Revisiting cardiac output estimated noninvasively from oxygen uptake during exercise: an exploratory hypothesis-generating replication study

Stringer et al. [J Appl Physiol (1985) 82: 908-912, 1997] developed a method from invasive data to estimate cardiac output during incremental exercise testing based on Fick's principle. The authors proposed that the arterio-mixed venous oxygen content difference increases linearly with percentage of maximal O2 consumption. We hypothesized an S-shaped pattern in the published data and calculated the inflection point of this curve and of the standard resting oxygen dissociation curve. Using a partial F test, we compared the linear model with a third-order polynomial model, which showed a better fit to the data [F(2,101) = 9.5, P < 0.001]. This finding was reproduced in a dataset published by Åstrand et al. in 1964 [F(2, 122) = 10.6, P < 0.001]. The inflection point of the curve coincided with the lactate acidosis threshold [first ventilatory threshold (VT1)] as measured by Stringer et al. (VT1 at 50% and inflection point at 56% [95% CI, 52.9 to 60.7] of maximal O2 consumption). The inflection point of the standard resting oxygen dissociation curve was calculated at a partial pressure of 21.5 mmHg and a saturation of 36%, matching the "critical capillary Po2" concept of Stringer et al. (21.2 mmHg). We conclude that the arterio-mixed venous oxygen content difference increases in an S-shaped manner with percentage of maximal oxygen consumption and that the inflection point of this curve may correspond to VT1 and that of the in vivo oxygen dissociation curve. Further research is needed to confirm these findings and improve the method.NEW & NOTEWORTHY In 1997, Stringer, Hansen, and Wasserman developed a method for estimating cardiac output during incremental exercise testing. They observed that the arterio-mixed venous oxygen content difference increases linearly with the percentage of maximal O2 consumption. This increase may be better modelled by an S-shaped function, the inflection point of which may be related to the first ventilatory threshold and the inflection point of the oxygen dissociation curve. This finding may help to improve the method.

A machine learning-based score for precise echocardiographic assessment of cardiac remodelling in hypertensive young adults

Aims

Accurate staging of hypertension-related cardiac changes, before the development of significant left ventricular hypertrophy, could help guide early prevention advice. We evaluated whether a novel semi-supervised machine learning approach could generate a clinically meaningful summary score of cardiac remodelling in hypertension.

Methods and results

A contrastive trajectories inference approach was applied to data collected from three UK studies of young adults. Low-dimensional variance was identified in 66 echocardiography variables from participants with hypertension (systolic ≥160 mmHg) relative to a normotensive group (systolic < 120 mmHg) using a contrasted principal component analysis. A minimum spanning tree was constructed to derive a normalized score for each individual reflecting extent of cardiac remodelling between zero (health) and one (disease). Model stability and clinical interpretability were evaluated as well as modifiability in response to a 16-week exercise intervention. A total of 411 young adults (29 ± 6 years) were included in the analysis, and, after contrastive dimensionality reduction, 21 variables characterized >80% of data variance. Repeated scores for an individual in cross-validation were stable (root mean squared deviation = 0.1 ± 0.002) with good differentiation of normotensive and hypertensive individuals (area under the receiver operating characteristics 0.98). The derived score followed expected hypertension-related patterns in individual cardiac parameters at baseline and reduced after exercise, proportional to intervention compliance (P = 0.04) and improvement in ventilatory threshold (P = 0.01).

Conclusion

A quantitative score that summarizes hypertension-related cardiac remodelling in young adults can be generated from a computational model. This score might allow more personalized early prevention advice, but further evaluation of clinical applicability is required.

Aerobic Exercise Training Response in Preterm-Born Young Adults with Elevated Blood Pressure and Stage 1 Hypertension: A Randomized Clinical Trial

Rationale: Premature birth is an independent predictor of long-term cardiovascular risk. Individuals affected are reported to have a lower rate of [Formula: see text]o2 at peak exercise intensity ([Formula: see text]o2PEAK) and at the ventilatory anaerobic threshold ([Formula: see text]o2VAT), but little is known about their response to exercise training. Objectives: The primary objective was to determine whether the [Formula: see text]o2PEAK response to exercise training differed between preterm-born and term-born individuals; the secondary objective was to quantify group differences in [Formula: see text]o2VAT response. Methods: Fifty-two preterm-born and 151 term-born participants were randomly assigned (1:1) to 16 weeks of aerobic exercise training (n = 102) or a control group (n = 101). Cardiopulmonary exercise tests were conducted before and after the intervention to measure [Formula: see text]o2PEAK and the [Formula: see text]o2VAT. A prespecified subgroup analysis was conducted by fitting an interaction term for preterm and term birth histories and exercise group allocation. Measurements and Main Results: For term-born participants, [Formula: see text]o2PEAK increased by 3.1 ml/kg/min (95% confidence interval [CI], 1.7 to 4.4), and the [Formula: see text]o2VAT increased by 2.3 ml/kg/min (95% CI, 0.7 to 3.8) in the intervention group versus controls. For preterm-born participants, [Formula: see text]o2PEAK increased by 1.8 ml/kg/min (95% CI, -0.4 to 3.9), and the [Formula: see text]o2VAT increased by 4.6 ml/kg/min (95% CI, 2.1 to 7.0) in the intervention group versus controls. No significant interaction was observed with birth history for [Formula: see text]o2PEAK (P = 0.32) or the [Formula: see text]o2VAT (P = 0.12). Conclusions: The training intervention led to significant improvements in [Formula: see text]o2PEAK and [Formula: see text]o2VAT, with no evidence of a statistically different response based on birth history. Clinical trial registered with www.clinicaltrials.gov (NCT02723552).

Aerobic exercise increases brain vessel lumen size and blood flow in young adults with elevated blood pressure. Secondary analysis of the TEPHRA randomized clinical trial

IMPORTANCE

Cerebrovascular changes are already evident in young adults with hypertension and exercise is recommended to reduce cardiovascular risk. To what extent exercise benefits the cerebrovasculature at an early stage of the disease remains unclear.

OBJECTIVE

To investigate whether structured aerobic exercise increases brain vessel lumen diameter or cerebral blood flow (CBF) and whether lumen diameter is associated with CBF.

DESIGN

Open, parallel, two-arm superiority randomized controlled (1:1) trial in the TEPHRA study on an intention-to-treat basis. The MRI sub-study was an optional part of the protocol. The outcome assessors remained blinded until the data lock.

SETTING

Single-centre trial in Oxford, UK.

PARTICIPANTS

Participants were physically inactive (<150 min/week moderate to vigorous physical activity), 18 to 35 years old, 24-hour ambulatory blood pressure 115/75 mmHg-159/99 mmHg, body mass index below 35 kg/m² and never been on prescribed hypertension medications. Out of 203 randomized participants, 135 participated in the MRI sub-study. Randomisation was stratified for sex, age (<24, 24-29, 30-35 years) and gestational age at birth (<32, 32-37, >37 weeks).

INTERVENTION

Study participants were randomised to a 16 week aerobic exercise intervention targeting 3×60 min sessions per week at 60 to 80 % peak heart rate.

MAIN OUTCOMES AND MEASURES

cerebral blood flow (CBF) maps from ASL MRI scans, internal carotid artery (ICA), middle cerebral artery (MCA) M1 and M2 segments, anterior cerebral artery (ACA), basilar artery (BA), and posterior cerebral artery (PCA) diameters extracted from TOF MRI scans.

RESULTS

Of the 135 randomized participants (median age 28 years, 58 % women) who had high quality baseline MRI data available, 93 participants also had high quality follow-up data available. The exercise group showed an increase in ICA (0.1 cm, 95 % CI 0.01 to 0.18, p =.03) and MCA M1 (0.05 cm, 95 % CI 0.01 to 0.10, p =.03) vessel diameter compared to the control group. Differences in the MCA M2 (0.03 cm, 95 % CI 0.0 to 0.06, p =.08), ACA (0.04 cm, 95 % CI 0.0 to 0.08, p =.06), BA (0.02 cm, 95 % CI -0.04 to 0.09, p =.48), and PCA (0.03 cm, 95 % CI -0.01 to 0.06, p =.17) diameters or CBF were not statistically significant. The increase in ICA vessel diameter in the exercise group was associated with local increases in CBF.

CONCLUSIONS AND RELEVANCE

Aerobic exercise induces positive cerebrovascular remodelling in young people with early hypertension, independent of blood pressure. The long-term benefit of these changes requires further study.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02723552, 30 March 2016.

Effect of moderate to high intensity aerobic exercise on blood pressure in young adults: The TEPHRA open, two-arm, parallel superiority randomized clinical trial

BACKGROUND

Exercise is advised for young adults with elevated blood pressure, but no trials have investigated efficacy at this age. We aimed to determine whether aerobic exercise, self-monitoring and motivational coaching lowers blood pressure in this group.

METHODS

The study was a single-centre, open, two-arm, parallel superiority randomized clinical trial with open community-based recruitment of physically-inactive 18-35 year old adults with awake 24 h blood pressure 115/75mmHg-159/99 mmHg and BMI<35 kg/m². The study took place in the Cardiovascular Clinical Research Facility, John Radcliffe Hospital, Oxford, UK. Participants were randomized (1:1) with minimisation factors sex, age (<24, 24-29, 30-35 years) and gestational age at birth (<32, 32-37, >37 weeks) to the intervention group, who received 16-weeks aerobic exercise training (three aerobic training sessions per week of 60 min per session at 60-80% peak heart rate, physical activity self-monitoring with encouragement to do 10,000 steps per day and motivational coaching to maintain physical activity upon completion of the intervention. The control group were sign-posted to educational materials on hypertension and recommended lifestyle behaviours. Investigators performing statistical analyses were blinded to group allocation. The primary outcome was 24 h awake ambulatory blood pressure (systolic and diastolic) change from baseline to 16-weeks on an intention-to-treat basis. Clinicaltrials.gov registered on March 30, 2016 (NCT02723552).

FINDINGS

Enrolment occurred between 30/06/2016-26/10/2018. Amongst the 203 randomized young adults (n = 102 in the intervention group; n = 101 in the control group), 178 (88%; n = 76 intervention group, n = 84 control group) completed 16-week follow-up and 160 (79%; n = 68 intervention group, n = 69 control group) completed 52-weeks follow-up. There were no group differences in awake systolic (0·0 mmHg [95%CI, -2·9 to 2·8]; P = 0·98) or awake diastolic ambulatory blood pressure (0·6 mmHg [95%CI, -1·4. to 2·6]; P = 0·58). Aerobic training increased peak oxygen uptake (2·8 ml/kg/min [95%CI, 1·6 to 4·0]) and peak wattage (14·2watts [95%CI, 7·6 to 20·9]) at 16-weeks. There were no intervention effects at 52-weeks follow-up.

INTEPRETATION

These results do not support the exclusive use of moderate to high intensity aerobic exercise training for blood pressure control in young adults.

FUNDING

Wellcome Trust, British Heart Foundation, National Institute for Health Research, Oxford Biomedical Research Centre.

Impaired myocardial reserve underlies reduced exercise capacity and heart rate recovery in preterm-born young adults

Aims 

We tested the hypothesis that the known reduction in myocardial functional reserve in preterm-born young adults is an independent predictor of exercise capacity (peak VO₂) and heart rate recovery (HRR).

Methods and results 

We recruited 101 normotensive young adults (n = 47 born preterm; 32.8 ± 3.2 weeks’ gestation and n = 54 term-born controls). Peak VO₂ was determined by cardiopulmonary exercise testing (CPET), and lung function assessed using spirometry. Percentage predicted values were then calculated. HRR was defined as the decrease from peak HR to 1 min (HRR₁) and 2 min of recovery (HRR₂). Four-chamber echocardiography views were acquired at rest and exercise at 40% and 60% of CPET peak power. Change in left ventricular ejection fraction from rest to each work intensity was calculated (EFΔ40% and EFΔ60%) to estimate myocardial functional reserve. Peak VO₂ and per cent of predicted peak VO₂ were lower in preterm-born young adults compared with controls (33.6 ± 8.6 vs. 40.1 ± 9.0 mL/kg/min, P = 0.003 and 94% ± 20% vs. 108% ± 25%, P = 0.001). HRR₁ was similar between groups. HRR₂ decreased less in preterm-born young adults compared with controls (−36 ± 13 vs. −43 ± 11 b.p.m., P = 0.039). In young adults born preterm, but not in controls, EFΔ40% and EFΔ60% correlated with per cent of predicted peak VO₂ (r² = 0.430, P = 0.015 and r² = 0.345, P = 0.021). Similarly, EFΔ60% correlated with HRR₁ and HRR₂ only in those born preterm (r² = 0.611, P = 0.002 and r² = 0.663, P = 0.001).

Conclusions 

Impaired myocardial functional reserve underlies reductions in peak VO₂ and HRR in young adults born moderately preterm. Peak VO₂ and HRR may aid risk stratification and treatment monitoring in this population.

Left atrial strain predicts cardiovascular response to exercise in young adults with suboptimal blood pressure

AIMS

To investigate the left ventricular response to exercise in young adults with hypertension, and identify whether this response can be predicted from changes in left atrial function at rest.

METHODS

A total of 127 adults aged 18-40 years who completed clinical blood pressure assessment and echocardiography phenotyping at rest and during cardiopulmonary exercise testing, were included. Measurements were compared between participants with suboptimal blood pressure ≥120/80mm Hg (n = 68) and optimal blood pressure <120/80mm Hg (n = 59). Left ventricular systolic function during exercise was obtained from an apical four chamber view, while resting left atrial function was assessed from apical four and two chamber views.

RESULTS

Participants with suboptimal blood pressure had higher left ventricular mass (p = 0.031) and reduced mitral E velocity (p = 0.02) at rest but no other cardiac differences. During exercise, their rise in left ventricular ejection fraction was reduced (p = 0.001) and they had higher left ventricular end diastolic and systolic volumes (p = 0.001 and p = 0.001, respectively). Resting cardiac size predicted left ventricular volumes during exercise but only left atrial booster pump function predicted the left ventricular ejection fraction response ( β = .29, p = 0.011). This association persisted after adjustment for age, sex, body mass index, and mean arterial pressure.

CONCLUSION

Young adults with suboptimal blood pressure have a reduced left ventricular systolic response to exercise, which can be predicted by their left atrial booster pump function at rest. Echocardiographic measures of left atrial function may provide an early marker of functionally relevant, subclinical, cardiac remodelling in young adults with hypertension.

Right ventricular morphology and function analysis in moderately preterm-born young adults

Background

Previous studies have shown that prematurity leads to altered right ventricular (RV) geometry and performance with persistent impairments in RV systolic function in young adulthood. It is unknown to what extent pulmonary physiology impacts these findings.

Purpose

To better quantify known alterations in RV morphology and function in preterm-born young adults and to determine to what extent these changes are influenced by the pulmonary circulation.

Methods

A total of 101 normotensive preterm-born (n=47, mean gestational age 32.8±3.2 weeks) and term-born (n=54) young adults were recruited. Echocardiography and cardiovascular magnetic resonance (CMR) imaging were performed to characterise RV morphology, RV function, pulmonary hemodynamics and RV-pulmonary arterial vascular (PA) coupling. CMR cine images were used to create a 3D computational atlas of the RV geometry and principal component analysis was undertaken to identify the key modes of shape variation. Spirometry was performed to assess lung function.

Results

RV CMR revealed a higher absolute and indexed RV mass (P&lt;0.05) and lower ejection fraction (54.90±5.17 versus 57.48±4.39%, P=0.008) in young adults born preterm. RV end-diastolic areas and volumes for CMR and echocardiography were lower in preterm-born compared to term-born young adults (P≤0.001). Principal component analysis of the computational atlas defined the anatomical modes of the RV geometry, with mode 1 accounting for 25.3% of the population variance. Preterm and term cohorts showed significant differences (P&lt;0.001) in mode 1, which represented a smaller and shorter RV cavity in the preterm group (Figure 1). Measurements of RV function by echocardiography, including RV fractional area of change (FAC) and tricuspid annular plane systolic excursion (TAPSE), were lower in preterm-born compared to term-born adults (P&lt;0.05). Despite lower pulmonary artery acceleration times (PAAT) in those born preterm (141.1±15.1 versus 159.2±21.6msec, P&lt;0.001), indicating increased pulmonary vascular resistance, the RV remained coupled to its pulmonary circulation (TAPSE/PAAT: 0.13±0.02 versus 0.14±0.03m/sec, P=0.153). Reduced RV performance in preterm-born individuals remained significant when adjusting for pulmonary function parameters (P&lt;0.05).

Conclusions

Multimodality cardiac imaging demonstrated that moderately preterm-born young adults exhibit structural and functional RV alterations, independent of lung physiology. Their RV remains hemodynamically coupled to its pulmonary circulation despite higher RV afterload, lower RV function and altered morphology.

Figure 1. Statistical shape model of the RV

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): British Heart Foundation

Physiological Stress Elicits Impaired Left Ventricular Function in Preterm-Born Adults

BACKGROUND

Experimental and clinical studies show that prematurity leads to altered left ventricular (LV) structure and function with preserved resting LV ejection fraction (EF). Large-scale epidemiological data now links prematurity to increased early heart failure risk.

OBJECTIVES

The authors performed echocardiographic imaging at prescribed exercise intensities to determine whether preterm-born adults have impaired LV functional response to physical exercise.

METHODS

We recruited 101 normotensive young adults born preterm (n = 47; mean gestational age 32.8 ± 3.2 weeks) and term (n = 54) for detailed cardiovascular phenotyping. Full clinical resting and exercise stress echocardiograms were performed, with apical 4-chamber views collected while exercising at 40%, 60%, and 80% of peak exercise capacity, determined by maximal cardiopulmonary exercise testing.

RESULTS

Preterm-born individuals had greater LV mass (p = 0.015) with lower peak systolic longitudinal strain (p = 0.038) and similar EF to term-born control subjects at rest (p = 0.62). However, by 60% exercise intensity, EF was 6.7% lower in preterm subjects (71.9 ± 8.7% vs 78.6 ± 5.4%; p = 0.004) and further declined to 7.3% below the term-born group at 80% exercise intensity (69.8 ± 6.4% vs 77.1 ± 6.3%; p = 0.004). Submaximal cardiac output reserve was 56% lower in preterm-born subjects versus term-born control subjects at 40% of peak exercise capacity (729 ± 1,162 ml/min/m2 vs. 1,669 ± 937 ml/min/m2; p = 0.021). LV length and resting peak systolic longitudinal strain predicted EF increase from rest to 60% exercise intensity in the preterm group (r = 0.68, p = 0.009 and r = 0.56, p = 0.031, respectively).

CONCLUSIONS

Preterm-born young adults had impaired LV response to physiological stress when subjected to physical exercise, which suggested a reduced myocardial functional reserve that might help explain their increased risk of early heart failure. (Young Adult Cardiovascular Health sTudy [YACHT]; NCT02103231).