Breathing patterns during varied activities.

The level of ventilation attained and breathing patterns adopted during activity have important implications for the distribution and deposition of particles that are inhaled. However, breathing patterns and levels of ventilation adopted during specific physical activities are unknown. We used a noninvasive means of measuring ventilation in subjects performing a variety of activities (bicycling, arm ergometry, lifting, and pulling) during unencumbered (no mouthpiece) breathing and while breathing through a mouthpiece. Minute ventilation (VE), tidal volume (VT), inspiratory time (TI), and total breathing cycle time (TT) were measured initially both spirometrically and from body surface displacements. When a mouthpiece was used, VE and breathing patterns were significantly altered during all activities such that VE, VT, and TT increased by 16, 34, and 20%, respectively. This mouthpiece effect was attenuated at the higher levels of VE. A task dependency of breathing pattern was also noted such that there was much greater variability of VT and TI for a given VE during the lifting activity compared with bicycling (coefficient of variation for VT of 0.39 +/- 0.09 vs. 0.20 +/- 0.07, P less than 0.01; and for TI of 0.38 +/- 0.08 vs. 0.21 +/- 0.08, P less than 0.01). We conclude that a mouthpiece significantly alters breathing pattern during varied types and intensities of activities, and breathing patterns may differ significantly from one activity to another. When the total dose of particulates inhaled in the lung are assessed, the mouthpiece effect and activity effect on breathing pattern must be considered.     

Postnatal maturation of ventilation and breathing pattern in kittens: influence of sleep

Ventilation and breathing pattern were studied in kittens at 1, 2, 3, 4, and 8 wk of life during quiet wakefulness (W), quiet sleep (QS), and active sleep (AS) with the barometric method. Tidal volume (VT), respiratory frequency (f), ventilation (VE), inspiratory time (TI), expiratory time (TE), mean inspiratory flow (VT/TI), and respiratory "duty cycle" (TI/TT) were measured. VT, VE, TI, TE, and VT/TI increased; f decreased and TI/TT remained constant during postnatal development in wakefulness and in both sleep states. No significant difference was observed between AS and QS for all the ventilatory parameters except TI/TT, which was greater in QS than in AS at 2 wk. VE was larger in W than in both AS and QS at all ages. This was mainly due to a greater f, TI/TT remaining constant. VT/TI, which represents an index of the central inspiratory activity, was larger in W than in sleep, VT not being significantly different whatever the stage of consciousness. The results of this study show that in the kitten 1) unlike in the adult cat, ventilation and breathing pattern are similar in QS and in AS; 2) in sleep, the central inspiratory drive appears to be independent of the type of sleep; and 3) in wakefulness, the increase of the central inspiratory activity could be related to important excitatory inputs.     

Effects of inspiratory resistive load on respiratory control in hypercapnia and exercise

Eight healthy young men underwent two separate steady-state incremental exercise runs within the aerobic range on a treadmill with alternating periods of breathing with no load (NL) and with an inspiratory resistive load (IRL) of approximately 12 cmH2O.1-1.s. End-tidal PCO2 was maintained constant throughout each run at the eucapnic or a constant hypercapnic level by adding 0-5% CO2 to the inspired O2. Hypercapnia caused a steepening, as well as upward shift, relative to the corresponding eucapnic ventilation-CO2 output (VE - VCO2) relationship in NL and IRL. Compared with NL, the VE - VCO2 slope was depressed by IRL, more so in hypercapnic [-19.0 +/- 3.4 (SE) %] than in eucapnic exercise (-6.0 +/- 2.0%), despite a similar increase in the slope of the occlusion pressure at 100 ms - VCO2 (P100 - VCO2) relationship under both conditions. The steady-state hypercapnic ventilatory response at rest was markedly depressed by IRL (-22.6 +/- 7.5%), with little increase in P100 response. For a given inspiratory load, breathing pattern responses to separate or combined hypercapnia and exercise were similar. During IRL, VE was achieved by a greater tidal volume (VT) and inspiratory duty cycle (TI/TT) along with a lower mean inspiratory flow (VT/TI). The increase in TI/TT was solely because of a prolongation of inspiratory time (TI) with little change in expiratory duration for any given VT. The ventilatory and breathing pattern responses to IRL during CO2 inhalation and exercise are in favor of conservation of respiratory work.(ABSTRACT TRUNCATED AT 250 WORDS)     

Breathing pattern during exercise in young black and Caucasian subjects

Lung volumes in sex-, age-, height-, and weight-matched Black subjects are 10-15% lower than those in Caucasians. To determine whether this decreased lung volume affected the ventilatory adaptation to exercise, minute ventilation (VE), its components, frequency (f) and tidal volume (VT), and breathing pattern were observed during incremental cycle-ergometer exercise. Eighteen Caucasian (age 8-30 yr) and 14 Black (age 8-25 yr) subjects were studied. Vital capacity (VC) was lower (P less than 0.001) in the Black subjects [90.6 +/- 8.6 (SD) vs. 112.9 +/- 9.9% predicted], whereas functional residual capacity/total lung capacity was higher (P less than 0.05). VE, mixed expired O2 and CO2, VT, f, and inspiratory (TI), expiratory (TE), and total respiratory cycle (TT) duration were measured during the last 30 s of each 2-min load. Statistical comparisons with increasing power output were made at rest and from 0.6 to 2.4 W/kg in 0.3-W/kg increments. VE was higher in Blacks at all work loads and reached significance (P less than 0.05) at 0.6 and 1.5 W/kg. VE/VO2 was also higher throughout exercise, reaching significance (P less than 0.01) at 1.2, 1.5, and 1.8 W/kg. The Black subjects attained any given level of VE with a higher f (P less than 0.001) and lower VT. TI and TE were shortened proportionately so that TI/TT was not different. Differences in lung volume and the ventilatory response to exercise in these Black and Caucasian subjects suggest differences in the respiratory pressure-volume relationships or that the Black subjects may breathe higher on their pressure-volume curve.     

Ventilation and respiratory pattern and timing in resting awake cats

The purpose of this study was to characterize the variability and patterns of spontaneous respiratory behaviour in awake cats. Respiration was measured in six cats over 80 or 90 min by the plethysmographic technique. In three cats, arterial blood gases were measured. Breath frequency (f) and tidal volume (VT) varied considerably breath-to-breath, although on average, these measurements as well as average ventilation remained relatively constant. The incidence of breath ventilation (VT X 60/TTOT) and VT were distributed unimodally but the incidence of breath f had a bimodal distribution. In the low f range, average f was 22.5 breaths/min, and in the high f range, average f was 41.6 breaths/min. The latter range appeared to be associated with purring. Inspiratory duration (TI) was less than expiratory duration (TE) at low f but exceeded TE at high f. For a given breath ventilation there was a predictable f and VT. At shorter TI (higher f) mean inspiratory flow, an index of central respiratory drive, increased but VT decreased. This study indicates that "normal" control respiratory behaviour in awake cats is better described by the range and pattern of breathing than by average values.     

Ventilatory response to fatiguing and nonfatiguing resistive loads in awake sheep

To study the changes in ventilation induced by inspiratory flow-resistive (IFR) loads, we applied moderate and severe IFR loads in chronically instrumented and awake sheep. We measured inspired minute ventilation (VI), ventilatory pattern [inspiratory time (TI), expiratory time (TE), respiratory cycle time (TT), tidal volume (VT), mean inspiratory flow (VT/TI), and respiratory duty cycle (TI/TT)], transdiaphragmatic pressure (Pdi), functional residual capacity (FRC), blood gas tensions, and recorded diaphragmatic electromyogram. With both moderate and severe loads, Pdi, TI, and TI/TT increased, TE, TT, VT, VT/TI, and VI decreased, and hypercapnia ensued. FRC did not change significantly with moderate loads but decreased by 30-40% with severe loads. With severe loads, arterial PCO2 (PaCO2) stabilized at approximately 60 Torr within 10-15 min and rose further to levels exceeding 80 Torr when Pdi dropped. This was associated with a lengthening in TE and a decrease in breathing frequency, VI, and TI/TT. We conclude that 1) timing and volume responses to IFR loads are not sufficient to prevent alveolar hypoventilation, 2) with severe loads the considerable increase in Pdi, TI/TT, and PaCO2 may reduce respiratory muscle endurance, and 3) the changes in ventilation associated with neuromuscular fatigue occur after the drop in Pdi. We believe that these ventilatory changes are dictated by the mechanical capability of the respiratory muscles or induced by a decrease in central neural output to these muscles or both.     

Driving and timing components of the breathing pattern during hypoxia in anaesthetized cats

The effects of acute hypoxic hypoxia elicited by N2 inhalation on the driving and timing components of the breathing pattern were studied in 18 adult anaesthetized cats. Two phases could be distinguished in the ventilatory response to acute hypoxia. During the first phase, mean inspiratory flow (VT/TI) increased exponentially up to 240% of the initial value. During the second phase, VT/TI gradually decreased, reaching the control values in the last preapnoeic breaths during the first exposure and remained higher than normal with earlier respiratory arrest in three repeated N2 inhalations. No significant changes could be observed in the timing component of breathing pattern (TI/TT) in the course of the first hypoxic exposure, and the changes in TI/TT did not exceed 7% in repeated attacks. This suggests that the shortening of both inspiratory and expiratory durations increased the breathing frequency up to 130% of its resting value. Moreover, tachypnoea was preserved until respiratory arrest. Accordingly, it is concluded that the decrease in ventilation with the appearance of apnoea during the second phase of N2 inhalation in anaesthetized cats is not due to a failure of respiratory timing, but to a depression of the driving mechanisms which are responsible for this phenomenon.     

Ventilatory response to high-frequency airway oscillation in humans

To investigate respiratory control during high-frequency oscillation (HFO), ventilation was monitored in conscious humans by respiratory inductive plethysmography during application at the mouth of high-frequency pressure oscillations. Studies were conducted before and after airway and pharyngeal anesthesia. During HFO, breathing became slow and deep with an increase in tidal volume (VT) of 37% (P less than 0.01) and inspiratory duration (TI) of 34% (P less than 0.01). Timing ratio (TI/TT) increased 14% (P less than 0.05) and respiratory frequency (f) decreased 12% (P less than 0.01). Mean inspiratory flow (VT/TI) did not change during HFO. Following airway anesthesia, VT increased only 26% during HFO (P less than 0.01), whereas significant changes in TI, TI/TT, and f were not observed. Pharyngeal anesthesia failed to diminish the effect of HFO on TI, TT, or f, although the increase in VT was reduced. These results indicate that 1) HFO presented in this manner alters inspiratory timing without affecting the level of inspiratory activity, and 2) receptors in the larynx and/or lower airways may in part mediate the response.     

Airway anesthesia effects on hypercapnic breathing pattern in humans

Minute ventilation (VE) and breathing pattern during an abrupt increase in fractional CO2 were compared in 10 normal subjects before and after airway anesthesia. Subjects breathed 7% CO2-93% O2 for 5 min before and after inhaling aerosolized lidocaine. As a result of airway anesthesia, VE and tidal volume (VT) were greater during hypercapnia, but there was no effect on inspiratory time (TI). Therefore, airway anesthesia produced an increase in mean inspiratory flow (VT/TI) during hypercapnia. The increase in VT/TI was compatible with an increase in neuromuscular output. There was no effect of airway anesthesia on the inspiratory timing ratio or the shape and position of the curve relating VT and TI. We also compared airway resistance (Raw), thoracic gas volume, forced vital capacity, forced expired volume at 1s, and maximum midexpiratory flow rate before and after airway anesthesia. A small (0.18 cmH2O X l-1 X s) decrease in Raw occurred after airway anesthesia that did not correlate with the effect of airway anesthesia on VT/TI. We conclude that airway receptors accessible to airway anesthesia play a role in hypercapnic VE.     

Effects of respiratory drive on upper airways in sleep apnea patients and normal subjects

We compared the changes in nasal and pharyngeal resistance induced by modifications in the central respiratory drive in 8 patients with sleep apnea syndrome (SAS) with the results of 10 normal men. Upper airway pressures were measured with two low-bias flow catheters; one was placed at the tip of the epiglottis and the other above the uvula. Nasal and pharyngeal resistances were calculated at isoflow. During CO2 rebreathing and during the 2 min after maximal voluntary hyperventilation, we continuously recorded upper airway pressures, airflow, end-tidal CO2, and the mean inspiratory flow (VT/TI); inspiratory pressure generated at 0.1 s after the onset of inspiration (P0.1) was measured every 15-20 s. In both groups upper airway resistance decreased as P0.1 increased during CO2 rebreathing. When P0.1 increased by 500%, pharyngeal resistance decreased to 17.8 +/- 3.1% of base-line values in SAS patients and to 34.9 +/- 3.4% in normal subjects (mean +/- SE). During the posthyperventilation period the VT/TI fell below the base-line level in seven SAS patients and in seven normal subjects. The decrease in VT/TI was accompanied by an increase in upper airway resistance. When the VT/TI decreased by 30% of its base-line level, pharyngeal resistance increased to 319.1 +/- 50.9% in SAS and 138.5 +/- 4.7% in normal subjects (P less than 0.05). We conclude that 1) in SAS patients, as in normal subjects, the activation of upper airway dilators is reflected by indexes that quantify the central inspiratory drive and 2) the pharyngeal patency is more sensitive to the decrease of the central respiratory drive in SAS patients than in normal subjects.     

Power spectral density of breathing pattern in patients with chronic obstructive lung disease

Newsom Davis and Stagg studying the interrelationship of the volume and time components of individual breaths in healthy resting man described a significant correlation between mean tidal volume (VT) and inspiratory time (TI) r = 0.704. The correlation between mean TI and expiratory time (TE) was lower, r = 0.381. Evaluation of these relationships and of the power spectral density of the breathing pattern was the aim of the present study. For breath by breath analysis we calculated power spectral density and cross correlations of VT, TI and TE. We found a significant correlation between VT and TI in 9 patients with global respiratory insufficiency (RI) (mean r = 0.52) and 7 patients with partial RI (mean r = 0.56). The correlation between TI, TE was lower, in 9 patients with global RI (mean r = 0.21) and 7 patients with partial RI (mean r = 0.35). The results of both groups did not differ from healthy subjects in power spectral density of the breathing pattern and in correlations of VT and TI as well as TI and TE.     

Ventilatory response during CO2 inhalation after airway anesthesia with lidocaine

Airway anesthesia with inhaled aerosolized lidocaine has been associated with increases in minute ventilation (VE) and mean inspiratory flow rate (VT/TI) during CO2 inhalation. However, it is unclear whether these increases are local effects of the anesthesia or systemic effects of absorbed and circulating lidocaine. To evaluate this 20 normal subjects were treated on separate days with aerosolized lidocaine, intravenous lidocaine, aerosolized control solution, or intravenous control solution, and the effects of each treatment on VE and VT/TI were determined and compared during room-air breathing and inhalation of 5% CO2-95% O2. None of the treatments altered VE or VT/TI during room-air breathing. Aerosolized lidocaine produced small (5.9-6.0%) increases in VE and VT/TI during CO2 inhalation, but these effects were not present after intravenous lidocaine despite equivalent lidocaine blood levels. We concluded that the increases in VE and VT/TI after aerosolized lidocaine were local effects of airway anesthesia rather than systemic effects of absorbed and circulating lidocaine.     

Effect of hilar nerve denervation on breathing and arterial PCO2 during CO2 inhalation

We determined the effects of denervating the hilar branches (HND) of the vagus nerves on breathing and arterial PCO2 (PaCO2) in awake ponies during eupnea and when inspired PCO2 (PICO2) was increased to 14, 28, and 42 Torr. In five carotid chemoreceptor-intact ponies, breathing frequency (f) was less, whereas tidal volume (VT), inspiratory time (TI), and ratio of TI to total cycle time (TT) were greater 2-4 wk after HND than before HND. HND per se did not significantly affect PaCO2 at any level of PICO2, and the minute ventilation (VE)-PaCO2 response curve was not significantly altered by HND. Finally, the attenuation of a thermal tachypnea by elevated PICO2 was not altered by HND. Accordingly, in carotid chemoreceptor-intact ponies, the only HND effect on breathing was the change in pattern classically observed with attenuated lung volume feedback. There was no evidence suggestive of a PCO2-H+ sensory mechanism influencing VE, f, VT, or PaCO2. In ponies that had the carotid chemoreceptors denervated (CBD) 3 yr earlier, HND also decreased f, increased VT, TI, and TT, but did not alter the slope of the VE-PaCO2 response curve. However, at all levels of elevated PICO2, the arterial hypercapnia that had persistently been attenuated, since CBD was restored to normal by HND. The data suggest that during CO2 inhalation in CBD ponies a hilar-innervated mechanism influences PaCO2 by reducing physiological dead space to increase alveolar ventilation.     

Human breathing patterns on mouthpiece or face mask during air, CO2, or low O2

Steady-state breathing patterns on mouthpiece and noseclip (MP) and face mask (MASK) during air and chemostimulated breathing were obtained from pneumotachometer flow. On air, all 10 subjects decreased frequency (f) and increased tidal volume (VT) on MP relative to that on MASK without changing ventilation (VE), mean inspiratory flow (VT/TI), or mean expiratory flow (VT/TE). On elevated CO2 and low O2, MP exaggerated the increase in VE, f, and VT/TE due to profoundly shortened TE. On elevated CO2, MASK exaggerated VT increase with little change in f. Increased VE and VT/TI were thus due to increased VT. During low O2 on MASK, both VT and f increased. During isocapnia, shortened TE accounted for increased f; during hypocapnia, increased f was related primarily to shortened TI. Thus the choice of a mouthpiece or face mask differentially alters breathing pattern on air and all components of ventilatory responses to chemostimuli. In addition, breathing apparatus effects are not a simple consequence of a shift from oronasal to oral breathing, since a noseclip under the mask did not change breathing pattern from that on mask alone.     

Airway obstruction during periodic breathing in premature infants

To characterize changes in pulmonary resistance, timing, and respiratory drive during periodic breathing, we studied 10 healthy preterm infants (body wt 1,340 +/- 240 g, postconceptional age 35 +/- 2 wk). Periodic breathing in these infants was defined by characteristic cycles of ventilation with intervening respiratory pauses greater than or equal to 2 s. Nasal airflow was recorded with a pneumotachometer, and esophageal or pharyngeal pressure was recorded with a fluid-filled catheter. Pulmonary resistance at half-maximal tidal volume, inspiratory time (TI), expiratory time (TE), and mean inspiratory flow (VT/TI) were derived from computer analysis of five cycles of periodic breathing per infant. In 80% of infants periodic breathing was accompanied by completely obstructed breaths at the onset of ventilatory cycles; the site of airway obstruction occurred within the pharynx. The first one-third of the ventilatory phase of each cycle was accompanied by the highest airway resistance of the entire cycle (168 +/- 98 cmH2O.l-1.s). In all infants TI was greatest at the onset of the ventilatory cycle, VT/TI was maximal at the midpoint of the cycle, and TE was longest in the latter two-thirds of each cycle. A characteristic increase and subsequent decrease of 4.5 +/- 1.9 ml in end-expiratory volume also occurred within each cycle. These results demonstrate that partial or complete airway obstruction occurs during periodic breathing. Both apnea and periodic breathing share the element of upper airway instability common to premature infants.     

Phonospirometry for noninvasive measurement of ventilation: methodology and preliminary results.

We measured tracheal flow from tracheal sounds to estimate tidal volume, minute ventilation (VI), respiratory frequency, mean inspiratory flow (VT/TI), and duty cycle (TI/Ttot). In 11 normal subjects, 3 patients with unstable airway obstruction, and 3 stable asthmatic patients, we measured tracheal sounds and flow twice: first to derive flow-sound relationships and second to obtain flow-volume relationships from the sound signal. The flow-volume relationship was compared with pneumotach-derived volume. When subjects were seated, facing forward and with neck rotation, flexion, and standing, flow-volume relationship was within 15% of pneumotach-derived volume. Error increased with neck extension and while supine. We then measured ventilation without mouthpiece or nose clip from tracheal sounds during quiet breathing for up to 30 min. Normal results +/- SD revealed tidal volume = 0.37 +/- 0.065 liter, respiratory frequency = 19.3 +/- 3.5 breaths/min, VI = 6.9 +/- 1.2 l/min, VT/TI = 0.31 +/- 0.06 l/s, and TI/Ttot = 0.37 +/- 0.04. Unstable airway obstruction had large VI due to increased VT/TI. With the exception of TI/Ttot, variations in ventilatory parameters were closer to log normal than normal distributions and tended to be greater in patients. We conclude that phonospirometry measures ventilation reasonably accurately without mouthpiece, nose clip, or rigid postural constraints.     

Effect of respiratory apparatus on respiration

A mouthpiece plus noseclip (MP + NC) is frequently used in performing measurements of breathing patterns. Although the effects the apparatus exerts on breathing patterns have been studied, the mechanism of the changes it causes remains unclear. The current study examines the effects on respiratory patterns of a standard (17-mm-diam) MP + NC during room air (RA) breathing and the administration of 2 and 4% CO2 in normal volunteers and in patients 2-4 days after abdominal operation. When compared with values obtained with a noninvasive canopy system, the MP + NC induced increases in minute ventilation (VE), tidal volume (VT), and mean inspiratory flow (VT/TI), but not frequency (f) or inspiratory duty cycle, during both RA and CO2 administration. The percentage increase in VE, VT, and VT/TI caused by the MP + NC decreased as the concentration of CO2 increased. During RA breathing, the application of noseclip alone resulted in a decrease in f and an increase in VT, but VE and VT/TI were unchanged. The changes were attenuated during the administration of 2 and 4% CO2. Reducing the diameter of the mouthpiece to 9 mm abolished the alterations in breathing pattern observed with the larger (17-mm) diameter MP.     

Impact of age on breathing and resistive pressure in people with and without sleep apnea.

We investigated the effect of age on breathing and total pulmonary resistance (RL) during sleep by studying elderly (>65 yr) and young (25-38 yr) people without sleep apnea (EN and YN, respectively) matched for body mass index (BMI). To determine the impact of sleep apnea on age-related changes in breathing, we studied elderly and young apneic patients (EA and YA, respectively) matched for apnea and BMI. In all groups (n = 11), breathing during periods of stable sleep was analyzed to evaluate the intrinsic variability of respiratory control mechanisms. In the absence of sleep apnea, the variability of the breathing was similar in the elderly and young [mean (+/- SD) coefficient of variation (CV) of tidal volume (VT); wake: EN 21.0 +/- 14.9%, YN 14.7 +/- 5.5%; sleep: EN 14.0 +/- 6.0%; YN 11.5 +/- 6.4%]. In patients with sleep apnea, breathing during stable sleep was more irregular, but there were no age-related differences (CV of VT; wake: EA 22.0 +/- 11.6%, YA 16.7 +/- 11.3%; sleep: EA 32.8 +/- 24.9%, YA 25.2 +/- 16.3%). In addition, EN tended to have a higher RL (n = 6, RL midinspiration, wake: EN 7.1 +/- 3.0; YN 9.1 +/- 6.4 cmH(2)O. l(-1). s, sleep: EN 17.5 +/- 11.7; YN 9.8 +/- 2.0 cmH(2)O. l(-1). s). We conclude that aging per se does not contribute to the intrinsic variability of respiratory control mechanisms, although there may be a lower probability of finding elderly people without respiratory instability.     

Effect of inspiratory muscle fatigue on perception of effort during loaded breathing

The present study examined the relationship between the intensity of the sense of effort during inspiratory threshold loading and the severity of inspiratory muscle fatigue. Studies were performed on normal subjects in whom the magnitude of airway pressure developed (Pm) and the duty cycle of breathing (TI/TT) were constrained to achieve a pressure-time integral (i.e., Pm/Pmax X TI/TT) 24% of maximum. In separate trials, the same pressure-time index (24%) was achieved using two widely different patterns of pressure magnitude and duty cycle to allow the effects of changes in the pattern of inspiratory muscle contraction on sensation and fatigue to be assessed. The intensity of the sense of effort was assessed using a category (Borg) scale. The severity of inspiratory muscle fatigue was assessed both from changes in the centroid frequency of the diaphragm electromyogram and from changes in the maximum static inspiratory pressure. Loaded breathing produced inspiratory muscle fatigue and a progressive increase in the sense of effort over time in all subjects. The rate at which the inspiratory muscles fatigued was the same with the two patterns of loading. In contrast, the rate of growth in the intensity of the sense of effort varied significantly as a function of the pattern of loaded breathing. The sense of effort increased at a faster rate with the high pressure-short duty cycle pattern of contraction as compared with the low pressure-long duty cycle pattern. As a result, the intensity of the sense of effort was not uniquely related to the severity of inspiratory muscle fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)     

Critique on application of diaphragmatic time-tension index to spontaneously breathing humans

Bellemare and Grassino (J. Appl. Physiol. 53: 1196-1206, 1982) have reported that the diphragmatic time-tension index (TTdi) (i.e., the product of mean transdiaphragmatic pressure/maximum transdiaphragmatic pressure and the inspiratory duty cycle) can be used as a predictor of diaphragmatic fatigue in humans. However, the publications of these authors do not directly address the question of whether inspiratory flow or transdiaphragmatic pressure should be used to calculate the inspiratory duty cycle. To gather data on this point, we computed TTdi by both methods in spontaneously breathing normal adult males (AMN) and age-matched males with chronic obstructive pulmonary disease (COPD) at rest and during treadmill exercise. During rest and exercise in both AMN and COPD, the fraction of the breathing cycle over which diaphragmatic tension was maintained (Tdi/TT) exceeded the fraction of the breathing cycle over which inspiratory airflow was maintained (TI/TT). Therefore, TTdi calculations using Tdi/TT were greater (P less than 0.05) than TTdi computations using TI/TT. However, this difference in TTdi values was relatively small (approximately 15%).