compared.

RESULTS

Mean bias (QRB-QICG) was -0.09 +/- 0.94 L/min. The largest deviation of QICG from QRB was +33%, and 83% of corresponding QICG values were within +/-20% of QRB result.

CONCLUSIONS

This device gives rapid, accurate, noninvasive Q measurements in children with CF.

Estimation of mixed venous PCO2 for determination of cardiac output in children

STUDY OBJECTIVES

Cardiac output (Q) can be estimated noninvasively during exercise by employing CO2-rebreathing techniques (equilibrium and exponential) to estimate the oxygenated mixed venous PCO2 (PvCO2). It has been found in adults and children that the equilibrium method underestimates Q as a result of overestimation of PvCO2, unless PvCO2 is "downstream corrected." In adults, it has been found that the exponential method does not require downstream correction and yields values similar to those obtained by the equilibrium method with downstream correction. The objectives of this study were as follows: to test whether the exponential method gives similar results to the equilibrium method with downstream correction in children; to verify that downstream correction is required in children; and to test whether a single equation could be used in adults and children to predict Q from oxygen consumption (VO2).

DESIGN

Descriptive.

SETTING

Exercise laboratory of a university hospital.

PARTICIPANTS

23 children (16 boys, 7 girls) with a mean age of 11.0 +/- 1.9 years (7.1 to 13.9 years), and 12 adults (7 men, 5 women) with a mean age of 33.6 +/- 7.2 years (24 to 48 years).

INTERVENTIONS

While performing steady-state exercise on an ergometer, PvCO2 was determined in 14 children using both the equilibrium and exponential methods, and in all other subjects using the equilibrium method alone.

MEASUREMENTS AND RESULTS

For the 14 children who underwent testing by both the equilibrium and exponential methods, the uncorrected equilibrium PvCO2 was significantly different from both the corrected PvCO2 and the exponential PvCO2. We found a strong relationship between Q (L/min), calculated using the downstream corrected values of PvCO2, and VO2 (L/min) (r2 = 0.95), and this relationship was similar to that obtained by dye dilution in other studies. When weight was included, it was determined that one equation could be used for children and adults: Q (L/min) = 1.42 + 5.80.VO2 (L/min) + 0.06.wt (kg), r2 = 0.97, SEY = 0.67.

CONCLUSIONS

CO2-rebreathing techniques can be used to determine Q in children; the exponential method gives values that are similar to the equilibrium method with the downstream correction; and one prediction can be used for Q in adults and children.

End-tidal estimates of arterial PCO2 for cardiac output measurement by CO2 rebreathing: a study in patients with cystic fibrosis and healthy controls

We set out to determine the effects of various estimates of arterial PCO2 (PaCO2) on calculation of cardiac output (Q) by the indirect Fick (CO2) method in healthy children and children with cystic fibrosis (CF), and to develop a prediction equation for children for PaCO2, based on end-tidal PCO2 (PetCO2). The study had 3 parts: 1) Twenty-three healthy children exercised lightly and moderately while arterialized capillary blood gases and PetCO2 were measured simultaneously so that a prediction equation for PaCO2 could be derived from PetCO2. Cardiac output was measured by CO2 rebreathing at each workload; different values for PaCO2 (measured in arterialized capillary blood, end-tidal, and PaCO2 derived from the Bohr equation assuming normal dead space) were used to calculate Q; 2) our equation PaCO2 = 0.647 PetCO2 + 12.4 was tested prospectively to measure Q in 9 healthy children; and 3) cardiac output based on arterialized capillary PaCO2 was compared with that based on Jones-corrected PetCO2 during light and moderate exercise in 16 CF patients whose forced expiratory volume in 1 second (FEV1), range from normal to 37% predicted. Our results have shown that in healthy children end-tidal based-estimates of PaCO2 tended to overestimate Q, whereas PaCO2 values derived by the Bohr equation and assuming normal dead space tended to underestimate Q, compared with Q calculated from directly measured PaCO2. Our prediction equation resulted in good agreement compared with directly measured PaCO2 when used to calculate Q (mean difference, +1.3%; range, +9% to -13%). CF patients with little or no airway obstruction had results similar to healthy controls, but those with severe airway obstruction had lower values for Q when PetCO2 was used instead of directly measured PaCO2. We conclude that estimates of PaCO2 from PetCO2 are not reliable in patients with moderately severe pulmonary disease due to CF. In healthy children, the prediction equation for PaCO2 from PetCO2 derived in the present study gives results superior to other bloodless methods currently in use for computation of Q by the indirect Fick (CO2) method.

Cardiorespiratory status after treatment for acute lymphoblastic leukemia

The use of certain chemotherapeutic agents is associated with dose-related cardiotoxicity and, potentially, with restrictive lung disease. Therefore, we assessed the cardiopulmonary status and exercise capacity of 19 patients (pts; 9M:10F) 1.1 to 7.1 years (mean 4.6 +/- 1.5 years) after successful treatment of acute lymphoblastic leukemia (ALL) with Dana Farber Cancer Institute protocols. As body mass and nutritional status may influence exercise capacity, we also evaluated their anthropometric status and the plasma levels of rapid turnover proteins. Seven pts designated as "standard risk for relapse" (SR) had received low cumulative doses of doxorubicin (50 +/- 21 mg/m2), while twelve pts at "high or very high risk for relapse" (HR/VHR) had received higher doses (349 +/- 16 mg/m2). The evaluations included a questionnaire, anthropometric assessments, echocardiography, pulmonary function studies, exercise testing, and nutritional assays. Patients' data were compared with published normative data or with control values from our laboratories. In addition, we compared SR pt data with HR/VHR pt data. No pt had overt symptoms or signs of cardiorespiratory compromise. The pts had a higher percent of body fat than age-matched healthy controls (29.7 +/- 7.9% vs. 20 +/- 6%; P < 0.001). On echocardiography, cardiac systolic function was within normal limits in all. However, HR/VHR pts had lower left ventricular (LV) shortening fractions than SR pts (P < 0.05). LV filling velocity, indicative of diastolic function (the E/A ratio), was normal in most pts. Pulmonary function studies were normal. Exercise capacity was below predicted in most cases but heart rates at peak exercise and leg muscle function were within normal limits, suggesting a deconditioned state. Plasma levels of rapid turnover proteins were also normal. Despite lack of overt morbidity in our pt population, subtle abnormalities persist in cardiac function while pulmonary function is normal. Longitudinal studies will identify if further abnormalities or overt morbidity develop. In later years, continuing obesity and a sedentary state may contribute to clinically relevant heart disease.