Signal-morphology impedance cardiography during incremental cardiopulmonary exercise testing in pulmonary arterial hypertension

Stroke volume and cardiac output during 6 minute-walk tests are strong predictors of maximal oxygen uptake in people after stroke

Background and objectives

The 6-minute walk test (6MWT) is a field test commonly used to predict peak oxygen consumption (VO2peak) in people after stroke. Inclusion of cardiodynamic variables measured by impedance cardiography (ICG) during a 6MWT has been shown to improve prediction of VO2peak in healthy adults but these data have not been considered in people after stroke. This study investigates whether the prediction of VO2peak can be improved by the inclusion of cardiovascular indices derived by impedance cardiography (ICG) during the 6MWT in people after stroke.

Methods

This was a cross-sectional study. Patients diagnosed with stroke underwent in random order, a maximal cardiopulmonary exercise test (CPET) and 6MWT in separate dates. Heart rate (HR), stroke volume (SV) and cardiac output (CO) were measured by ICG during all tests. Oxygen consumption was recorded by a metabolic cart during the CPET. Recorded data were subjected to multiple regression analyses to generate VO2peak prediction equations.

Results

Fifty-nine patients, mean age 50.0±11.7 years were included in the analysis. The mean distance covered in the 6MWT (6MWD) was 294±13 m, VO2peak was 19.2±3.2 ml/min/kg. Mean peak HR, SV and CO recorded during 6MWT were 109±6 bpm, 86.3±8.8 ml, 9.4±1.2 L/min and during CPET were 135±14 bpm, 86.6±9 ml, 11.7±2 L/min respectively. The prediction equation with inclusion of cardiodynamic variables: 16.855 + (-0.060 x age) + (0.196 x BMI) + (0.01 x 6MWD) + (-0.416 x SV6MWT) + (3.587 x CO 6MWT) has a higher squared multiple correlation (R2) and a lower standard error of estimate (SEE) and SEE% compared to the equation using 6MWD as the only predictor.

Conclusion

Inclusion of SV and CO measured during the 6MWT in stroke patients further improved the VO2peak prediction power compared to using 6MWD as a lone predictor.

Cardiodynamic variables measured by impedance cardiography during a 6-minute walk test are reliable predictors of peak oxygen consumption in young healthy adults

Accurate prediction of aerobic capacity is necessary to guide appropriate exercise prescription. It is common to use 6-minute walk distance (6MWD) to predict peak oxygen uptake (VO2peak) in the clinical environment. The aim of this study was to determine whether prediction of VO2peak can be improved by the inclusion of cardiovascular indices derived by impedance cardiography (ICG) during the 6MWT. A total of 62 healthy university students aged 21±1 years completed in separate days, a cardiopulmonary exercise test (CPET) and two 6MWTs (30 min apart), during which heart rate (HR), stroke volume (SV) and cardiac output (CO) were measured by ICG (PhysioFlow® PF07 EnduroTM). The CPET was conducted with the Ergoselect 200 Ergoline and oxygen consumption measured by a MasterScreenTM CPX breath-by-breath metabolic cart. Multiple regression analyses were conducted to generate VO2peak prediction equations using 6MWD with, or without the cardiovascular indices recorded at the end of the best performed 6MWT as predictor variables. The mean peak HR (bpm), SV (ml) and CO (L/min) recorded during 6MWT were 156±18, 95.6±9, 15±2.8 and during CPET were 176±16, 91.3±8, 16.2±2.7, respectively. Analyses revealed the following VO2peak prediction equation: VO2peak = 100.297+(0.019x6MWD)+(-0.598xHR6MWT)+(-1.236xSV6MWT) + (8.671 x CO6MWT). This equation has a squared multiple correlation (R2) of 0.866, standard error of the estimate (SEE) of 2.28 mL/kg/min and SEE:VO2peak (SEE%) of 7.2%. Cross-validation of equation stability using predicted residual sum of squares (PRESS) statistics showed a R2 (Rp2), SEE (SEEp) and SEEp% of 0.842, 2.38 mL/kg/min and 7.6% respectively. The minimal shrinkage of R2 implied regression model stability. Correlation between measured and predicted VO2peak using this equation was strong (r = 0.931, p<0.001). When 6MWD alone was used as the predictor for VO2peak, the generated equation had a lower R2 (0.549), and a higher SEE (4.08 mL/kg/min) and SEE% (12.9%). This is the first study which included cardiac indices during a 6MWT as variables for VO2peak prediction. Our results suggest that inclusion of cardiac indices measured during the 6MWT more accurately predicts VO2peak than using 6MWD data alone.

Predictors of low exercise cardiac output in patients with severe pulmonic regurgitation

BACKGROUND AND OBJECTIVES

Chronic pulmonic regurgitation (PR) following repair of congenital heart disease (CHD) impairs right ventricular function that impacts peak exercise cardiac index (pCI). We aimed to estimate in a non-invasive way pCI and peak oxygen consumption (pVO₂) and to evaluate predictors of low pCI in patients with significant residual pulmonic regurgitation after CHD repair.

METHOD

We included 82 patients (median age 19 years (range 10-54 years)) with residual pulmonic regurgitation fraction >40%. All underwent cardiac MRI and cardiopulmonary testing with measurement of pCI by thoracic impedancemetry. Low pCI was defined <7 L/min/m².

RESULTS

Low pCI was found in 18/82 patients. Peak indexed stroke volume (pSVi) tended to compensate chronotropic insufficiency only in patients with normal pCI (r=-0.31, p=0.01). Below 20 years of age, only 5/45 patients had low pCI but near-normal (≥6.5 L/min/m²). pVO₂ (mL/kg/min) was correlated with pCI (r=0.58, p=0.0002) only in patients aged >20 years. Left ventricular stroke volume in MRI correlated with pSVi only in the group of patients with low pCI (r=0.54, p=0.02). No MRI measurements predicted low pCI. In multivariable analysis, only age predicted a low pCI (OR=1.082, 95% CI 1.035 to 1.131, p=0.001) with continuous increase of risk with age.

CONCLUSIONS

In patients with severe PR, pVO₂ is a partial reflection of pCI. Risk of low pCI increases with age. No resting MRI measurement predicts low haemodynamic response to exercise. Probably more suitable to detect ventricular dysfunction, pCI measurement could be an additional parameter to take into account when considering pulmonic valve replacement.

Noninvasive investigation of the cardiodynamic response to 6MWT in people after stroke using impedance cardiography

This is a cross-section observational study that investigated the cardiodynamic response to a 6-minute walk test (6MWT) in patients after stroke using impedance cardiography (ICG). Patients diagnosed with stroke were invited to participate in a 6MWT on consecutive days. Heart rate (HR), cardiac output (CO), stroke volume (SV) and cardiac index (CI) were measured by ICG using the PhysioFlow^® PF07 Enduro^TM at 1-second intervals for 10 minutes prior to, during and for 10 minutes after each 6MWT. Oxygen saturation, perceived exertion score (modified Borg scale) and the distance covered at the end of each 6MWT were recorded. Twenty-nine patients (mean age 55.6±10.9 years) completed the study. The mean duration of stroke after diagnosis was 14.4±19.1 months. There were no differences in the measured data between the first and second 6MWT (mean intraclass correlation coefficient (ICC) range: 0.87–0.95). The 6 minute walk distance (6WMD) covered in the two 6MWTs was 246±126 and 255±130m respectively (p>0.05). Mean measured data for each subject at rest, and at the end of the better performed 6MWT were, respectively: HR 78±11 and 100±18 bpm; CO 5.5±1.2 and 8.9±2.6 l/min, SV 71.3±16 and 89.3±18.6 ml/beat and CI 3.0±0.6 and 4.9±1.3 l/min/m². After commencement of the 6MWT, the increase in SV took 30 sec before the rise approaching a plateau, whereas HR, CO and CI continued to rise steeply for 90 sec before leveling off to a steady rise. After completion of the 6MWT, all parameters had returned to baseline by a mean of 3.5 min. Sub-group analysis showed that the increase in cardiac output was predominantly contributed by an increase in heart rate in participants diagnosed with stroke for less than 1 year, whereas both stroke volume and heart rate contributed similarly to the increase in cardiac output in participants with diagnosis of stroke for longer than 1 year. Systolic blood pressure (SBP) and diastolic blood pressure (DBP) both returned to baseline within 2 minutes post 6MWT. HR recorded at the end of the 6MWT was 60.8±10.6% of the predicted maximal heart rate and perceived exertion score was 5±2. Correlations between 6MWD and HR, and between 6MWD and SV were weak, with correlation coefficients Spearman’s rho (r_s) =0.46, and 0.42, respectively (p<0.05). Correlation between 6MWD and CO and CI were higher (r_s= 0.66 and 0.63, respectively (p<0.01)). This is the first study to report cardiac responses during a 6MWT in stroke patients. ICG is a reliable, non-invasive, repeatable method of measuring cardiodynamic data in stroke patients.

Effects of lung deflation induced by tiotropium/olodaterol on the cardiocirculatory responses to exertion in COPD

BACKGROUND

Hyperinflation has been associated with negative cardiocirculatory consequences in patients with chronic obstructive pulmonary disease (COPD). These abnormalities are likely to worsen when the demands for O₂ increase, e.g., under the stress of exercise. Thus, pharmacologically-induced lung deflation may improve cardiopulmonary interactions and exertional cardiac output leading to higher limb muscle blood flow and oxygenation in hyperinflated patients with COPD.

METHODS

20 patients (residual volume = 201.6 ± 63.6% predicted) performed endurance cardiopulmonary exercise tests (75% peak) 1 h after placebo or tiotropium/olodaterol 5/5 μg via the Respimat® inhaler (Boehringer Ingelheim, Ingelheim am Rhein, Germany). Cardiac output was assessed by signal-morphology impedance cardiography. Near-infrared spectroscopy determined quadriceps blood flow (indocyanine green dye) and intra-muscular oxygenation.

RESULTS

Tiotropium/olodaterol was associated with marked lung deflation (p < 0.01): residual volume decreased by at least 0.4 L in 14/20 patients (70%). The downward shift in the resting static lung volumes was associated with less exertional inspiratory constraints and dyspnoea thereby increasing exercise endurance by ~50%. Contrary to our premises, however, neither central and peripheral hemodynamics nor muscle oxygenation improved after active intervention compared to placebo. These results were consistent with those found in a subgroup of patients showing the largest decrements in residual volume (p < 0.05).

CONCLUSIONS

The beneficial effects of tiotropium/olodaterol on resting and operating lung volumes are not translated into enhanced cardiocirculatory responses to exertion in hyperinflated patients with COPD. Improvement in exercise tolerance after dual bronchodilation is unlikely to be mechanistically linked to higher muscle blood flow and/or O₂ delivery.

Cardiac output measurement during exercise in COPD: A comparison of dye dilution and impedance cardiography

INTRODUCTION

Impedance cardiography (IC) derived from morphological analysis of the thoracic impedance signal is now commonly used for noninvasive assessment of cardiac output (CO) at rest and during exercise. However, in Chronic Obstructive Pulmonary Disease (COPD), conflicting findings put its accuracy into question.

OBJECTIVES

We therefore compared concurrent CO measurements captured by IC (PhysioFlow: CO_IC ) and by the indocyanine green dye dilution method (CO_DD ) in patients with COPD.

METHODS

Fifty paired CO measurements were concurrently obtained using the two methods from 10 patients (FEV₁ : 50.5 ± 17.5% predicted) at rest and during cycling at 25%, 50%, 75% and 100% peak work rate.

RESULTS

From rest to peak exercise CO_IC and CO_DD were strongly correlated (r = 0.986, P < 0.001). The mean absolute and percentage differences between CO_IC and CO_DD were 1.08 L/min (limits of agreement (LoA): 0.05-2.11 L/min) and 18 ± 2%, respectively, with IC yielding systematically higher values. Bland-Altman analysis indicated that during exercise only 7 of the 50 paired measurements differed by more than 20%. When data were expressed as changes from rest, correlations and agreement between the two methods remained strong over the entire exercise range (r = 0.974, P < 0.001, with no significant difference: 0.19 L/min; LoA: -0.76 to 1.15 L/min). Oxygen uptake (VO₂ ) and CO_DD were linearly related: r = 0.893 (P < 0.001), CO_DD = 5.94 × VO₂ + 2.27 L/min. Similar results were obtained for VO₂ and CO_IC (r = 0.885, P < 0.001, CO_IC = 6.00 × VO₂ + 3.30 L/min).

CONCLUSIONS

These findings suggest that IC provides an acceptable CO measurement from rest to peak cycling exercise in patients with COPD.

A study of clinical and physiological relations of daily physical activity in precapillary pulmonary hypertension

Daily physical activity is reduced in precapillary pulmonary hypertension (PH), but the underlying mechanisms are inadequately explored. We sought to investigate clinical and physiological relations of daily physical activity and profile differences between less and more active patients with precapillary PH. A prospective, cross-sectional study of 20 patients with precapillary PH who undertook 1) a comprehensive clinical assessment, 2) a preliminary treadmill test, 3) 7-day monitoring of daily walking intensity with triaxial accelerometry, and 4) a personalized treadmill test corresponding to the individual patient mean daily walking intensity with real-time physiological measurements. Significant clinical correlations with individual patient mean walking intensity [1.71 ± 0.27 (SD) m/s²] were observed for log-transformed N-terminal probrain natriuretic peptide (log NT-proBNP; r = -0.75, P = <.001), age (r = -0.70, P = 0.001), transfer factor for carbon monoxide %predicted (r = 0.51, P = 0.022), and 6-min walk distance (r = 0.50, P = 0.026). Significant physiological correlations were obtained for heart rate reserve (r = 0.68, P = 0.001), quadriceps tissue oxygenation index (Q-[Formula: see text]; r = 0.58, P = 0.008), change in Q-[Formula: see text] from rest (r = 0.60, P = 0.006), and ventilatory equivalent for oxygen uptake (r = -0.56, P = 0.013). Stepwise multiple regression analyses retained log NT-proBNP (R² = 0.55), heart rate reserve (R² = 0.44), and Q-[Formula: see text] (R² = 0.13) accounting for a significant variance in individual walking intensity. Less active patients had greater physical activity-induced cardiopulmonary impairment, worse quadriceps oxygenation profile, and compromised health-related quality of life compared with more active patients. These preliminary findings suggest a significant relation between right ventricular and peripheral muscle oxygenation status and reduced daily physical activity in precapillary PH. Further research is warranted to unravel the physiological determinants, establish clinical predictors, and identify beneficial interventions.NEW & NOTEWORTHY Daily physical activity holds promise to be a meaningful, patient-related outcome measure in pulmonary hypertension. In this study, novel findings in a representative sample of patients with precapillary pulmonary hypertension link reduced daily walking activity, as measured by triaxial accelerometry, with compromised right ventricular and pulmonary vascular status, peripheral muscle oxygenation, and health-related quality of life, providing a preliminary insight into the physiological mechanisms and clinical predictors of daily physical activity in precapillary pulmonary hypertension.

Cardiac hemodynamic response to the 6-minute walk test in young adults and the elderly

BACKGROUND

Exercise capacity is evaluated using the 6-minute walk test in various diseases. Variety in the distances walked was also shown in healthy subjects. Moreover, age-related influences on cardiac hemodynamic response to the 6-minute walk test have not been clarified. The purpose of this study was thus to investigate the hemodynamic response to the 6-minute walk test and to detect factors related to the distance walked in healthy subjects.

METHODS

Thirteen young adults (age 20.5 ± 0.7 years, BMI 22.0 ± 4.3) and 26 elderly individuals (age 60.2 ± 6.1 years, BMI 21.7 ± 2.2) were enrolled to measure real-time hemodynamic responses using non-invasive impedance cardiography during the 6-minute walk test.

RESULTS

Stroke volume was higher in the young than in the elderly and reached a plateau within 30 s of starting to walk in all subjects. An increase in heart rate took more than 1 min in the elderly, while it took less than 30 s in the young, which resulted in slower increases in cardiac output and cardiac index in the elderly. There was no difference in the distance in the 6-minute walk test between the young and the elderly. The distance walked was correlated with heart rate, cardiac output, and cardiac index, but not with stroke volume, at the end of the 6-minute walk test.

CONCLUSIONS

The distance walked appeared to depend on increased cardiac output based on heart rate, but did not appear to be limited by stroke volume, in healthy subjects.

Value of impedance cardiography during 6-minute walk test in pulmonary hypertension

BACKGROUND

Methods that predict prognosis and response to therapy in pulmonary hypertension (PH) are lacking. We tested whether the noninvasive estimation of hemodynamic parameters during 6-minute walk test (6MWT) in PH patients provides information that can improve the value of the test.

METHODS

We estimated hemodynamic parameters during the 6MWT using a portable, signal-morphology-based, impedance cardiograph (PhysioFlow Enduro) with real-time wireless monitoring via a bluetooth USB adapter.

RESULTS

We recruited 48 subjects in the study (30 with PH and 18 healthy controls). PH patients had significantly lower maximum stroke volume (SV) and CI and slower cardiac output (CO) acceleration and decelerations slopes during the test when compared with healthy controls. In PH patients, CI change was associated with total distance walked (R = 0.62; P < 0.001) and percentage of predicted (R = 0.4, P = 0.03), HR recovery at 1 minute (0.57, P < 0.001), 2 minutes (0.65, P < 0.001), and 3 minutes (0.66, P < 0.001). Interestingly, in PH patients CO change during the test was predominantly related to an increase in SV instead of HR.

CONCLUSIONS

Estimation of hemodynamic parameters such as cardiac index during 6-minute walk test is feasible and may provide useful information in patients with PH. Clin Trans Sci 2013; Volume #: 1-7.