Abstract
The effect of high altitude (HA) on exercise-induced diaphragm fatigue in normal subjects was examined.
Eight normal subjects completed an incremental exercise test at sea level (SL) and at 3,325 m. Before (baseline), during, and after exercise (recovery), maximal transdiaphragm pressure (Pdi,sniff), breathing pattern, and diaphragmatic effort (PTPdi) were measured. Arterialized blood lactate was measured at baseline and during recovery.
At maximal exercise (WRmax) Pdi,sniff fell to 72% and 61% of baseline at SL and HA respectively, recovering to baseline in 60 min at SL, and >60 min at HA. At the 5th min of recovery, circulating lactate was six-fold and seven-fold baseline at SL and HA, respectively. The time course of circulating lactate recovery was as for Pdi,sniff. At WRmax PTPdi was 80.74±9.87 kPa·s−1 at SL and 64.13±8.21 kPa·s−1 at HA. HA WRmax compared to isowork rate, SL data showed a lower Pdi,sniff (8.90±0.68 versus 11.24±0.59 kPa) and higher minute ventilation (117±11 versus 91±13 L·min−1), PTPdi being equal.
To conclude, in normal subjects hypoxia-related effects, and not an increase in diaphragm work, hastens exercise-induced diaphragm fatigue and delays its recovery at high altitude compared to sea level.
Exercise limitation at high altitude (HA) has been extensively studied using sophisticated techniques (Swan-Ganz catheterization, inert gas diffusion, etc.) 1, 2. However, data concerning respiratory muscle function are scanty nevertheless, exercise on either cycle-ergometer or treadmill at sea level (SL) has been shown to produce diaphragm fatigue in healthy subjects 3– as measured with volitional (Mueller and sniff) 3, 4, 6 and objective (bilateral phrenic nerve stimulation; BPNS) manoeuvres 3–. Therefore, the aim was to investigate exercise-induced diaphragm fatigue at HA. HA might well resemble normobaric hypoxia which is known to hasten diaphragm fatigue during exercise 3. However, at HA the density of air decreases, resulting in decreased airway flow resistance 7 and reduced work of breathing. The authors sought to identify possible different physiological responses to exercise at HA compared to normobaric hypoxia 3.
Methods
Subjects
Eight healthy male subjects (age 38±2 yrs) born and living at SL, not acclimatized to HA, gave their consent to perform the study.
Measurements
Flow (V') was measured with a mass flow sensor (Sensormedics, Yorba Linda, CA, USA) and volume (V) was obtained by integrating the flow signals. The mass flow sensor was calibrated with a 3 L syringe before each test.
Oesophageal (Poes) and gastric (Pga) pressure changes were measured as previously reported 8. Airway opening pressure (Pao) was measured via a side-port placed on a two-way breathing valve (720068, Jaeger, Würzburg, Germany) connected to the mass flow sensor. Transpulmonary (Pl) and transdiaphragmatic (Pdi) pressures were obtained by subtracting Poes from Pao and Pga, respectively. Pl, V' and V data were used to calculate dynamic lung elastance (EL,dyn) and lung resistance (Rl) 9.
Maximal oesophageal and transdiaphragmatic pressures were measured with the Mueller 10 (Poes,max and Pdi,max, respectively) and the sniff 10 (Poes,sniff and Pdi,sniff, respectively) manoeuvres. End-expiratory lung volume (EELV) before sniffs was monitored using end-expiratory Poes 11, 12. To assess diaphragmatic length, Pga was continuously monitored 11. Criteria for accepting and correcting Poes,sniff for abdominal muscle contractions (Poes,sniff corr) were those described by Kyroussis et al. 12. The sniff showing the greatest pressure deflection in each condition and fulfilling all criteria 12 was selected for analysis. Figure 1⇓ shows a representative record of sniffs during the protocol.
Representative recordings of maximal inspiratory pressures measured with the sniff manoeuvre in subject 6 at SL. a-d) maximal inspiratory oesophageal (Poes,sniff), e-h) gastric (Pga,sniff) and i-l) transdiaphragmatic (Pdi,sniff) pressures, measured at baseline (a, e, i), maximal exercise (b, f, j), and at the 1st min (c, g, k) and 60th min of recovery (d, h, l).
To estimate the diaphragmatic and respiratory muscle effort, the pressure-time product for diaphragm (PTPdi) and respiratory muscles (PTPoes) over 1 min were computed 8. Since diaphragm external work rate (W'di) has been shown to be a better index of diaphragm effort than PTPdi when inspiratory flow is not constant 13, the former was also computed, by multiplying PTPdi by the actual mean inspiratory flow 13.
Oxygen consumption (V'O2), and carbon dioxide production (V'CO2) were measured breath-by-breath (Vmax29; Sensormedics, Yorba Linda, CA, USA). Gas analysers were calibrated before each test. Heart rate (HR) was measured from a 12-lead electrocardiogram.
Arterialized venous blood 14 was obtained to estimate Pa,CO2 (ABL 300; Radiometer, Copenhagen, Denmark) and measure lactate (2300 STAT; Yellow Spring Instrument, Yellow Spring, OH, USA). Pa,CO2 and barometric pressure (Pb) were used to calculate the alveolar oxygen partial pressure (PA,O2) 15. A finger oximeter (BIOX 3760; Datex-Ohmeda, Division Instrumentarium Corp., Helsinki, Finland) measured arterial O2 saturation (Sa,O2).
Protocol
Measurements were performed at baseline, during an incremental exercise test to volitional tolerance on cycle-ergometer (Ergo-metrics 800S; Sensormedics, Yorba Linda, CA, USA) and during the hour after exercise. The protocol was performed both at 3,325 m of altitude (Rifugio Torino, Monte Bianco, Italy) and at SL. At HA, Pb and resting PA,O2 were 66.93±0.14 and 7.60±0.27 kPa respectively. At HA, the subjects were studied within a few hours of the ascent (cableway), sitting on the cycle-ergometer with the thorax kept in the same position throughout the study.
Baseline measurements
Before exercise, spirometry showed normal values 16 in every instance. The sniff and Mueller manoeuvres were practised until both appeared reproducible (±5%) and fulfilled all inclusion criteria. Then subjects started to breathe through the mouthpiece until they felt comfortable with the measurement apparatus. At this point, breathing pattern was recorded for 1 min, followed by measurements of HR, blood samples, and maximal inspiratory pressures.
Exercise
After 3 min of unloaded pedalling, the work rate (WR) was increased by 20–30 watts·min−1 to volitional tolerance and/or until maximal predicted HR (220 - age) was reached 15. Incremental loads differed between subjects to keep the exercise within 10 min 15. In the last 30 s of unloaded pedalling and between the 30th and 45th second of every minute during exercise, V'O2, V'CO2, HR and breathing pattern were measured; the last 15 s of each minute were used to perform sniffs (mouthpiece off, mouth closed). Sa,O2 was measured throughout exercise.
Recovery
After exercise, the breathing pattern and sniffs were recorded each minute for the first five min and at the 10th, 15th, 30th, 45th and 60th min. The Mueller manoeuvre was performed and arterialized blood samples were obtained starting from the fifth min.
Additional experiments
On a separate day, four subjects repeated the exercise at SL using cervical magnetic stimulation (CMS) to obtain twitch maximal transdiaphragmatic pressure (Pdi,tw) and the degree of voluntary neural activation during maximal voluntary efforts by the twitch occlusion technique 17. Right and left diaphragmatic electromyograms (EMEdi) were obtained using surface electrodes (Ref. 9013L0202; Dantec Medical, Tonsbakken, Denmark) connected to an electromyograph (Mystro+ MS20, Medelec, Woking, Surrey, UK) 18, amplified and band pass filtered (band width 20 Hz–5 kHz).
CMS was performed by a Magstim 200 stimulator equipped with a circular (doughnut shaped) 90 mm coil with a maximum magnetic field of 2.3 Tesla (Magstim, Whitland, Dyfed, UK) 18, 19. Stimulation amplitude was 100% of the maximal output of the stimulator. Supramaximal stimulation was verified according to Similowski et al. 19. Stimuli were delivered at end-expiration with the airways closed.
Pdi,tw and, consecutively, twitch occlusion were performed firstly 10 min after end-exercise to avoid twitch potentiation, and then after 60 min of recovery.
Data and statistical analysis
All data are reported as mean±sem. A paired t-test 20 was used to compare data, i.e. maximal WR (WRmax), iso-WR data at SL and HA, and for comparison between Pdi,max and Pdi,sniff. One-way analysis of variance (ANOVA) 20 was used for determination of differences in mean values for Pdi,sniff, lactate and V'E during baseline, exercise and recovery, and, when allowed by the F-value, the significance between measures was computed using Fisher's Protected least significant difference (PLSD) test. Significance was set at p<0.05.
Results
Sa,O2, Pa,CO2, and pH data are shown in table 1⇓. At SL, pH became acidotic at WRmax despite the marked hyperventilation (decrease in Pa,CO2). Sa,O2 remained constant throughout exercise. At HA, all subjects showed hypocapnia and hypoxaemia (Sa,O2 <90%) at baseline; both increased at maximal exercise.
Percentage of O2 saturation of arterial blood (Sa,O2) as estimated by pulse oximetry, arterialized blood pH and CO2 partial pressure (Pa,CO2), dynamic lung elastance (EL,dyn), and pulmonary resistance (RL) for normoxic (sea level; SL) and hypoxic (high altitude; HA) exercise
Pdi,sniff and Poes,sniff were compared to Pdi,max and Poes,max, respectively, and no difference was found. Therefore only data based on the sniff manoeuvre is reported.
Maximal exercise
Table 2⇓ shows WR, metabolic parameters, HR, and breathing pattern at maximal exercise. WRmax was similar at SL and HA, even though three subjects reached a lower WR at HA. At SL the subjects reached predicted maximal V'O2 (V'O2,max) and HR (HRmax). At HA both V'O2,max and HRmax were lower (12% and 6%, respectively), though maximal V'CO2 (V'CO2,max) was similar. El was similar at SL and HA (table 1⇑). Rl was lower at HA compared to SL (−11%, table 1⇑). The breathing pattern at WRmax was similar at SL and HA. Figure 2a⇓ shows average Pdi,sniff values. Pdi,sniff was lower at WRmax compared to baseline (SL: average 72%, range 61–80%; HA: average 61%, range 37–80%) in all but one test at SL (94%). Pdi,sniff had a positive gastric component (Pdi,sniff>Poes,sniff) before exercise in all instances. By contrast, at WRmax, the gastric component of Pdi,sniff became always negative (Pdi,sniff< Poes,sniff) at HA, while at SL it was negative in six subjects and reduced, but still positive in the remaining two. At SL but not at HA Pdi,sniff recovered to baseline within 60 min.
Mean±sem of: a) the maximal transdiaphragm pressure (Pdi,sniff), and of b) the pressure-time product for the diaphragm expressed as a fraction of the oesophageal pressure-time product (PTPdi/PTPoes) at baseline (B), maximal exercise (M), and during 60 min of recovery (R1-R60) measured at sea level (○) and at high altitude (•). During maximal exercise the PTPdi/PTPoes ratio dropped below unity indicating that the majority of the work of breathing had been shifted from the diaphragm to the accessory inspiratory muscles. *: p<0.05 condition versus B. (sea level versus high altitude)
Measurements during maximal exercise at sea level and high altitude
At SL, Poes,sniff was higher at WRmax (11.34± 0.49 kPa) than at baseline (9.97±0.39 kPa), whereas at HA it was equal (WRmax: 10.36±0.78 kPa; baseline: 8.99±0.29 kPa). Poes,sniff corr showed similar results at WRmax (SL: 11.24±0.39 kPa; HA: 10.17±0.68 kPa).
At SL, PTPdi averaged 80.74±9.87 kPa·s−1 over 1 min at WRmax, all subjects being on or above the fatigue threshold of 53.76–58.65 kPa·s−1 over 1 min 4. PTPdi at WRmax was lower at HA than at SL (64.13±8.21 kPa·s−1 over 1 min), but five subjects were still above and three lay just below the fatigue threshold (42.03–44.97 kPa·s−1 over 1 min). PTPdi/PTPoes ratio fell below unity at WRmax both at SL and at HA (but was lower at HA), and recovered to baseline within 5 min in both conditions (fig. 2b⇑).
At SL, lactate increased from 0.9±0.1 mEq·L−1 at rest to 6.2±0.7 mEq·L−1 at the 5th min of recovery, regaining baseline at 60 min (1.0±0.1 mEq·L−1). At HA, lactate increased from baseline (0.9±0.1 mEq·L−1) to the 5th min of recovery (6.7±0.6 mEq·L−1) but, as with Pdi,sniff, lactate did not recover to baseline within 60 min (1.3±0.2 mEq·L−1, p<0.05). In contrast to Pdi,sniff and lactate, V'E reached baseline (V'E at SL: 12.8±1.6 L·min−1; V'E at HA: 15.3±2.3 L·min−1) within 15 min at SL (15.2±2.4 L·min−1) and within 30 min at HA (14.9±1.5 L·min−1).
Isowork rate
WRmax was lower at HA in three subjects. To data at isowork rate conditions (iso-WR) compared in each subject, WRmax at HA was matched in these three subjects with a similar WR obtained at SL. At iso-WR, Pdi,sniff was significantly lower at HA (fig. 3a⇓), Poes,sniff never being below baseline (fig. 3b⇓). V'E was higher at HA (117.2±11.4 L·min−1) than at iso-WR SL (91.3± 13.3 L·min−1) because of higher frequency (35.9±3.1 and 30.6±2.1 breaths·min−1, respectively, p<0.05). Similar results were obtained for mean inspiratory flow (WRmax HA: 3.64±0.41 L·s−1; iso-WR SL: 3.03±0.29 L·s−1, p<0.05). Notwithstanding the higher V'E at HA, PTPdi and W'di were similar at iso-WR (fig. 4⇓).
Mean±sem of: a) maximal transdiaphragmatic (Pdi,sniff) and b) oesophageal (Poes,sniff) pressures measured at baseline (B) and at isowork rate (iso-WR) at sea level (SL; ○) and at high altitude (HA; •). Iso-WR: workload at SL matched to maximal workload reached at HA. The reduction of Pdi,sniff at iso-WR with respect to baseline was greater at HA. By contrast, Poes,sniff increased slightly both at SL and HA at iso-WR. *: p<0.05 B versus iso-WR (SL versus HA).
Mean±sem of: a) pressure-time product for the diaphragm (PTPdi) and of b) the mechanical work performed by the diaphragm (W'di). W'di is the product of PTPdi and mean inspiratory flow (VT/tI). These parameters are shown at baseline (B) and at isowork rate (iso-WR) measured at sea level (SL; ○) and at high altitude (HA; •). Iso-WR: workload at SL matched to maximal workload reached at HA. PTPdi was similar at SL and HA at both B and iso-WR. The same finding applied to W'di which takes into account changes in diaphragm effort induced by flow rate variations. These data indicate that the effort of the diaphragm at iso-WR was similar both at SL and HA. *: p<0.05 B versus iso-WR.
Volitional versus objective measurements
In four subjects retested at SL, both Pdi,tw and Pdi,sniff decreased significantly 10 min after exercise, and recovered 60 min after exercise (table 3⇓). All subjects showed near complete activation of the diaphragm, as no significant increase of Pdi due to CMS was found (fig. 5⇓). Pdi,sniff was similar to Pdi,max in any condition (fig. 5⇓), showing near complete activation of the diaphragm also with this manoeuvre.
Maximal transdiaphragmatic pressure obtained with a “sniff” manoeuvre a, c, e, g, i, k) and with a Mueller manoeuvre to which a single supramaximal twitch was superimposed (b, d, f, h, j, l) in a representative healthy subject at rest a–d), at 10 min (e–h), and at 60 min (i–l) after the end of an exhaustive incremental exercise on cycle-ergometer performed at sea level. Electromyograms of the right hemidiaphragm in response to supramaximal cervical phrenic nerve stimulation (EMEdi) are also provided. The superimposition of supramaximal magnetic stimulation (indicated by the electrical artefact in the EMEdi tracing) on maximal quasistatic efforts never evoked a further increase in maximal transdiaphragmatic pressure. Note that Pdi obtained with the “sniff” manoeuvre was similar to that obtained during the twitch occlusion test in every condition.
Volitional and objective measurements of diaphragmatic performance before and after exercise at sea level in four subjects
Discussion
The present data show that diaphragmatic force generating capacity (FGC) at maximal whole-body exercise was impaired at HA compared to equivalent work rates at SL, hypoxia alone being the predominant causative factor. This study also shows that, similarly to endurance exercise 3, 4, exhaustive incremental exercise can produce diaphragm fatigue at both SL and HA.
Technical considerations
BPNS is considered to have the best potential as a diagnostic test 21. However, the sniff manoeuvre was primarily used because it can be performed during exercise, at variance with BPNS that requires preparation (5–8 min) after exercise 3.
Poor subject effort might impair the sniff manoeuvre 21. All subjects had previous experience in performing the manoeuvre and were very well motivated. Moreover, sniff intrasubject variability was low (mean coefficient of variation at baseline: 2.3±1.0%, range: 0.1–5.9%). Tachypnoea and general exhaustion could limit Pdi,sniff performance 21. However, Pdi,sniff recovered to baseline later than the breathing pattern, both at SL and HA. Furthermore, in contrast to Pdi,sniff, Poes,sniff did not decrease at maximal exercise, suggesting maximal inspiratory effort even in these extreme conditions.
Pdi,sniff depends on lung volume 22. The end-expiratory Poes was monitored to ensure that sniff manoeuvres were performed at similar EELV, end-expiratory Poes being within 0.15 kPa of baseline just before sniffs in any tested condition. Similar results were obtained with end-expiratory Pga. These data suggest that diaphragmatic length was reasonably constant before Pdi,sniff manoeuvres.
Finally, according to Travaline et al. 23, Pdi,sniff and Pdi,max reliability was tested at SL by CMS in a subset of four subjects on a separate day. The trend of Pdi,sniff decay was similar in the two tests, Pdi,sniff being within ±5% at each workload tested. Similarly to Pdi,sniff, Pdi,tw was on average 27% lower than baseline 10 min after exercise (table 3⇑), suggesting that Pdi,sniff was accurate in detecting losses of diaphragm FGC, at least in the condition tested. It was extrapolated that similar results could be obtained at HA.
Diaphragm fatigue
A decreased diaphragm FGC was found at maximal exercise both at SL and HA, which recovered to baseline late after exercise (fig. 2⇑a), finding that fits the accepted definition of muscle fatigue 21. These data extend to exhaustive incremental exercise, the evidence that diaphragm fatigue can be generated in normal subjects by endurance exercise 4–, 24.
At end-exercise, Pdi,sniff decreased, whereas Poes,sniff did not or increased compared to baseline (figs. 1 and 3⇑⇑). These data are consistent with data obtained at, for example, the first minute of recovery (fig. 1⇑). It may be argued that this observation cannot be explained by fatigue, since both the diaphragm and the extradiaphragmatic muscles serve to reduce the Poes. However, Pdi,sniff was not only reduced compared to baseline, but it also became lower than Poes,sniff at end-exercise in almost all the conditions tested (see previously). This suggests diaphragm displacement into the rib cage during maximal manoeuvres, since end-expiratory abdominal pressure (Pab) was close to baseline values at the beginning of the Pdi,sniff manoeuvre (i.e. abdominal muscles phasically inactive). Finally, prolonged forced muscle contractions (similar to those developed by the respiratory muscles during exercise) can cause the phenomenon of potentiation 25. In this condition, a Poes,sniff increase at end-exercise would be expected, but not a decrease in Pdi,sniff (rather it should increase too).
The presented finding contrasts with Levine et al. 26 who reported, in preliminary experiments, no sign of diaphragmatic fatigue at end-incremental exercise. However, they provided neither references nor data to support their statement. Short incremental exercise may appear too light to produce diaphragm fatigue. However, increased diaphragm work generated by sustained hyperpnoea at rest, comparable to that found in the present study, has been demonstrated to cause significant diaphragm fatigue 4. Moreover, end-exercise pH and circulating lactate concentrations similar to those found in the present study have been shown to increase the amount of hyperpnoea-induced diaphragm fatigue 4.
Central, transmission and contractile fatigue have been described 10. In the four subjects in whom the twitch occlusion technique 17 was performed, no signs of central fatigue were detected at least at SL (fig. 5⇑). In the same subjects the M-wave was preserved after exercise, indicating absence of transmission fatigue in the condition tested. Finally, the long lasting duration of Pdi,sniff decay after exercise at SL and HA (fig. 2⇑), and the postexercise reduction in twitch pressure observed at SL, suggests that contractile fatigue did occur 3, 4, 11.
Effects of high altitude on diaphragm fatigue
Incremental exhaustive exercise at 3,325 m altitude worsened the FGC compared to iso-WR SL (fig. 3⇑) notwithstanding similar effort of the diaphragm (fig. 4⇑). At HA, significant hypoxia was found. Comparable hypoxaemia has been shown to worsen exercise-induced diaphragm fatigue in normobaric conditions 3. Increased diaphragm work was also found during hypoxic exercise at SL, suggesting that its combination with decreased O2 transport impairs diaphragm FGC in this condition 3. By contrast, it was found that, although V'E was higher at HA than at iso-WR SL, PTPdi as well as W'di (fig. 4⇑) and PTPoes were equal, indicating that in the experimental condition hypoxia per se hastened exercise-induced diaphragm fatigue.
EELV changes affect inspiratory muscle effort estimation 13. EELV was not directly assessed, but it is generally accepted that in young healthy subjects it does not exceed its baseline value during exercise 27. Conversely, the lower density of the air at HA (resulting in decreased airway flow resistance) could explain the increased inspiratory muscle efficiency in generating V'E 7. Indeed, similarly to previous studies 7, Rl decreased at HA approaching predicted values 28.
Not only did hypoxia impair independently exercise-induced diaphragm fatigue at HA, but it also influenced respiratory muscle interaction during exercise. Aliverti et al. 29 have shown that rib cage and abdominal muscles are progressively activated during exercise, allowing the diaphragm to act as a flow generator. Similar results were obtained in the present study at SL. Indeed, PTPdi/PTPoes ratio fell below unity at maximal exercise, indicating a strong activation of extra-diaphragmatic respiratory muscles (fig. 2b⇑). At HA, end-exercise PTPdi/PTPoes ratio (fig. 2b⇑) as well as absolute values of PTPdi significantly decreased compared to SL, suggesting further recruitment of accessory respiratory muscles to assist impending failure of the diaphragm. These observations are indirectly confirmed by studies on hyperoxic exercise that showed a decrease of accessory muscle activation compared to normoxic exercise 24.
At HA, but not at SL, diaphragm recovery from fatigue took longer than 60 min (fig. 2a⇑). It was associated with lower (three subjects) or equal (five subjects) external work at HA, with significantly reduced end-exercise PTPdi, and with blood lactate higher than baseline 60 min after exercise. As lactate clearance is delayed by hypoxia 30, and the increased level of circulating lactate and other metabolites has been implicated in the fatigue process 31, it might well be that hypobaric hypoxia and not increased workload, represents the major cause of the delay in recovery from exercise-induced diaphragm fatigue at HA.
In conclusion, exercise at high altitude challenges the respiratory muscles as a whole. In fact, not only can high altitude enhance diaphragm fatigue during exhaustive incremental exercise, but it also overloads accessory inspiratory muscles on account of hypobaric hypoxia alone, at least during acute exposure to the altitude tested. Whether this could affect exercise capability, particularly when ventilatory requirements become enormous (e.g. in acclimatized subjects at extreme altitudes), is still questionable. In fact, the effects of hypoxaemia are not confined to the respiratory muscles, since changes in oxygen supply affect locomotor muscles as well 24. Further work is required to assess whether exercise-induced diaphragm dysfunction has any role in determining exercise limitation at high altitude.
Acknowledgments
The authors gratefully acknowledge the excellent work of E. Visetti in preparing the laboratory in the Rifugio Torino and assisting in the measurements performed there, and of M. Marucco, President of the “Soccorso Alpino Italiano” Regione Piemonte, Italy, who organized the teams staying at Monte Bianco. The authors also warmly thank L. Bucciardini, M. Carone, M. Linden, U. Zummo, and L. Spagnolatti for their very useful assistance. They are also indebted to C. Miscio, from Dept of Neurology for her help in performing studies with magnetic stimulation. Finally, the authors would also like to thank R. Allpress for her help in the preparation of this manuscript.
- Received March 27, 2000.
- Accepted November 23, 2000.
- © ERS Journals Ltd