INTRODUCTION
Obesity is a metabolic problem. Its prevalence continues to increase in the developed world, and is reaching almost epidemic proportions.1,2 Chronic obesity is associated with an increased left ventricular mass3 and with high cardiovascular morbidity and mortality.4,5 Its effects on cardiac function, however, are still controversial. While some authors describe alterations of systolic6,7 or diastolic8,9 function, others indicate cardiac function to be normal.10,11
The cardiopulmonary exercise test offers objective measurements of functional capacity and cardiac reserve. Several studies have evaluated the exercise capacity of obese patients, but results have been contradictory. Some authors believe obese people to have a cardiopulmonary response within normal limits, but that their exercise capacity is compromised by the large body mass they have to carry.12,13 Others indicate that they have reduced aerobic capacity compared to people of normal weight, their fat mass interfering with cardiac and pulmonary function and limiting their aerobic response to excescise.14-18 Some of the discre-pancies in the results of these studies might be due to the different methodologies employed, and because they examined different populations with different ages and degrees of obesity.
Using treadmill exercise test and gas analysis, the present cross-sectional study prospectively analyzed cardiopulmonary functional capacity in a group of patients with morbid obesity (MO), and in a group of healthy, volunteer controls of normal body weight.
PATIENTS AND METHODS
Study group
The study subjects were 55 patients of both sexes, all of whom suffered OM. All were receiving treatment at the Nutritional Disorder Unit of our hospital and were included in a bariatric surgery program. MO was defined as a having a body mass index (BMI) equal to or greater than 40 kg/m². All patients had suffered obesity for more than 15 years. In nine patients, exercise tests could not be performed because of the physical difficulty experienced in walking. The BMI of these latter patients was significantly greater than those who were able to undergo exercise tests (BMI 57.7 ± 10 compared to 50 ± 10 kg/m²; P<.001). Of those who did undergo this testing, 15 were excluded because of high blood pressure (their results were, however, reserved for future studies). This was to try to control for the potential negative influence that this variable might have on cardiac function. Thirty one MO patients with normal blood pressure therefore made up the final sample (56% of the initial population).
The control group was made up of 30 healthy, normotensive volunteers of normal body weight (BMI<27 kg/m²). These were recruited from the patients' families and from among healthcare personnel via advertisement of the study. Pairing with the OM group was performed on the basis of age (± 5 years) and height (± 5 cm). Before starting, all participants underwent physical examination and a normal 12 lead electrocardiogram. None of the participants practiced sport regularly nor did they take any type of medication that might interfere with the exercise test results.
The study protocol was assessed and accepted by the Clinical Trials and Research Committee of our hospital. All participants received detailed information about the aim of the study and the methods to be used. They all provided written consent to participate.
Cardiopulmonary exercise test
A symptom-limited cardiopulmonary exercise test (Enraf Nonius Holland ergometer) with analysis of respiratory gases was performed at least 3 h after breakfast. After trying several different tests with a patient weighing 244 kg, an appropriate experimental protocol (a modification of Balkes protocol19) was designed. The belt speed and the gradient settings were: stage 12.5 km/h, 0%; stage 22.5 km/h, 2%; stage 32.5 km/h, 4%; stage 42.5 km/h, 6%; stage 52.5 km/h, 8%; stage 6-3 km/h, 10%; stage 7-3 km/h, 12%; stage 83 km/h, 14%; stage 93 km/h, 16%; stage 103 km/h, 18%; stage 113.5 km/h, 20%; stage 123.5 km/h, 22%; stage 133.5 km/h, 24%; stage 143.5 km/h, 25%. From this point on, both belt speed and gradient were held constant. Each stage lasted 2 min. The patients were asked to keep going until they could continue no longer.
Cardiac frequency (CF) was monitored by continuous electrocardiographic recording. Blood pressure was monitored at the beginning of the test and every 2 mi-nutes thereafter, during both exercise and recovery, using a sphygmomanometer attached to the arm. An ergospirometer (Mintjarth 4, Holland) with a Hans Rudolf one-way mask was used for the analysis of gases expired during rest and exercise. Tidal volume (TV, in mL), breathing frequency (BF, in breaths per min), ventilation per minute(VE, in L/min), oxygen consumption O2 (VO2, in mL/min), carbon dioxide production (VCO2, in mL/min) and respiratory quotient (RQ = VCO2/VO2) were measured every 30 s. Before each session, the system was calibrated using standard gases with known O2 and CO2 concentrations. The metabolic equivalent (ME) is a unit of oxygen consumption at rest with the subject sitting20 (3.5 mL of O2 per kg of body weight per min [mL/kg/min]).
The efficiency of the cardiovascular system during exercise was evaluated by the O2 pulse (the amount of O2 consumed during a complete cardiac cycle; calculated by dividing O2 consumption by the cardiac frequency[VO2/CF]). If an individual's VO2 is expressed according to the principle of Fick:21
If cardiac usage is equal to the stroke volume multiplied by the CF, then O2 pulse equals the stroke volume multiplied by the arterio-venous difference in O2. Given that during exercise this difference has a physiological limit20 of 15-17 vol/%, if a large physical effort is made then the O2 pulse allows the behavior of the stroke volume to be evaluated.
The maximum theoretical cardiac frequency (MTCF) is calculated using the algorithm
MTCF and RQ were used to determine the effort made.22
Body densitometry
Total body mass, fat mass and lean mass were measured by dual densitometry with an x-ray source using a Lunar Prodigy densitometer (Lunar Corp., Madison, WI, USA). Precision controls were performed daily using an external calibrator. The margin of error for total body mass was 1%.
Statistical analysis
Categorical variables were expressed as percentages. Quantitative values were expressed as mean ± SD. The Student t test was used to compare means, and Persons χ² test to assess gender proportions. The influence of obesity was studied by repeated measures analysis of variance (RM ANOVA). To compare the O2 pulse between the groups, the 25th, 50th and 75th percentiles were calculated for each subject, as well as baseline and maximum values. RM ANOVA was used to test the hypothesis that VO2, VE, CF and systolic blood pressure (SBP) vary differently throughout the exercise test in patients and in controls. Significance was set at P<.05. All analyses were performed using SPSS statistical software (version 10.0.6) for Windows.
RESULTS
Baseline characteristics
No differences were seen between the groups with respect to age, sex or height. Patients with MO had significantly greater weight, BMI, and lean and fat masses (Table 1). Table 2 shows the results for the parameters recorded at rest and during maximum effort. When baseline and maximum values are taken into account, ventilation patterns (BF, TV and VE) were no different between groups. Although under baseline conditions the SBP was significantly higher in the patients, the maximum SBP reached by both groups was similar.
The duration of exercise endured by the patients was shorter than that endured by the controls (14 ± 3 compared to 27 ± 4 min; P<.001). The distance patients traveled was therefore much shorter (661 ± 175 m compared to 1.363 ± 290 m; P<.001) (Table 2).
Variables during exercise
Important differences were seen in the behavior of the CF, SBP, VO2 and VE curves (Figure 1). From the beginning, and throughout exercise, the patients showed marked increases in the values of these variables. Compared to the control group, this determined an upward shift of their curves steeper slopes. This reflects the patients' greater energy consumption. After 4 min of exercise walking at 2.5 km/h on a 2% gradient (Table 3), the patients reached 75% of their maximum CF, 86% of their maximum blood pressure and 58% of their peak VO2, whereas control subjects had only reached 57, 75 and 34% respectively. After 14 min of exercise, when the patients had all ended the test through exhaustion, they were consuming 2.17 L/min of O2-almost double that seen in the controls (1.12 L/min) (Table 3). Since the abscissa represents time, the graphs for all these variables were shorter in patients, corresponding to the shorter duration of their tests (Figure 1).
Fig. 1. Behavior of the different variables during exercise. From the outset, the patients showed higher values in general than those of controls. CF indicates cardiac frequency; SBP, systolic blood pressure; VE, ventilation per minute; VO2, oxygen consumption; beats/min, beats per minute. Points are means, bars are standard errors.
Baseline and final VO2 were higher in the patients (Figure 1 and Table 2). In both situations, if VO2 is corrected for body weight, the relationship inverts and VO2 becomes much greater for the control group (Table 2). However, VO2 per kg of lean mass was the same in baseline conditions in both groups and, although the maximum was slightly lower in the patients, no significant differences were seen between the two groups during exercise.
The baseline, maximum (Table 2) and in-exercise O2 pulse values of patients were significantly greater than those of the controls. This variable was always higher in the patients whether comparisons were made for the 25th, 50th or 75th percentiles, at rest, or at the point of maximum effort (P<.001) (Figure 2). However, when O2 pulse was calculated after correcting for VO2 for lean body mass, the differences between the groups disappeared (Figure 2).
Fig. 2. The upper figure compares the O2 pulse values (VO2/CF) of the two groups using a box chart. Values for the patient group are much higher (P<.001). The lower figure shows how these differences disappear when O2 pulse is calculated after having corrected VO2 for lean body mass (VO2/kg lean body mass/CF)(NS). CF indicates cardiac frequency; VO2, oxygen consumption.
When exercise was finished, the controls had reached 95% of their MTCF, and their RQ was 1 (Table 2); therefore the effort made by these subjects was practically their maximum. When the patients reached the end of exercise, however, they had only reached 86% of their MTCF and their RQ was 0.87 (Table 2); therefore, they had not reached the limit of their cardiopulmonary capacity and their effort was sub-maximum. The gradients of the patients' VO2 and CVO2 curves during exercise were almost parallel (Figure 3); the expected increase in CO2 production with increased O2 consumption--seen in the control group--did not occur (Figure 3).
Fig. 3. The upper figure shows that throughout exercise, the VCO2 of patients is lower than their VO2. This suggests that their effort was not maximum. The lower figure shows how, in the control group at the end of effort, VCO2 was greater than VO2. Points represent the mean; bars are standard error. L/min indicates liters per minute; VCO2, production of carbon dioxide; VO2, oxygen consumption.
DISCUSSION
The patients endured the exercise test for much shorter times than the controls-the former therefore covered only half the distance achieved by the latter. As soon as effort began, the patients had higher CF, SBP, VO2 and VE levels (Figure 1), showing them to consume more energy from the beginning of exercise. This might be needed to move their much heavier bodies.23 When walking at 2.5 km/h and with only a very slight gradient, the patients had already reached 58% of their maximum VO2. In contrast, the controls had only reached 34%. These results agree with those reported by other authors.18,23,24 For these patients, a simple walk therefore exacted a metabolic output much greater than that required of the normal weight controls. After 14 minutes of exercise, when the patients could no longer continue the test (and their effort ended), the controls had consumed only 5 ME, i.e., the VO2 needed to perform the basic activities of daily life.20
Although the majority of authors agree on the limitation of effort by obese people, controversy remains with respect to their cardiopulmonary capacity. Some authors consider it to be normal12,13 while others believe it to be affected.14-18 The patients in the present study showed higher O2 pulse rates during exercise. Bearing in mind that O2 pulse depends on stroke volume and the arterio-venous difference in O2,25 and given that during maximum exercise the latter is similar in obese and normal weight people,26 the higher O2 pulse values of the patients must correspond to a greater stroke volume.27 This res ponse has also been described in people who practice top level sport.28 For this reason, it is indicated by some that obese people are physically more able because of the training that carrying their excess weight provides.29 On the contrary, when the stroke volume is incapable of increasing in response to exercise, the O2 pulse is low.30
The controversy surrounding cardiopulmonary response to exercise in obese people stems from the lack of agreement on how to compare populations with different body sizes. When the absolute VO2 of different populations with different weights is compared, their is wide consensus that the heaviest individuals will have the greatest O2 consumption. But if VO2 is corrected for body weight, those who are obese show much lower values. This criterion has been used to argue that their cardiopulmonary functional capacity is deficient.14,18 Howe-ver, the normalization of variables by weight for obese people has been criticized by several authors for not taking into account the different metabolic needs of the various body tissues.31,32 Recently, it has been suggested that lean body mass might be a better variable to use since it is metabolically very active and correlates strongly with VO2.12,33 In the present study, when the O2 pulse of the two groups is compared after correcting VO2 for lean mass (Figure 2), the differences between the groups disappear. This supports the idea that cardiopulmonary capacity is similar in both groups, and, therefore, normal. The small capacity the patients showed for exercise is due to the high metabolic cost of their daily life activities. Their greater O2 consumption is insufficient to compensate for the overload of their fat mass, as shown by their low VO2 per kg body weight figures (Table 2).
RQ is equivalent to the carbon dioxide produced divided by the oxygen consumed. At high levels of exercise, the production of CO2 is greater than VO2 and, therefore, the RQ is greater than 1. This is one of the parameters used to determine the level of effort.20 Reaching the MTCF is another indicator of having reached the limit of cardiovascular capacity. In the patients, the production of CO2 throughout the test was always lower than O2 intake, and their RQ at the end of exercise was below 0.9 (Figure 3). Further, only 86% of MTCF was reached. Therefore, the patients finished their effort without having reached the maximum limit of their cardiopulmonary capacity. The present study does not allow us to determine whether this is due to a subjective sensation of poor tolerance to effort,34 the incapacity to perform functions in anaerobiosis,35 or an alteration in pulmonary fucntion36,37. Hulens18 obtained the same results--in that particular study, only 18% of patients ended their effort due to skeletomuscular discomfort.
Limitations of the study
Since only 56% of the original obese population was studied, it could be argued that the present results are biased since the least affected subjects were those chosen. However, those who were analyzed were a wide selection and showed an acute degree of obesity.
Since the patients did not reach their maximum cardiopulmonary capacity and only managed a sub-maximum effort, this study does compare two groups with different effort levels. In any event, the patients showed normal cardiopulmonary capacity for the effort they made.
CONCLUSIONS
The patients finished the test only having made a sub-maximum effort. Despite this, they showed cardiopulmonary capacity within the normal limits for the effort made. After correcting VO2 for lean body mass, the O2 pulse of the patients was no different from that of the normal weight controls. However, as soon as exercise began, the patients showed high energy consumptionnecessary to move their large mass. This metabolic cost determines the reduced exercise capacity they suffer, as reflected in the short duration of their tests.
Este estudio forma parte de un proyecto de investigación aprobado por el FIS (expediente 99/1021), con el título: Alteraciones de la anatomía y función cardíaca en pacientes con obesidad mórbida. Modificaciones tras la pérdida ponderal secundaria a cirugía bariátrica.
Correspondence: Dr. L. Serés García.
Servicio de Cardiología. Hospital Universitario Germans Trias i Pujol.
Carretera de Canyet, s/n. 08916 Badalona. Barcelona. España.
E-mail: seres@hugtip.scs.es
