Effects of the self-contained breathing apparatus on ...

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Effects of the self-contained breathing apparatus on left-ventricular function at rest and during graded exercise

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by 142.158.254.202 on 02/27/13 For personal use only.

Jonathan R. Mayne, Mark J. Haykowsky, Michael D. Nelson, Timothy C. Hartley, Scott J. Butcher, Richard L. Jones, and Stewart R. Petersen

Abstract: The purpose of this study was to examine the effects of the self-contained breathing apparatus (SCBA) on leftventricular (LV) function at rest and during mild- to moderate-intensity exercise, using 2-dimensional echocardiography. Twenty-three healthy male volunteers exercised on a stair-climber at work rates equivalent to 50%, 60%, 70%, and 80% of peak oxygen consumption. Esophageal pressure LV diastolic and systolic cavity areas, and myocardial areas were acquired during the final minute of each stage of exercise. As expected, the esophageal pressure response during SCBA breathing revealed significantly lower (more negative) inspiratory pressures and higher (more positive) expiratory pressures and, consequently, higher pressure swings, than free breathing (FB). End-diastolic cavity area (EDCA) and end-systolic cavity area (ESCA) were lower with the SCBA than with FB. LV contractility was higher (p < 0.05) with the SCBA, which can partially be explained by decreases in end-systolic wall stress. Therefore, the SCBA was found to decrease LV preload during moderate-intensity exercise, but did not negatively affect stroke area because of a similar reduction in ESCA. Key words: intrathoracic pressure, left-ventricular function, graded exercise, self-contained breathing apparatus. Re´sume´ : Cette e´tude se propose d’analyser au moyen de l’ećhocardiographie bidimensionnelle les effets d’un appareil respiratoire autonome (SCBA) sur la fonction du ventricule gauche (LV) au repos et au cours d’un exercice physique d’intensite´ le´ge`re a` mode´reé. Vingt-trois hommes participent volontairement a` une seánce d’exercice sur un simulateur d’escalier a` une intensite´ correspondant a` 50 %, 60 %, 70 % et 80 % du consommation d’oxyge`ne de pointe. Durant la dernie`re minute de chaque stade d’effort, on mesure la pression œsophagienne, le volume du myocarde et du LV au cours de la diastole et de la systole. Selon toute attente, la pression œsophagienne est significativement plus faible (plus ne´gative) durant l’inspiration et plus forte (plus positive) durant l’expiration dans le SCBA; en d’autres mots, les ećarts de pression sont nettement plus importants comparativement a` la respiration libre (FB). Comparativement a` la FB, le volume du LV a` la fin de la diastole (EDCA) et celui a` la fin de la systole (ESCA) est plus petit avec le SCBA. Avec le SCBA, la contractilite´ du ventricule gauche est supe´rieure (p < 0,05), ce qui peut eˆtre explique´ en partie par la diminution de la tension te´le´diastolique de la paroi. En conse´quence, le SCBA diminue la prećharge du LV au cours d’un exercice physique d’intensite´ mode´reé, mais ne modifie pas ne´gativement la surface d’e´jection (SA), et ce, a` cause de la diminution semblable du ESCA. Mots-cle´s : pression intrathoracique, fonction du ventricule gauche, exercice d’intensite´ progressive, appareil respiratoire autonome. [Traduit par la Re´daction]

Introduction Firefighters wear self-contained breathing apparatus (SCBA) for protection from hazardous airborne pollutants. However, studies from our laboratory (Eves et al. 2005; Dreger et al. 2006) have shown that the Scott 4.5 SCBA reduces maximal aerobic power (V_ O2 max). Most of the reduction in V_ O2 max and work capacity has been attributed to

attenuated peak ventilation, secondary to a higher expiratory breathing resistance (Eves et al. 2005). The higher expiratory resistance leads to increases in the work of breathing with the SCBA during submaximal exercise, when ventilation exceeds approximately 80 Lmin–1 (Butcher et al. 2006, 2007). Expiratory time has also been shown to lengthen during strenuous submaximal and maximal exercise with the SCBA (Eves et al. 2003, 2005; Butcher et al. 2007).

Received 10 October 2008. Accepted 10 February 2009. Published on the NRC Research Press Web site at apnm.nrc.ca on 16 July 2009. J.R. Mayne, M.D. Nelson, T.C. Hartley, and S.R. Petersen.1 Faculty of Physical Education and Recreation, University of Alberta, Edmonton, AB T6G 2H9, Canada. M.J. Haykowsky. Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB T6G 2R7, Canada; Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, AB T6G 2G4, Canada. S.J. Butcher. School of Physical Therapy, University of Saskatchewan, Saskatoon, SK S7N 0W3, Canada. R.L. Jones. Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB T6G 2R7, Canada. 1Corresponding

author (e-mail: [email protected]).

Appl. Physiol. Nutr. Metab. 34: 625–631 (2009)

doi:10.1139/H09-029

Published by NRC Research Press


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Recent work by Stark-Leyva et al. (2004) has shown that an expiratory load applied to healthy subjects during submaximal cycling reduced stroke volume and cardiac output. The authors reasoned that the mechanism behind this finding was a reduction in venous return caused by a systematic decline in filling pressure. Other research has shown that stroke volume is reduced with the application of lower body negative pressure, but can be restored with inspiratory loading (Convertino et al. 2005; Ryan et al. 2008). The SCBA has previously been shown to increase esophageal pressure during moderate-intensity exercise (Butcher et al. 2006, 2007); however, the impact that the SCBA may have on left-ventricular (LV) function is currently unknown. Therefore, the purpose of this study was to investigate LV function (with the SCBA and while free breathing (FB)), using 2-dimensional echocardiography at rest and during mildto moderate-intensity exercise. It was hypothesized that the increased esophageal pressure associated with the SCBA would negatively affect LV function.

Materials and methods Subjects Twenty-three physically active men provided written informed consent to participate in the study, which had previously been approved by an institutional ethics review board. None of the subjects smoked or had any known health concerns. Subjects over the age of 40 years were screened by a physician and completed an exercise stress test with 12-lead electrocardiogram prior to enrollment. Subject characteristics are shown in Table 1. Experimental procedures All testing took place in the same laboratory (20–22 8C) at the University of Alberta (Edmonton, Atla.). On the first visit, demographic data were collected (height, mass, age) and spirometry testing was performed (Medgraphics, St. Paul, Minn.), using standard procedures, in accordance with the 2005 American Thoracic Society guidelines (Miller et al. 2005). Subjects were familiarized with the research techniques and completed an incremental exercise test to exhaustion wearing firefighter protective equipment, including the SCBA (Scott 4.5, Scott Health and Safety, Monroe, N.C.). All testing was done on a motorized stair-climber (Stepmill 7000 PT, Nautilus Inc., Vancouver, Wash.). Pilot work determined that use of the stair-climber maintained the subject in a stable position enabling the acquisition of echocardiographic images and other physiological data. For the incremental test, the stepping rate began at 40 stepsmin–1, and was increased by 10 stepsmin–1 every 2 min until volitional exhaustion. Subjects were allowed light contact with the handrail to maintain balance; however, a tight grip of the handrail was not permitted. Expired gases were ducted through a Plexiglas cone attaching the SCBA regulator to a metabolic cart (TrueOne, ParvoMedics, Salt Lake City, Utah), as described elsewhere (Eves et al. 2002). Peak oxygen consumption (V_ O2 peak) was defined as the highest 60 s oxygen consumption value recorded before volitional exhaustion. After completion of the incremental exercise test, stepping rates, which corresponded to 50%, 60%, 70%, and 80% of V_ O2 peak, were identified for the subsequent experiment.

Appl. Physiol. Nutr. Metab. Vol. 34, 2009 Table 1. Physical and physiological characteristics of the 23 subjects. Variable Age (year) Height (cm) Mass (kg) V_ O2 peak (mLkg–1min–1) V_ O2 peak (Lmin–1) FVC (L) FVC (% predicted) FEV1 (L) FEV1 (% predicted) FEV1/FVC FEV1/FVC (% predicted)

Mean (±SD) 35.6±10.6 182.3±7.6 82.7±10.2 41.9±7.5 3.5±0.6 5.7±0.9 105.5±10.8 4.3±0.6 98.0±8.7 0.77±0.05 93.4±7.1

Min 22.0 170.0 64.8 26.9 2.3 4.1 88.5 3.1 83.8 0.65 70.0

Max 54.0 195.5 102.9 54.8 4.4 7.7 130.5 5.7 115.9 0.89 103.3

Note: FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; V_ O2 peak, peak oxygen consumption.

On the second visit to the laboratory, the subject, dressed in firefighter protective equipment and breathing through the SCBA, practiced the work rates chosen for the experiment. This session confirmed that the exercise intensities were appropriate for each subject and allowed the subject to become familiar with the experimental protocol. On the third visit to the laboratory, each subject completed 2 sets of 5 stages (rest, 50%, 60%, 70%, and 80% V_ O2 peak , in that order), with the order of sets randomized for breathing condition (either SCBA or FB). The 2 sets were separated by at least 1 h of rest. Subjects were dressed in firefighter protective equipment and carried the SCBA for each set; however, subjects did not breathe through the SCBA during the FB trial. Within the set, each stage was 4 min in duration followed by 4 min of standing rest. Immediately following the final workload (80% V_ O2 peak), the subjects removed the SCBA and firefighter protective equipment and remained in the laboratory dressed in normal exercise attire. During the rest period, the subjects were provided with cold water and sports drinks. The 4 min duration of each stepping interval was designed to allow for sufficient time to obtain stable physiological responses to the work intensity and sufficient time for data collection. Overall, the design allowed for the study of both breathing conditions (SCBA and FB) in 1 visit to the laboratory. Measurements Blood samples were collected prior to each trial to quantify any hematological changes that might have occurred between trials. A 21 gauge needle was used to draw 2 mL of whole blood, via venapuncture, which was collected in a Vacutainer containing lithium heparin. Blood was analyzed for hemoglobin, hematocrit, mean corpuscular volume, and mean corpuscular hemoglobin content. During each trial, expired gas samples were collected from either the SCBA or a low resistance valve (Hans Rudolph, Kansas City, Mo.) in a Tissot Spirometer (ChainCompensated Gasometer, Warren E Collins Inc., Braintree, Mass.) during the last 30 s of each 4 min stage. For the SCBA condition, the expired tubing was attached to the Published by NRC Research Press


Mayne et al.

Plexiglas cone for the final 30 s of each stage. For the FB condition, the low resistance valve and expired tubing was placed in the subject’s mouth for the final 30 s of each stage. The displacement of the water-sealed bell was used to calculate minute ventilation (ambient temperature and pressure, saturated with water), which was subsequently corrected to body tempterature, ambient pressure, saturated. Prior to each test, the calibration of the Tissot was verified using a 3 L syringe (Hans Rudolph). Esophageal pressure was measured using a 10 cm esophageal balloon catheter (Ackrad Laboratories Inc., Cranford, N.J.) inserted through the nasal cavity and positioned into the lower third of the esophagus, as per standard procedure (Milic-Emili et al. 1964). Using the procedure described by Butcher et al. (2007), the balloon catheters were passed through a sealed port in the facemask during the SCBA condition. The esophageal balloon catheter was connected to a pressure transducer (MP45 ± 50 cm H2O, Validyne, Northridge, Calif.), which was input into an amplifier (Model MC1-3, Validyne). Data were collected at 200 Hz with an automated digital chart recorder (PowerLab/8SP, ADInstruments, Castle Hill, New South Wales, Australia). Respiratory rate was calculated from the pressure tracings. Esophageal pressure was continuously recorded throughout the rest and exercise stages. Peak inspiratory and expiratory pressures were recorded near the end of each stage of the exercise protocol, but prior to the connection and collection of expired gases into the Tissot to avoid any influence from the hose and spirometer. Esophageal pressure swing, as an estimate of total work of breathing (Butcher et al. 2006), was calculated as peak expiratory esophageal pressure minus peak inspiratory esophageal pressure. Echocardiographic images (Sonos 5500, Agilent Technologies, Andover, Mass.) were acquired simultaneously with records of esophageal pressure during the final minute of each work load, prior to the collection of expired gases with the Tissot Spirometer, using 2-dimensional transthoracic echocardiography at the level of the mid-papillary muscles (parasternal short axis). All images were collected by an experienced sonographer who was kept naı¨ve to the hypothesis. Images were analyzed offline by the same experienced sonographer in the order they were collected; measuring end-diastolic and end-systolic cavity area, and end-systolic myocardial areas (EDCA, ESCA, and ESMA, respectively). Within-subject coefficient of variation, in our laboratory, for EDCA and ESCA during upright exercise has previously averaged 7.1% and 8.1%, respectively (Nelson et al. 2009). The average of 3 cardiac cycles was used for each measurement. Systolic blood pressure (SBP) was measured concurrently with echocardiography using a stethoscope and sphygmomanometer on the right arm. End-systolic blood pressure was calculated as SBP 0.9. The following 5 calculations were performed using known formulas (Haykowsky et al. 2001): SA ¼ EDCA ESCA where SA is stroke area (in cm2), SA EDCA where fractional area change (FAC) indicates LV systolic function, FAC ¼

627 Table 2. Means (±SE) mass, hematological data, and temperature prior to exercise trials with the self-contained breathing apparatus (SCBA) or free-breathing (FB) for 23 subjects. Variable Mass (kg) Hematocrit Hemoglobin (gL–1) Mean corpuscular volume (fL) Mean corpuscular hemoglobin (gL–1) Temperature (8C)

Pre SCBA 82.4±2.2 0.44±0.00 147.4±1.5 89.4±0.6 341.3±0.9 37.0±0.1

Pre FB 82.4±2.2 0.44±0.01 149.0±1.6 89.5±0.6 341.7±0.9 36.9±0.1

ESTMP ¼ ESP mean esophageal pressure where ESTMP is LV end-systolic transmural pressure, LV elastance ¼

ESTMP ESCA

where LV elastance is a surrogate for contractility, and ESCA is measured as mm Hgcm–2, ESWS ¼ 1:33 ESTMP

ESCA ESMA

where ESWS is end-systolic wall stress. Heart rate was recorded every minute using a telemetric heart rate monitor (FS1 receiver and T-31 transmitter, Polar Electro Canada Inc., Lachine, Que.). Core temperature was recorded by telemetry from an ingested capsule and a VitalSense monitor (Mini-Mitter Inc., Bend, Ore.). Ratings of perceived exertion (Borg 1982) and perceived respiratory distress (Morgan and Raven 1985) were recorded at the end of each work load. Perceived thermal distress was also recorded at this time, according to a 9-point scale. This scale ranged from the lowest value of 1 (my body temperature is comfortable) to the highest value of 9 (the heat is unbearable). Analysis A 2-way analysis of variance for repeated measures was used to measure changes between trials throughout each work bout. Upon detection of a main effect, Tukey’s post hoc test was performed to define each difference. Student’s t test was used to detect differences between hematological data for each condition. All statistical analysis was performed using Statistica 7.0 (StatSoft Inc., Tulsa, Okla.). Significance was accepted at p < 0.05. Data are presented as means ± standard error (SE), unless indicated otherwise.

Results All subjects completed the experimental protocol. Because 2 subjects could not tolerate the esophageal balloon catheter, the esophageal pressure data are reported for 21 subjects. Images from 2 subjects were of poor quality and were discarded; therefore, echocardiography data are reported for 21 subjects. As shown in Table 2, there were no differences in pretest body mass, hematological data, or core temperature as subjects began the 2 exercise sets. Published by NRC Research Press

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Appl. Physiol. Nutr. Metab. Vol. 34, 2009


Fig. 1. Heart rate, end-diastolic cavity area (EDCA), end-systolic cavity area (ESCA), end-systolic wall stress (ESWS), left-ventricular (LV) contractility (end-systolic transmural pressure (ESTMP)/ESCA), peak expiratory esophageal pressure (PesExp), fractional area change (FAC), and stroke area (SA). Data are collapsed across all intensities to describe the main effect between the self-contained breathing apparatus (SCBA) and free-breathing (FB) conditions. Values are means ± SE; n = 21. *, Significant difference between conditions. bpm, beats per minute.

Effect of SCBA on cardiorespiratory function Main condition effects are reported in Fig. 1. Heart rate was found to be significantly higher (p < 0.05) with the SCBA than with FB (128.3 ± 2.9 vs. 122.0 ± 2.6 beatsmin–1, respectively). As expected, peak expiratory esophageal pressure was significantly higher with the SCBA than with FB (7.6 ± 0.6 vs. 2.4 ± 0.8 cm H2O, respectively) (Fig. 1), which resulted in a significant reduction in EDCA (LV preload), compared with FB. Stroke area was unaffected by the SCBA, which can be explained by a significant reduction (p < 0.05) in LV ESCA, ESMA, end-systolic wall stress, and an increase in LV fractional area change and LV contractility (Table 3). Peak inspiratory esophageal pressure was also significantly more negative with the SCBA than with FB (–16.7 ± 0.9 vs. –12.7 ± 0.9 cm H2O), resulting in a significant difference in esophageal pressure swing (24.3 ± 0.7 vs. 15.1 ± 0.9 cm H2O, respectively) between conditions (Fig. 2). Mean esophageal pressure was, however, higher with the SCBA (Table 3). No differences were found between conditions for core temperature, minute ventilation, perceived exertion, or respiratory and thermal distress (ratings of perceived exertion, perceived respiratory distress, perceived thermal distress).

Effect of exercise intensity on cardiorespiratory function As expected heart rate, SBP, perceived exertion, respiratory and thermal distress, minute ventilation, and respiratory rate increased (p < 0.05) in response to increases in intensity. As minute ventilation and respiratory rate increased to support metabolic demand, peak expiratory and inspiratory esophageal pressure increased (from 1.3 ± 0.4 to 7.7 ± 0.6 cm H2O) and decreased (from –8.6 ± 0.3 to –18.6 ± 0.7 cm H2O), respectively. Esophageal pressure swing, a surrogate for work of breathing (Butcher et al. 2006), also significantly increased with exercise intensity (Fig. 2). EDCA increased from rest to exercise (p < 0.05), and then tended to decrease with each successive workload. By the final workload, the reduction in EDCA was significantly lower than rest, 50%, and 60% V_ O2 peak. ESCA decreased significantly throughout exercise (Table 3). End-systolic wall stress increased at the onset of exercise, but tended to decrease with each progressive workload and, similar to the EDCA response, the decrease was significant (p < 0.05) at the final workload (Table 3). SBP increased and mean esophageal pressure decreased (more negative) with each workload (p < 0.05), which led to an increase in ESTMP. Published by NRC Research Press

Mayne et al.

629 Table 3. Esophageal pressure, heart rate, and echocardiography data (means ± SE) for each trial at rest and for each exercise workload with a self-contained breathing apparatus (SCBA) or free breathing (FB) (n = 21).


Variable

Rest

50%

60%

70%

80%

Mean

Mean Pes (mm Hg) SCBA –2.2±0.4 FB –3.1±0.4

–3.5±0.6 –3.7±0.8

–3.8±0.6 –3.9±0.8

–3.6±0.5 –4.0±0.9

–3.7±0.6 –4.2±0.8

–4.6±0.5{ –5.1±0.7

EDCA (cm2) SCBA 16.5±0.7 FB 16.9±0.7

17.2±0.5 17.7±0.4

16.7±0.6* 17.7±0.6

15.2±0.6* 16.6±0.7

14.3±0.8 15.1±0.7

16.0±0.6{ 16.9±0.6

6.2±0.3* 6.9±0.4

5.2±0.2* 6.1±0.3

ESCA (cm2) SCBA FB

8.1±0.4* 8.9±0.5

7.0±0.3* 7.9±0.4

4.6±0.2* 5.3±0.3

6.2±0.3{ 7.0±0.4

ESMA (cm2) SCBA 21.2±0.5 FB 21.2±0.6

20.9±0.6 21.0±0.6

20.7±0.7 21.1±0.7

19.6±0.7 20.2±0.7

19.2±0.7 20.1±0.7

20.3±0.7{ 20.7±0.7

ESWS (kdynecm–2) SCBA 55.9±2.9* FB 60.2±3.4

60.1±3.7 63.9±3.8

61.3±3.8 64.4±4.0

56.6±3.7* 63.7±2.9

53.5±2.9 57.3±3.2

57.5±3.4{ 61.9±3.5

ESTMP (mm Hg) SCBA 110.9±3.2 FB 108.2±1.8

132.7±3.5 127.3±2.9

150.0±3.5 142.8±2.7

FAC SCBA FB

156.7±4.5 156.0±3.7

166.3±4.2 163.2±4.1

143.3±3.8 139.5±3.0

0.59±0.02 0.55±0.02

0.62±0.02 0.61±0.02

0.65±0.02 0.62±0.02

0.67±0.02 0.64±0.01

0.61±0.02{ 0.58±0.02

ESTMP/ESCA (mm Hgcm–2) SCBA 15.0±1.4 20.3±1.8* FB 13.1±1.1 16.7±0.9

25.2±1.6* 21.4±1.3

30.7±1.5* 26.3±1.9

37.7±1.9* 31.4±1.9

25.8±1.6{ 21.8±1.5

0.51±0.01 0.48±0.02

Heart rate (beatsmin–1) SCBA 82.3±3.2 FB 75.0±2.7 SA (cm2) SCBA FB

8.4±0.4 8.0±0.3

118.1±2.6 111.7±2.4

131.2±2.9 124.6±2.7

146.6±3.0 141.0±2.6

163.4±2.7 158.4±2.4

10.2±0.6 9.9±0.5

10.5±0.6 10.8±0.6

10.0±0.5 10.5±0.7

9.7±0.7 9.8±0.6

128.3±2.9{ 122.0±2.6 9.7±0.6 9.8±0.6

Note: Pes, esophageal pressure; EDCA, end-diastolic cavity area; ESCA, end-systolic cavity area; ESMA, endsystolic myocardial area; ESWS, end-systolic wall stress; ESTMP, end-systolic transmural pressure; FAC, fractional area change; SA, stroke area. *Significantly different than FB condition. { Significant main condition effect (p < 0.05).

Stroke area was higher during exercise than during rest, and no differences were found between workloads (Table 3). Effect of the SCBA and exercise intensity on cardiorespiratory function Under the conditions of mild or moderate exercise, the SCBA was found to affect esophageal pressure more as exercise intensity was increased (Fig. 2). Furthermore, ESMA decreased (p < 0.05) and LV contractility increased (p = 0.06) with the SCBA, contributing to the maintenance of stroke area during exercise.

Discussion This investigation was designed to simulate a task common to firefighting (climbing stairs) at a range of intensities most frequently observed during fire suppression duties

(Gledhill and Jamnik. 1992). The novel results of this investigation are that the SCBA decreased LV preload (EDCA) secondary to a higher peak expiratory esophageal pressure, and stroke area was maintained through a reduction in ESCA secondary to an increase in LV contractility (ESTMP/ESCA) and a decrease in end-systolic wall stress. Therefore, while LV function was altered during exercise up to 80% V_ O2 peak with the SCBA, the heart can seemingly compensate for changes in preload caused by the SCBA. Effects of SCBA on LV function In accordance with previous findings (Butcher et al. 2006; 2007), we demonstrated a significant difference in esophageal pressure with the SCBA, compared with FB (the control) during mild to moderate exercise. Specifically, with the SCBA, peak inspiratory pressures are lower (more negative) and peak expiratory pressures are higher (more posiPublished by NRC Research Press

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Fig. 2. Mean (± SE) expiratory esophageal pressure (PesExp) (top) and inspiratory esophageal pressure (PesIns) (bottom), at rest and during exercise at a work intensity equivalent to 50%, 60%, 70%, and 80% of peak oxygen consumption (V_ O2 peak) with the selfcontained breathing apparatus (SCBA) or free-breathing (FB) conditions (n = 21). *, Significant difference between trials for esophageal pressure (Pes) swing (Pesmax – Pesmin); {, significant difference between trials (p < 0.05).

Appl. Physiol. Nutr. Metab. Vol. 34, 2009

duration of the 10 min workloads. Direct comparison of these 2 studies remains difficult, as the degree of expiratory loading was greater in the experiment by Stark-Leyva et al. (2004). Furthermore, the SCBA results in a significantly lower (more negative) inspiratory pressure, which was not a feature of the expiratory loading model used in the previous experiment. Limitations Two-dimensional echocardiography is subject to both movement and respiratory artifact, which may limit the use of this technique in some exercise applications. However, our laboratory has reported a small within-subject coefficient of variation during exercise for fractional area change of 8.7% (Stickland et al. 2006) and for EDCA and ESCA of 7.1% and 8.1%, respectively (Nelson et al. 2009). There is also possibly a bias toward images from the expiratory portion of the duty cycle because of less respiratory artifact.

Conclusion

tive), which leads to a significantly greater esophageal pressure swing than FB (p < 0.05). As exercise intensity increases, total work of breathing also increases, resulting in an altered pressure environment in which the heart must function. As shown in Table 3, at work rates above 60% V_ O2 peak (corresponding to minute ventilation in the range of 60–90 Lmin–1), EDCA was reduced in the SCBA condition due to the significantly greater positive esophageal pressure required to exhale against the external resistance. Contrary to our hypothesis, LV systolic function was increased in the presence of reduced preload, resulting in maintenance of stroke area. This result can be explained by the increased contractility and reduced LV systolic wall stress we found (Table 3). However, we did not observe a reduction in ESTMP, a surrogate for afterload, which might be expected. While esophageal pressure swing was significantly greater with the SCBA (Fig. 1), mean esophageal pressure, which is used to estimate ESTMP, was not different between conditions (Table 3). Therefore, in the presence of a similar end-systolic pressure, transmural pressure was maintained between conditions. The maintenance of stroke area is, therefore, explained by the significant increase in LV systolic function (LV contractility and fractional area change), and not a reduction in transmural pressure. Our results show that despite a significant reduction in LV preload when exercising with the SCBA, stroke area is maintained by an increase in LV systolic function. These results differ from those of Stark-Leyva et al. (2004), where a constant expiratory load (10 cm H2O) caused a reduction in stroke volume at rest and during cycling exercise at 40% and 70% V_ O2 peak. In that experiment, the reduction in stroke volume was apparent within 30 s of the application of the expiratory resistance, and remained reduced over the

In conclusion, the SCBA regulator increases expiratory and inspiratory esophageal pressure, compared with FB, during mild- to moderate-intensity exercise. The changes observed in esophageal pressure have the potential to negatively impede stroke area and cardiac area output, if not for increases in LV contractility. However, these findings support the notion that under times of reduced preload caused by increased intrathoracic pressure, contractility is increased to maintain stroke area. Thus, it appears that during stepping exercise up to 80% V_ O2 peak, a healthy heart can compensate for reductions in preload by increasing LV systolic function.

Acknowledgements The authors thank all the volunteers for their time and effort. A special thanks to Allen MacLean for his technical expertise. This project was supported by the Department of National Defence, the Canadian Forces Personnel Support Agency, and the Canadian Forces Fire Marshal.

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