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. 2000 Jul;84(1):46–52. doi: 10.1136/heart.84.1.46

Oxygen uptake versus exercise intensity: a new concept in assessing cardiovascular exercise function in patients with congenital heart disease

T Reybrouck 1, L Mertens 1, S Brusselle 1, M Weymans 1, B Eyskens 1, J Defoor 1, M Gewillig 1
PMCID: PMC1729398  PMID: 10862587

Abstract

OBJECTIVE—To assess the relation between exercise intensity and oxygen uptake during graded exercise in paediatric patients who underwent surgical repair of congenital heart disease, and to compare it with conventional measures of aerobic exercise function.
DESIGN—Cross sectional study. Exercise testing was performed on a treadmill and gas exchange was measured on a breath by breath basis.
PATIENTS—29 patients who underwent an atrial switch operation for transposition of the great arteries (TGA) (mean (SD) age at testing 10.3 (2.5) years) and 30 patients who underwent total repair of tetralogy of Fallot (TF) (age 12.1 (3.3) years) performed graded exercise testing. Exercise responses were compared with data obtained in 24 normal controls (age 11.4 (2.6) years).
RESULTS—The slope of oxygen uptake versus exercise intensity averaged 1.50 (0.64) ml O2/min2/kg in the patients with TGA and 1.68 (0.75) ml O2/min2/kg after TF repair, both lower (p < 0.005) than in normal controls (2.42 (0.68) ml O2/min2/kg). The lower slope of oxygen uptake was correlated with a subnormal value for ventilatory anaerobic threshold, which averaged 78.0 (13.3)% of normal in TGA and 85.1 (10.6)% in TF. This was associated with a steeper slope (p = 0.001) of carbon dioxide output versus oxygen uptake above the ventilatory anaerobic threshold in TGA (1.26 (0.20)) and TF (1.20 (0.18)) compared with the normal controls (1.05 (0.13)), and also a steeper slope of ventilation versus carbon dioxide in TGA (47.0 (15.4)) and TF (41.5 (13.7)) than in the controls (30.3 (8.5)).
CONCLUSIONS—Calculation of the steepness of the slope of oxygen uptake versus exercise intensity is a valid measurement of oxygen flow to the exercising tissues, which may be limited in congenital heart disease.


Keywords: congenital heart disease; exercise testing; oxygen uptake; oxygen uptake kinetics

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Figure 1  .

Figure 1  

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Typical example of the slope of carbon dioxide versus oxygen uptake during graded treadmill exercise in a 14 year old boy with atrial switch operation for transposition of the great arteries. The data represent average values for 10 second intervals. S1 (= 0.79) is the slope of carbon dioxide versus oxygen uptake below the ventilatory anaerobic threshold, and S2 (= 1.23) is the slope of carbon dioxide versus oxygen uptake above the ventilatory threshold up to a heart rate of 170 beats/min. V̇CO2 = carbon dioxide output, expressed as ml/min/kg; V̇O2 = oxygen uptake, expressed as ml/min/kg.

Figure 2  .

Figure 2  

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Relation between oxygen uptake and heart rate during graded exercise in patients with transposition of the great arteries and atrial switch (solid circles) and patients with repair of tetralogy of Fallot (solid squares), compared with normal controls (open circles). The data points represent mean values for the different patient groups.

Figure 3  .

Figure 3  

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Typical example of the slope of oxygen uptake v exercise intensity during graded treadmill exercise in a 14 year old normal control (open circles ) and in a 16 year old patient after repair for tetralogy of Fallot (solid circles). The data represent average values of oxygen uptake in 10 second intervals. Data collected during the first minute of exercise were omitted because of breathing irregularity at onset of exercise.

Figure 4  .

Figure 4  

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Slopes of oxygen uptake v exercise intensity (time) during graded treadmill exercise for normal controls (NL; open circles), patients after tetralogy of Fallot repair (TF; solid squares), and patients with transposition of the great arteries and atrial switch (TGA; solid circles). The slopes represent the mean value for each group compared for subsequent levels of exercise (inclination of the treadmill). Data collected during the first minute of exercise were omitted because of breathing irregularity at the onset of exercise. The data points represent average values and 95% confidence intervals of the mean for oxygen uptake (ml O2/min/kg).

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