Noninvasive positive pressure ventilation enhances the effects of aerobic training on cardiopulmonary function

Purpose The purpose of this study was to determine the effect of aerobic training under noninvasive positive pressure ventilation (NPPV) on maximal oxygen uptake (V˙O2max). Methods Ten healthy young male volunteers participated in the study. Before the training, stroke volume (SV) and cardiac output (CO) were measured in all subjects under 0, 4, 8, and 12 cmH2O NPPV at rest. Then, the subjects exercised on a cycle ergometer at 60% of pre-training V˙O2max for 30 min daily for 5 consecutive days with/without NPPV. The 5-day exercise protocol was repeated after a three-week washout period without/with NPPV. The primary endpoint was changes in V˙O2max. The secondary endpoints were changes in SV, CO, maximum heart rate (HRmax), maximum respiratory rate (RRmax), maximum expiratory minute volume (VEmax) and the percent change in plasma volume (PV). Results NPPV at 12 cmH2O significantly reduced SV and CO at rest. V˙O2max significantly increased after 5 days training with and without NPPV, but the magnitude of increase in V˙O2max after training under 12 cmH2O NPPV was significantly higher than after training without NPPV. VEmax significantly increased after training under NPPV, but not after training without NPPV. HRmax and RRmax did not change during training irrespective of NPPV. The percent change in PV was similar between training with and without NPPV. The 5-day training program with NPPV resulted in greater improvement in V˙O2max than without NPPV. Conclusions Aerobic training under NPPV has add-on effects on V˙O2max and exercise-related health benefits in healthy young men.

Introduction Good cardiopulmonary function is associated with health benefits; a lower risk of all-cause mortality [1][2][3] and a higher physical work capacity [4,5]. Cardiopulmonary function is often tested to assess fitness, development and appraisal of exercise and training programs. Thus, assessment of cardiopulmonary function is of interest to researchers and clinicians alike. Maximal oxygen uptake ( _ VO 2max ) is considered the criterion measure of cardiopulmonary function [6]. First established by Hill and Lupton [7], _ VO 2max represents the integrated response of the cardiovascular, respiratory and muscular systems to take up, distribute and utilize oxygen during exercise to volitional exhaustion and is one of the most widely used diagnostic tests for both athletic and clinical population groups [8][9][10]. Estimates of _ VO 2max obtained using maximal exercise protocols are typically based on a performance measure such as time or distance covered [5,[11][12][13] or in cycle ergometer and peak work rate [14].
The _ VO 2max is reduced by prolonged bed rest. Saltin et al. [15] reported that _ VO 2max in 5 healthy 20-years-old men was reduced by an average of 28% after three weeks of bed rest. Long-term bed rest conditions increase the stroke volume (SV) and cardiac output (CO) due to increased venous return from the lower body. The persistent increase in SV and CO induces a decrease in plasma volume (PV) and causes cardiac atrophy, with subsequent fall in _ VO 2max [16]. The circulatory condition induced by application of negative pressure to the lower parts of the body while in supine position, mimics the fall in venous return during upright posture with low SV and CO associated with the effects of gravity. Watenpaugh et al. [17] demonstrated that daily supine lower body negative pressure (LBNP) treadmill exercise at 41-65% of _ VO 2max during 15 days of bed rest can preserve peak _ VO 2 at pre-bed rest levels. Noninvasive positive pressure ventilation (NPPV) is a non-invasive treatment used for patients with sleep apnea syndrome and chronic obstructive pulmonary disease [18][19][20]. NPPV is delivered through a nose/full-face mask instead of endotracheal intubation. In addition to its effect on the respiratory system, NPPV also alters the cardiovascular-circulatory system and including falls in SV and CO [21].
Based on the above background, we hypothesized that aerobic training under NPPV improves the cardiopulmonary function, compared with aerobic training alone. The primary outcome of the present study was changes in _ VO 2max . The purpose of this study was to determine the effects of NPPV under rest conditions on circulatory status, and the effects of the combination of NPPV and aerobic training on cardiopulmonary function, including _ VO 2max .

Aerobic training
In this cross-over design study, aerobic exercise was performed with and without NPPV (mode; CPAP, at inspired oxygen concentration [FiO 2 ] of 21%). Subjects performed on a cycle ergometer exercise in an upright position at 60% of pre-training _ VO 2max for 30 min daily for 5 consecutive days either with or without NPPV. The exercise protocol was repeated with the alternate combination of NPPV after a three-week washout period. NPPV was used in random order among the participating subjects. The exercise was performed at 1700-1900 in an airconditioned room with the temperature set at 28˚C. HR was continuously monitored during the exercise. BP and Borg Scale were measured before and at the end of the exercise on the first day of training with and without NPPV. Subjects were not allowed to drink any fluid during exercise.

Measured variables
_ VO 2max , maximum heart rate (HR max ), maximum respiratory rate (RR max ) and maximum expiratory minute volume (VE max ) were measured 24 hour before the first training (baseline) and 24 hour after last training (post-training). _ VO 2max , HR max , RR max and VE max were measured with graded exercise using a cycle ergometer in an upright position. _ VO 2 , HR, RR and VE were monitored continuously by expiration gas analyzer (Aeromonitor AE300S; Minato, Tokyo). After baseline measurements at rest for 3 min, the subject started pedaling at 60 cycles/min without load. The exercise intensity was increased by 50 W every 3 min to 150 W and, higher than this intensity, by 20 W every 1 min until exhaustion. _ VO 2max , HR max , RR max and VE max were determined by averaging the three largest consecutive values at the end of exercise. The ergometer seat and handlebar heights were recorded for each individual subject during the baseline measurements and were used during the post-training measurements. Resting systolic blood pressure (mmHg) 116.3±7.1 Resting diastolic blood pressure (mmHg) 65.4±8.6 Resting mean blood pressure (mmHg) 82.4±7.6 Data are mean ± SD. https://doi.org/10.1371/journal.pone.0178003.t001 Blood samples were collected at baseline and post-training from the antecubital vein using heparinized tubes, to measure hemoglobin and hematocrit. The percent change in PV was calculated from the hematocrit and hemoglobin concentrations using the following equation: ΔPV (%) = 100 × (Hb post /Hb C ) × {[1 -(Hct C /100)]/[1 -(Hct post /100)]} − 100, where ΔPV is the percent change in PV, Hb C is baseline hemoglobin concentration, Hb post is post-training hemoglobin concentration, Hct C is baseline hematocrit, and Hct post is post-training hematocrit [22].

Statistical analysis
Differences in SV, CO, BP and HR during different NPPV values recorded in supine position were analyzed by one-way repeated measures analysis of variance followed by Tukey-Kramer's test. The Student's paired t-test was used to examine for differences between before and after exercise, pre-and post-training, and training under NPPV and without NPPV for each parameter. Data were expressed as mean±SD. A P value <0.05 was considered statistically significant. All statistical analyses were performed using statistical analysis software (Graph Pad Prism 6). We calculated the statistical power and the appropriate sample size to detect significant differences that need to be observed in this study. The statistical power was 61.6%, and the necessary sample size was 10 samples.

Discussion
The followings are the major two findings of present study; 1) 5-day aerobic training (ergometer exercise in an upright position at 60% of pre-training _ VO 2max for 30 min/day) significantly increased _ VO 2max with and without NPPV, but the magnitude of increase was significantly higher with 12 cmH 2 O NPPV than without NPPV in healthy young males. 2) NPPV of 12 cmH 2 O significantly reduced SV and CO at rest. These findings suggest that NPPV at 12 cmH 2 O can reduce SV and CO, and that the same NPPV can further enhance the cardiopulmonary beneficial effects of aerobic training.
Positive pressure ventilation (PPV) with PEEP decreases SV and CO, but not BP and HR [23]. The major mechanism of SV and CO reduction is a decrease in venous return to the right heart secondary to increased intrathoracic pressure [24][25][26]. Our study also demonstrated that NPPV of 12 cmH 2 O reduced SV and CO at rest but did not change BP or HR.
_ VO 2max is calculated by multiplying maximal cardiac output by maximal arterial-venous O 2 difference ( _ VO 2max = CO max × a-vO 2 diff max) [9,27]. In addition, CO max is also calculated by multiplying maximal SV (SV max ) by HR max . Research suggests that vigorous aerobic training (60-84% _ VO 2max ) results in a significant increase in cardiopulmonary function [28]. Moreover, vigorous aerobic exercise also increases SV by increasing blood volume and strength of cardiac contraction, leading to improvement in _ VO 2max [9,29]. In the present study, 5-day vigorous aerobic exercise (cycle ergometer exercise at 60% of pre-training _ VO 2max for 30 min/day) also significantly increased _ VO 2max with and without NPPV. In addition, the same program increased PV, but not HR max . We assume that the increased _ VO 2max after training with and without NPPV was probably induced by increases in blood volume and strength of cardiac contraction.
In the present study, the magnitude of increase in _ VO 2max after the 5-day aerobic training at 12 cmH 2 O NPPV was significantly higher than that during the same length aerobic training without NPPV, though HR max did not increase after either of the two protocols. As described above, _ VO 2max is estimated by multiplying SV max by HR max and maximal arterial-venous O 2 difference ( _ VO 2max = SV max × HR max × a-vO 2 diff max). The results suggest that the increases in SV max and/or a-vO 2 diff max after training under NPPV could be larger than after training without NPPV.
The 5-day aerobic training significantly increased VE max under NPPV only but not under the control condition. VE represents the product of tidal volume multiplied by respiratory frequency. Because the fastest respiratory rate is limited, any increase in VE max is considered as a function of tidal volume, i.e., improvement in respiratory muscle contraction [30]. Resting ventilation is achieved by the contraction of inspiratory muscle activity with little or no expiratory muscle activity. During exercise, the associated hyperventilation involves increased inspiratory and expiratory muscle activities [31][32][33]. Respiratory muscle activity plays an important role in ventilatory control and plays an important role in respiratory response during exercise https://doi.org/10.1371/journal.pone.0178003.g004 [34]. Previous studies reported that respiratory muscle training using respiratory resistance increased VE during exercise as well as respiratory muscle strength [35]. In the present study, the increase in VE max after the 5-day aerobic training under NPPV could probably include NPPV-related increase in expiratory resistance. The increase in VE max probably improved alveolar ventilation volume, and increased a-vO 2 diff max. Therefore, the larger increase in _ VO 2max after the 5-day aerobic training under NPPV compared with without NPPV could be related to improvement in alveolar ventilation volume and a-vO 2 diff max.
Aerobic training increased PV. The latter contributes to the increase in SV max and _ VO 2max after aerobic training [9,29]. In the present study, the 5-day aerobic training also increased PV, and the percent increase in PV was not influenced by NPPV. Therefore, the larger increase of _ VO 2max after 5-day aerobic training under NPPV compared with no NPPV is probably not due to changes in PV.
Several investigators reported that aerobic training without NPPV improves the strength of cardiac contraction and increases both SV max and _ VO 2max [9,29]. The pulmonary capillarywedge pressure is similar to left ventricular end-diastolic pressure during 10 cmH 2 O PEEP or less, but left ventricular end-diastolic pressure was decreased by over 10 cmH 2 O PEEP [36]. The decrease in left ventricular end-diastolic pressure probably explains the reductions in SV and CO. In the present study, during aerobic exercise with 12 cmH 2 O NPPV, the absolute value of BP was maintained and end-diastolic pressure of the left ventricle should decrease. Thus, the total pressure production by the myocardia during exercise under NPPV should be greater than without NPPV. We assumed that the relative afterload in the myocardia would increase and the cardiac workload should be elevated during exercise with NPPV. Based on these suggestions, exercise under 12 cmH 2 O NPPV could increase cardiac stress, with resultant improvement in cardiac contraction. The larger increase in _ VO 2max after 5-day aerobic training under NPPV (compared to under no NPPV) would be at least in part due to improvement in the strength of cardiac contraction.
Middle-aged and elderly people with cardiopulmonary dysfunction, low _ VO 2max and obesity are prone to develop adult-related diseases, e.g., diabetes mellitus, cardiovascular disease and dyslipidemia [37,38]. Therefore, exercise improves _ VO 2max and is important in preventing the development of adult diseases in middle-aged and elderly people. In the present study, aerobic exercise training under NPPV further improved _ VO 2max . Aerobic exercise training under NPPV could be beneficial clinically in preventing adult-related diseases. On the other hand, cardiopulmonary function is poor in individuals with physical disabilities, e.g., spinal cord injury, partly due to low physical activities of daily living [39]. Furthermore, cardiopulmonary function in astronauts during space flight is reduced due to microgravity, and prevention of such reduction is important in the field of space medicine [40]. We believe that aerobic exercise training under NPPV can prevent cardiopulmonary dysfunction in disabled people and astronauts.
The present study has certain limitations. We could not measure directly differences in SV max between aerobic training under NPPV and no-NPPV due to technical difficulty. Because of this, we could not directly compare the difference in the present study. Moreover, the subjects included in the present study were healthy young men, and the results may not be applicable to children, women and elderly people. Further studies are needed to measure SV max directly after overcoming these technical difficulties, and also to evaluate the response to different conditions of exercise stress and length, as well as the response in females and males of different age groups.
In conclusion, the present study examined the effects of aerobic exercise training under NPPV as a new endurance training method. The results showed that 5-day aerobic endurance training at 60% of pre-training VO 2max for 30 min/day under NPPV resulted in greater improvement of VO 2max than training without NPPV in healthy young men. The results suggest that aerobic exercise training under NPPV has an add-on effect on VO 2max and exerciserelated health benefits in healthy young men.
Supporting information S1 File. Raw data of the present study. (DOCX)