Pediatric Pulmonology 16:31-35 (1993)

Gender Related Differences in Airway Tone in Children

Louis 1. Landau, MD, Wayne Morgan, YD, Karen S. McCoy, YD, and Lynn M. Taussig, MD

Summary. The effects of gender, volume history, and inhaled atropine and isoproterenol on lung mechanics were assessed in 16 normal boys and 14 normal girls using lung volumes, flow- volume curves, and oscillatory resistances. Flows were measured from full and partial forced expiratory flow-volume curves. Six girls and 6 boys were studied before and after inhaled atropine, and 10 boys and 8 girls before and after inhaled isoproterenol. Girls demonstrated a significant increase jn flows on full and partial curves with a deep inspiration vmax-partial 0.73 2 0.34 (SD) to Vmax-fullO.80 2 0.37 and 0.83 2 0.20 to 1.06 2 0.29 TLCls in each group] and following inhalation of isoproterenol on the partial curves only (0.73 2 0.34 to 0.93 5 0.40 TLC/s). Boys showed a small but significant increase in Vmax with isoproterenol on full curves but not on partial curves. Following atropine, boys demonstrated a significant increase in Vmax on partial flow-volume curves (0.78 & 0.28 to 1 .OO 2 0.35 TLC/s) and a significant decrease in specific respiratory resistance (7.6 -C 2.7 to 5.1 2 0.9 cmH,O/s), whereas girls had no such changes. These data suggest that boys have greater resting airway tone than girls and that this tone is less responsive to deep inspiration and isoproterenol independently, although a combina- tion of isoproterenol and a deep inspiration will produce increased flows in boys. Atropine reduces airway tone predominantly in boys, suggesting that the increased resting airway tone in boys is partially mediated via the vagus nerve. Pediatr Pulmonol. 1993; 16:31-35. 6 1993 Wiley-Liss, Inc.

Key words: Lung mechanics; atroplne; volume history; children; lsoproterenol.

INTRODUCTION

Previous studies have described a variety of sex-related differences in the mechanical properties of the lung in children. These include larger lung volumes for boys than girls of comparable height14; larger size-corrected flows for girls536; greater respiratory resistances for boys than girls7*'; larger dead-space/lung size ratios for girls'; and increased flows on full vs. partial expiratory flow-volume curves for girls. l o Boys can generate greater maximum inspiratory and expiratory pressures than girls. ' I Thurl- beck'* reported that alveolar dimensions did not differ between boys and girls, and that the larger lung volume was due to a greater total number of alveoli. There did not appear to be more alveoli per unit of peripheral airway. It has been postulated that girls have larger airways (com- pared to the size of their lung parenchyma) than do boys. However, the physiologic and/or anatomic bases for the larger size-corrected flows in girls are not known.

A deep inspiration has been shown to dilate airways and may decrease elastic recoil pressure of the lung (PstL).'3-'s The effect on flows will depend on the rela- tive contributions of each of these changes. In dogs, induced bronchoconstriction of peripheral airways, but not of central airways, can be overcome by a deep inspi- ration.6 Atropine produces no significant change in PstL but can reduce the degree of hysteresis of the airwayIs and increase flows, thought to occur by dilatation of 0 1993 Wiley-Liss, Inc.

predominantly central airways. ' 7918 Isoproterenol prefer- entially dilates the smaller peripheral airway^'^,'^ and appears not to alter Pst, when given by inhalation, al- though very high doses may cause some loss of recoil.''

The present study was undertaken to assess gender differences in airway tone in normal children by volume history maneuvers and by the inhalation of atropine and isoproterenol.

From the Pediatric Pulmonary Section, Department of Pediatrics; Res- piratory Sciences Center, Westend Laboratories; Steele Memorial Children's Research Center; Arizona Health Sciences Center, Tucson, Arizona.

Received September 8, 1992; (revision) accepted February 4, 1993.

This work was supported by SCOR grant 14136 from the NIH (NHLBI) and a Clinical Fellowship (Dr. McCoy) from the Cystic Fibrosis Foundation.

Address correspondence and reprint requests to: Dr. L.M. Taussig, Department of Pediatrics, Arizona Health Sciences Center, 1501 N. Campbell Avenue, Tucson, AZ 85724.

Dr. K.S. McCoy is now at Ohio State University, Columbus Chil- dren's Hospital.

32 Landau et al.

MATERIALS AND METHODS Subjects

Thirty untrained 8- to 10-year-old prepubescent chil- dren were studied. Each had a negative history for neona- tal respiratory diseases, bronchiolitis, croup, pneumonia, wheezing, cough for more than 3 weeks duration, and systemic illness or symptoms suggestive of allergic air- ways disease. There was no strong family history of asthma or allergic rhinitis for any of the subjects. In addition, each subject had been free of a respiratory tract infection (including “colds”) for at least 3 weeks prior to the study. The study was approved by the Human Sub- jects Committee of the Arizona Health Sciences Center. Consent was obtained from the subjects and their parents.

Lung Volumes Lung volumes were measured in a whole-body vol-

ume-displacement plethysmograph,20 which includes a Krogh spirometer with a frequency response flat to 10 Hz. Thoracic gas volume (TGV) at functional residual capacity (FRC) was calculated. Total lung capacity (TLC) was calculated as the sum of the TGV + inspiratory ca- pacity; the mean of 2-3 such values not differing by more than 10 percent was used for determining TLC. Residual volume (RV) was calculated as the difference between the TLC and the largest vital capacity (VC) obtained at any time during the testing procedure. Lung volumes were measured pre and post atropine, and pre and post isoproterenol .

Flow-Volu me Curves Flow volume curves were measured in the whole-body

plethysmograph. Flows were measured with a #3 Fleisch pneumotachograph, linear to 9 Us; volumes were mea- sured plethysmographically . Flow-volume curves were displayed on a Tektronix 5 103N oscilloscope and photo- graphed for subsequent measurements.

Standard maximal expiratory flow-volume curves (full) were performed following inhalation to TLC, and partial expiratory flow-volume curves (partial) were per- formed from end-tidal inspiration with a subsequent inha- lation to TLC to demonstrate that the VC was reproduc- ible. All full and partial curves were obtained after two or more tidal breaths with the same FRC point on the oscil- loscope. These volume history maneuvers were com- pleted in random order with at least a 20-30 second pause of tidal breathing between successive individual maneu- vers to avoid unwanted volume history effects.

Maximal flows at 50% vital capacity (Vmax509b vc) were measured from curves matched at TLC. Flows were also measured at other absolute lung volumes (Vmax2s4h vc, Vmax508 TLC, and Vmax4”% .,-Lc,) but not reported. Results were essentially the same as those measured at Vmaxs,l,fi v c . The Vmax measurements on each curve

(full and partial, pre and post drug) were made at isovol- umes points determined by measuring the same absolute volumes down from TLC on each maneuver, since TLC did not change after inhalation of the two drugs. All flows were normalized for lung size by expressing them as TLC/s.

Respiratory Resistance Total respiratory resistance was determined by the

forced oscillation method. A 6 Hz oscillating sine wave was generated by piston displacement ( I 00 mL volume). Flows and integrated volume were determined by a Fleisch #3 pneumotachograph (linear to 9 Ws). Pressure at the mouthpiece was measured by a Validyne MP45-I transducer ( 2 100 cm H20) . Volume, flow, and pressure were recorded on a Hewlett-Packard recorder. Flow and pressure envelopes were drawn at mid-inspiratory tidal volume,21 and the average of five to ten such calculations (breaths) was used to derive total respiratory resistance. Specific respiratory system resistance SR,, was computed by multiplying mean resistance (cm H20/L/s) by thoracic gas volume at FRC, yielding units of cm H,O/s.

Drug Inhalation Studies After completing the baseline studies, 6 boys and 6

girls were given nebulized atropine sulfate (Elkins-Sinn, Inc., 50 pg/kg), delivered by a Hudson nebulizer, model 1720 (>75% of particles 0.5-5 pm) driven by air (ap- proximately 10 Wmin) for 15 minutes. Following the atropine inhalation and an additional 15 minute wait, the entire series of pulmonary function tests was repeated. Ten boys and 8 girls were given sequential breaths of 1:200 isoproterenol from a metered dose inhaler until the pulse increased by 20% or a maximum of 7 breaths had been given. All children received between 3 and 7 puffs, with a mean of 4 puffs. There was no significant differ- ence in the number of puffs given to boys and girls, and both groups had a significant increase in pulse rate (pre drug, 86 * 15; post drug, 118 2 25 bpm, P < 0.05).

Statistical Analysis Statistical evaluation was by paired-sample t analysis

for baseline vs.post-volume history maneuver and post drug comparisons, and the Wilcoxon matched-pairs signed-rank analysis for comparison of the non-normally distributed S q , at baseline and post drug. Two-tailed tests were used, P values <0.10 are reported and those <0.05 were accepted as significant.

RESULTS

The male and female groups were very similar with respect to age and height (Table 1). Nonetheless, the boys had higher baseline values than girls for TLC (P < 0.05) and VC (P < 0.05) but not for volume-corrected flows.

Gender and Airway Tone 33

TABLE 1-Demographic and Baseline Pulmonary Function Data (Mean 2 SD)

Function

Number Age (years) Height (cm) TLC (L) vc (L) y m a ~ 5 o s v-fulfull (TLC/s) Vmax,,, .-partial (TLCls) SR, (cmH,O/s)

*P < 0.05.

Atropine group Isoproterenol group

Boys Girls Boys Girls

6 6 10 8 9.5 f 1.2 9.5 f 0.6 9.3 f 1.0 10.9 f 0.7 136 f 5 134 f 9 135 f 8 139 5 2

3.03 f 0.47 2.45 rt 0.44* 3.10 & 0.46 2.79 & 0.65 2.62 f 0.39 2.09 f 0.33* 2.73 f 0.43 2.48 f 0.53 0.86 2 0.26 1.06 2 0.29 0.89 2 0.22 0.80 f 0.37 0.78 f 0.28 0.83 f 0.20 0.88 2 0.17 0.73 ? 0.34 7.6 f 2.7 5.3 2 3.2 4.7 2 1.2 4.1 % 1.4

TABLE 2-Pulmonary Function (Mean f SD) Before and After Atropine and lsoproterenol

Pre Post Pre Post Function atropine atropine isoproterenol isoproterenol

Partial VmaxJOc vc (TLC~S)

FUII VmaxJO, vc (TLC~S)

SR, (cm H,O/s)

Girls TLC (L)

vc (L)

Partid \jmax509h vc (TLC/S)

Full Vmax 50, vc (TLCls)

SR,, (cm H,O/s)

3.03 & 0.47

2.62 f 0.39

0.78 -+ 0.28

2.86 +- 0.28

2.48 2 0.36

I .OO f 0.35

NS

NS

P = 0.003

NS NS NS

0.86 f 0.26 7.6 2 2.7

0.91 5 0.27 5.1 f 0.9

P = 0.04

2.45 2 0.44

2.09 f 0.33

0.83 f 0.20

2.56 2 0.46

2.08 2 0.33

0.97 k 0.33

NS

NS

NS

NS P = 0.05 NS

I .06 f 0.29 5.3 2 3.2

I .07 2 0. I6 4.2 2 1.2

NS

3.105 0.46 3.11 ? 0.44 NS

2.73 f 0.43 2.72 2 0.32 NS

0.88 f 0.17 0.95 2 0.26 NS

P = 0.03 NS NS

0.89 5 0.22 4.7 f 1.2

0.99 2 0.28 4.8 ? 1.3

NS

2.79 5 0.65

2.48 -t 0.53

0.73 % 0.34

2.56 5 0.46

2.54 ? 0.54

0.93 % 0.40

NS

NS

P=001

NS P = 0.04 NS

0.80 f 0.37 4.1 f 1.4

0.85 5 0.38 3.9 2 1.3

NS

NS, no significant differences.

S k s appeared higher for boys but was not significantly different due to considerable variability.

Volume History Effects (Table 2) There was no significant difference in baseline vol-

ume-corrected flows at any lung volume between boys and girls. At baseline, boys and girls both had higher flows on maneuvers involving a previous deep inspira- tion, such as the full curves compared to the partial ma- neuvers, but significant increases were seen only in the two groups of girls (0.83 k 0.20 (SD) to 1.06 k 0.29 pre-atropine and 0.73 * 0.34 to 0.80 2 0.37 TLCls pre- isoproterenol). This effect of volume history was not seen following atropine or isoproterenol for either gender.

Atropine Effects There were no changes in lung volumes following atro-

pine. Both boys and girls had increased isovolume flows on post-atropine maneuvers compared to baseline (Table 2), but the change was most consistent and only statisti- cally significant in the boys during partial maneuvers (Vmax - partial 0.78 f 0.28 to 1.00 2 0.35 TLC/s; P < 0.005). Boys also showed a decrease in SR, from 7.6 * 2.7 to 5.1 * 0.9 cmH,O/s (P < 0.05), which was not seen in girls.

lsoproterenol Effects There were no significant changes in lung volumes or

SR, following isoproterenol (Table 2). Isoproterenol did

34 Landau et al.

not produce increased flows on partial curves in boys but did result in a significant though small increase in Vmax for boys on the full forced expiratory maneuver (0.89 * 0.22 to 0.99 ? 0.28 TLC/s, P <: 0.05). In girls, there was a significant increase in Vmax after isoprotere- no1 on the partial curves (0.73 ? 0.34 to 0.93 2 0.40 TLC/s; P < 0.01), which was not seen with the full forced expiratory maneuvers.

DISCUSSION

The most important new findings from this study are the significantly greater increases in Omax on partial forced expiratory maneuvers and decreases in specific respiratory resistance following atropine in boys com- pared with girls. Although the girls consistently demon- strated some increase in flows and decreases in resistance after atropine, the changes were smaller than in boys and not statistically significant. Thus, there appears to be a sex-related quantitative difference in the response of air- ways to atropine during childhood.

In the present study, significantly higher flows on full curves (following a deep inspiration) compared with par- tial maneuvers were observed in girls but not in boys, confirming our previous observation. ’’ Isoproterenol also resulted in greater flows with the partial maneuvers in girls but not for boys. However, the combination of isoproterenol and a full inspiration led to increased flows on full curves in the boys but not in girls. The increase of flows with a deep inspiration in the full maneuvers (as compared to partial curves) was less consistent after atro- pine or isoproterenol, in both boys and girls.

Some investigators have reported that a rapid inspira- tion has a greater effect on reducing airway resistance than a slow inspiration,22 although others could find no effect of rate of inspiration or length of breath-hold at TLC.” Duggan et aLz3 demonstrated that the effect was due to the PstL generated and not the volume change. In this study maneuvers were standardized with a fast in- spiratory flow and no breath-hold.

Our data suggest that boys have higher resting airway tone in either equal size or smaller airways than girls. This increased tone in boys is not affected by a deep inspiration or isoproterenol independently, but is de- creased by atropine and by the combination of isoprotere- no1 and a deep inspiration. Nadel and Tierney“ first showed that airway resistance in normal adults decreased after a deep inspiration. It was subsequently shown that a full inspiration produced higher maximum expiratory flowsz5 and overcame the fall in flow following induced bronchospasm. 26 The deep inspiration has a greater effect on increasing airway caliber than on reducing lung recoil, and it is likely that this effect is occurring predominantly in the smaller airways, compatible with reports of the site of action of isoproterenol in previous studies.”-lS The

failure to overcome the increased tone by either a deep inspiration or isoproterenol alone, as seen in the boys in this study, suggests that any small airway tone is not easily reduced. A significant change in boys required a combination of the two procedures.

Increased flows have been described in normal adults following the inhalation of atropine, ‘7*27 but the gender differences noted in this study have not been previously reported. Atropine is thought to act predominantly on the larger central airways,” and the findings in this study would suggest that boys have increased tone in these major airways, which may be responsible, in part, for the previously reported higher airway resistance, lower vol- ume corrected flows, and decreased dead space in

There was no evidence that the boys in this study had asthma or a predisposition to asthma. Subjects were cho- sen because they had low risk factors for asthma, lung function measurements were normal, and the response to isoproterenol was not in the range considered diagnostic for asthma. In addition, Barnes et a1.’* have reported that asthmatics fail to increase flows and may even decrease flows with a deep inspiration when compared to normal subjects.

These findings may help to explain certain clinical observations during childhood. The increased tone noted in the males in this study may be a marker for the in- creased prevalence and/or severity of lower airway ob- struction, including asthma and bronchiolitis, which have been consistently observed in boys.

REFERENCES

I . Bjure J . Spirometric studies in normal subjects: Ventilatory capac- ities in healthy children 7-17 years of age. Acta Paediatr. 1963;

2. DeMuth GR, Howatt WF, Hill BM. The growth of lung function.

3. Dickman ML, Schmidt CD, Gardner RM. Spirometric standards for normal children and adolescents (ages 5 years through 18 years). Am Rev Respir Dis. 1971; 104:68M87.

4. Hsu KHK. Jenkins DE, Hsp BP, et al. Ventilatory functions of normal children and young adults-Mexican American, white and black I. spirometry. J Pediatr. 1979; 95:14-23.

5. Leeder SR, Swan AV, Peat JK, et al. Maximum expiratory flow- volume curves in children: Changes with growth and individual variability. Bull Eur Physiopath Resp. 1977; 13:249-260.

6. Taussig LM. Maximal expiratory flows at functional residual ca- pacity: A test of lung function for young children. Am Rev Respir Dis. 1977; 116:1031-1038.

7 . Doershuk CF, Fisher BJ, Matthews LW. Specific airway resis- tance from the perinatal period into adulthood. Am Rev Respir Dis. 1974; 109:452457.

8. Godfrey S, Kamburoff PL, Nairn JR, et al. Spirometry, lung volumes and airway resistance in normal children aged 5-18 years. Br J Dis Chest. 1970; 64:15-24.

9 . Hutchinson A, Sum A, Demis T, Erben A, Landau 1. Moment analysis of multiple breath nitrogen washout in children. Am Rev Respir Dis. 1982; 125:28-32.

53:232-240.

PedkdtriCS. 1965; 35(sUppl):187.

Gender and Airway lone 35

on lung mechanics in normal man. J Appl Physiol. 1979; 46:217- 226.

20. DeBois AB, Botelho SY, Bedell GN, et al. A rapid plethysmo- graphic method for measuring thoracic gas volume: A comparison with a nitrogen washout method for measuring functional residual capacity in normal subjects. J Clin Invest. 1956; 35:322-326.

21. Goldman M, Knudson R. Mead J, et al. A simplified measure- ment of respiratory resistance by forced oscillation. J Appl Phys- iol. 1970; 28: 11 3-1 16.

22. Hida W, Arai M, Shindoh C, Liu Y-N, Sasaki H, Takishima T. Effect of inspiratory flow rate on bronchomotor tone in normal and asthmatic subjects. Thorax. 1984; 39:8&92.

23. Duggan CJ, Chan J, Whelan AK, Berend N. Bronchodilatation induced by deep breaths in relation to transpulmonary pressure and lung volume. Thorax. 1990; 45:93&934.

24. Nadel JA, Tiemey DF. Effect of a previous deep inspiration on airway resistance in man. J Appl Physiol. 1961; 16:717-719.

25. Green M, Mead J. Time dependence of flow-volume curves. J Appl Physiol. 1974; 37:793-797.

26. Bouhuys A, Hunt VR, Kim BM, Zapletal A. Maximum expiratory flow rates in induced hronchoconstriction in man. J Clin Invest.

27. Vincent NJ, Knudson R, k i t h DE, et al. Factors influencing pulmonary resistance. J Appl Physiol. 1970; 29:236-243.

28. Barnes PJ, Gribbin HR, Osmanliev D, Pride NP. Partial flow- volume curves to measure bronchodilator dose-response curves in normal humans. J Appl Physiol. 1981; 50:1193-1997.

1969;48:1159-1168.

10. Taussig LM, Cota K, Kaltenbom W. Different mechanical prop- erties of the lung in boys and girls. Am Rev Respir Dis. 1981; 123:640-643.

11. Wilson JH, Cooke NT, Edwards RIJT, Spiro SE. Predicted nor- mal values for maximal respiratory pressures in Caucasian adults and children. Thorax. 1984; 39535-538.

12. Thurlbeck WM. Postnatal human lung growth. Thorax. 1982; 37:564-571.

13. Wellman JJ, Brown R, Ingram RH, Jr., et al. Effect of volume history on successive partial expiratory flow-volume maneuvers. J Appl Physiol. 1976; 47:153-158.

14. Fish JE, Arkin MG, Kelly JF, Peterman VI. Regulation of bron- chomotor tone by lung inflation in asthmatic and non-asthmatic subjects. J Appl Physiol. 1981; 50:1079-1086.

15. Fairshter RD. Airway hysteresis in normal subjects and individu- als with chronic air flow obstruction. J Appl Physiol. 1985;

16. Drazen JM, Loring SH, Jackson AC, et al. Effects of volume history on airway changes induced by histamine and vagal stimu- lation. J Appl Physiol. 1979; 47:657465.

17. IngramRL, Jr., Wellman JJ, McFadden ER, Jr., Mead J. Relative contributions of large and small airways to flow limitation in normal subjects before and after atropine and isoproterenol. J Clin Invest. 1977; 59:696-703.

18. Hensley MJ, O’Cain CF, McFadden ER, Jr., Ingram RHJ. Distri- bution if bronchodilation in normal subjects: Beta agonist versus atropine. J Appl Physiol. 1978; 45:778-782.

19. DeTroyer A, Yemault J, Rodenstein D. Effects of vagal blockade

58: 1505-15 10.