Transient Airway Cooling Modulates Dry-Air-induced and Hypertonic Aerosol-induced Bronchoconstriction 1- 3
ARTHUR N. FREED, STEVEN D. FULLER, and CHARLES E. STREAM
More than a decade ago it was suggested that either airway heat or water loss initiated exercise-induced asthma (1-3). Today, the mechanism responsible for this form of airflow-induced bronchoconstriction (AlB) remains unknown. Some investigators believe that airway cooling initiates AlB, and a rewarminginduced hyperemia leads to airway obstruction (4, 5). Although a few studies have indirectly addressed this hypothesis (6, 7), none has demonstrated a cause and effect relationship between changes in airway surface temperature, a subsequent rebound hyperemia and edema, and airway obstruction. Other investigators suggest that changes in airway fluid osmolality resulting from evaporativewater loss initiates mediator releaseand triggers AlB (8-10). Although the inability to measure changes in osmolality remains problematic, many studies provide indirect support for this latter hypothesis. For example, hypertonic-induced bronchoconstriction (HIB) in asthmatic subjects does correlate wellwith AlB (10-13). Peripheral airway responses to local insufflation of dry air through sublobar segments of anesthetized dogs has been previously characterized and described as a model of exercise-induced asthma (14). Although peripheral airways in this canine model are exposed to unidirectional airflow, responses are strikingly similar to those reported for human subjects undergoing exercise or hyperpnea (4). The time course over which AlB develops and subsides is similar (14), and the degree of airway cooling is correlated with the magnitude of the response (15).Epithelial cell damage is associated with AlB in both species, and mediator profiles in postchallenge canine bronchoalveolar lavage fluid are similar to those recently reported in humans after hyperpnea challenge (16-18). As in humans, humidified air (14, 15), ~-agonists (19), methylxanthines (20),and muscarinic receptor antagonists (15) can reduce or abolish AlB. In addition, the attenu358
SUMMARY Airflow-Induced bronchoconstrlctlon (AlB) may be Initiated In asthmatic patients by Inhaling dry air during eucapnlc hyperventilation or exercise. Hypertonic aerosol-Induced bronchoconstrlctlon (HIB) also occurs In these patients, but It differs from AlB by exhibiting a faster time course. Although AlB and HIB probably Increase airway fluid osmollllty, only AlB Is associated with Ilrway cooling. In light of the slmlllrities between our canine model and human AlB, weexamined peripheral airway responses to dry Ilr and hypertonic aerosol challenge. Specifically, we studied the magnitude Ind time course ofthese responses In In In situ, Isolated, perfused lobe In which airway temperature was Independently controlled. At body temperature, HIB peaked Immediately after challenge, whereas transient airway cooling during lerosal challenge delayed HIB. In contrast, airway cooling attenuated AlB but did not liter Its time course. Hypocapnia- Ind hlstemlne-Induced response. were not affected by airway cooling, suggesting thlt smooth muscle function was not Impaired. To the extent that the mechanisms producing AlB In dogs and In human. are similar, our results suggest that (1) changes In airway fluid osmollllty Initiate AlB, (2) AlB = HIB + Cooling, and (3) exercise-Induced asthma results from an Imbalance between an excitatory pathway stimulated by airway drying and In Inhibitory pathway Initiated by airway cooling. AM REV RESPIR DIS 1991; 144:358-362
ation of AlB via inhibition of cyclooxygenase that was first described in dogs (16) was recently confirmed in asthmatic humans (21). Finally, as in humans, a positive correlation between HIB and AlB has been demonstrated in this canine model (10-13, 17, 22). The comparison of human (4) and canine data (23)leads us to hypothesize that AlB results from an imbalance between two opposing mechanisms: an excitatory pathway stimulated by airway drying and an inhibitory pathway initiated by airway cooling (23). Although many similarities exist between AlB and HIB, there is a marked difference in the time course of these two responses (22). Unlike AlB, HIB occurs in the absence of airway cooling and is unopposed by this apparent inhibitory stimulus. This coldassociated inhibition may account for the slow onset of constriction that characterizes AlB in our canine model (14,22), and it may explain the differential responses to aerosol and dry air challenges reported in asthmatics (10, 11,24). In this study, we demonstrate that transient cooling (1) attenuates peripheral lung constriction to dry air, (2) reduces and delays the response to a hypertonic aerosol challenge, and (3) does not alter responses to either hypocapnia or to aerosolized histamine, indicating that smooth muscle function is not impaired.
Methods Dogs were handled in accordance with the standards established by the U.S.Animal Welfare Acts set forth in DHEW (NIH) guidelines and the Policy and Procedures Manual published by The Johns Hopkins University School of Hygiene and Public Health's Animal Care and Use Committee. Male mongrel dogs (mean weight ± SE = 18.3 ± 0.6 kg, n = 14) were anesthetized with pentobarbital sodium (30 mg/kg), and supplemented with pentobarbital (30 mg) and pancuroniurn bromide (1 mg) as needed. The dog was ventilated through a tracheal tube with a constant-volume respirator. End-expiratory CO 2 was monitored with a CO 2 analyzer (Beckman LB-2; Beckman Instruments, Fullerton, CAl and maintained between4 and 5 0J0 by adjusting the respiratory frequency. Rectal temperature was monitored and maintained with a warming pad during the course of the experiment. Heart rate (HR) and mean arterial pressure (pa) weremonitored through-
(Received in originalform September 4, 1990 and in revised form January 2, 1991) 1 From the Department of Environmental Health Sciences, Division of Physiology, The Johns Hopkins University, Baltimore, Maryland. 2 Supported in part by Grants R29 HL-39406 and SCOR Grant HL-37119 from the National Heart, Lung, and Blood Institute. 3. Correspondence and requests for reprints should be addressed to Dr. Arthur N. Freed, 7006 Hygiene, 615 North Wolfe Street, Baltimore, MD 21205-2179.
AIRWAY COOLING MODULATES AIRWAY REACTIVITY
out all experimental trials via a catheter placed in the femoral artery.
compressedair (C0 2 = O%t 200ml/min). The above challenges wereperformed at body temperature and compared with responses to Measurement of Bronchoconstriction in identical challenges done in the same wedged the Lung Periphery segment but accompanied by transient coolA fiberoptic bronchoscope (Model BFA-4B2; ing from 39° to 29° C only during the period Olympus Corp. of America, New Hyde Park, of challenge. A Taw of 29° C represents the NY) with an outer diameter of 5.5 mm was . lower limit of temperature change typically inserted into the tracheal tube and visually recorded during dry-air challenge when using guided until it wedged in a peripheral airway this technique (23). in the in situ isolated left lower lobe (LLL). Statistical Analyses Room temperature 5070 CO 2 in air was delivered to this wedged segment at constant flow Pb data were analyzed using repeated mea(200 mllmin) through one lumen of a dual sures ANOVA and Duncan's multiple range lumen catheter inserted through the suction test. Pretreatment and post-treatment Pat port of the bronchoscope. The other lumen mean pulmonary artery pressure, and Tpa was used to monitor end-expiratorysublobar werecompared using paired t tests. Unpaired airway pressure (Pb), an indicator of airway t tests were used to compare Tpa to Taw. All resistance, which was recorded and compared values represent mean ± SE. Statistical sigbefore and after exposure of the wedged seg- nificance was judged at p < 0.05 in all cases. ment to dry airflow, hypertonic saline, hypocapnia, and histamine. In one animal, a Results specially modified 5-F Swan-Ganz thermodiEffect of Transient Sub/obar Cooling lution catheter with its thermistor exposed at on Responses to Dry-air Challenge the tip was threaded through the suction port of the bronchoscope and used to simultane- When LLL Tpa was maintained at 38.7 ously measure airway wall temperature (Taw) ± 0.2 0 C, Pb increased 60 ± 116,10 (n = and Pb during dry air challenge (15). 6) 2 min after challenge with dry air. In In Situ Isolated Lobe Preparation Blood flow to the LLL was controlled using an open-chest preparation that was previously described in detail (23).Briefly,a catheter was placed in the branch of the pulmonary artery (PA) entering the LLL. Lobar arterial pressure (Ppa) and perfusate temperature (Tpa) were measured at the tip of the PA catheter using a modified thermodilution catheter. Blood for perfusing the LLL was drawn from the femoral vein by a rotary pump set at 150 mllmin and passed through a bubble trap and a heat exchanger perfused with 39° C water to maintain the blood at body temperature (Tb), Blood was rapidly cooled by switching the intake of the heat exchanger to a water bath set at 29° C. The LLL was rapidly rewarmed by reversing this procedure.
Experimental Protocols Dry-air challenge was done by increasing the 200ml/min baseline dry airflow with 5070 CO 2 to either I t500 or 2tOOO mllmin for 2 min. This period of increased airflow is analogous to a period of exercise or hyperventilation (14). After airflow was returned to 200 ml/min, Pb was recorded at 30 s and at 2 and 5 mint and then every5 min until Pb returned to baseline. Histamine (30 or 50 ug/ml) or 14.4% NaCI (4AOO mOsm/kg) was deliveredthrough the bronchoscope to the obstructed lung segment in the form of aerosols generated by a DeVilbissUltra Neb 100ultrasonic nebulizer (DeVilbiss CO. t Somerset, PA). The duallumen catheter was temporarily removedfrom the bronchoscope, and the aerosol was delivered through the suction port using 5% CO 2 in air flowing at 200 mllmin for a 6O-s period. Wedged peripheral lung segments were made hypocapnic for a 2-min period by switching the standard 5% CO 2 in air with
contrast, when Tpa was decreased to 29.2 ± 0.10 C during the course of the dry-air challenge, Pb increased only 37 ± 13070 2 min after exposure, and responses were significantly attenuated (p < 0.01) throughout the 15-min postchallenge period when compared with values recorded at normal Tpa (figure 1). HR (205 ± 14 versus 210 ± 12 beats/mint p = 0.759), Pa. (83 ± 5 versus 84 ± 3 mm Hg, p = 0.930), mean pulmonary artery pressure (16 ± 2 versus 16 ± 2 mm Hg, p = 0.782), and Tpa (38.7 ± 0.2 versus 38.6 ± 0.10 C, p = 0.247) were not significantly different directly preceding warm and cold challenges, respectively (n = 6).
Comparison of Perfusate Temperature to Airway Wall Temperature Dry air challenge with transient cooling was repeated four times in one animal in which Tpa and Taw were simultaneously recorded. Tpa fell 9.0 ± 0.02 0 C, from 38.6 ± 0.02 to 29.6 ± 0.04 0 C during the 2-min challenge. Taw trailed Tpa by 6 ± 0.8 s and dropped 8.8 ± 0.18 0 C from 38.7 ± 0.02 to 29.9 ± 0.17 0 C during this same period. There were no significant differences between Tpa and Taw or delta 'Ipa and delta Taw. Tpa rewarmed at a centigrade rate of 0.12 ± O.1/s,which was significantly faster (p = 0.014) than the rewarming rate of 0.11 ± O.1/s for Taw. Thus, Tpa x 0.88 = Taw. For our dry air study (figure 1), Tpa rewarmed over the first 60-s postchallenge period, from 29.2 ± 0.12 to
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Time (min) Fig. 1. The effect oftransient cooling on sublobar pressure (Pb) during a 2-min dry-air challenge. A. Pb before and after dry-air challenges (vertical bar). B. Temperature of blood (Tpa) perfusing the lobe before, during. and after challenge (n = 6. mean ± SE). Double asterisks indicate p = 0.01. Open circles = warm; closed circles = cold.
37.6 ± 0.33 0 C at a rate of 0.14 ± O.OI/s (n = 6). This corresponds to an estimated Taw rewarming rate of 0.12 ± O.Ol/s. For purposes of comparison, the average Taw rewarming rate was calculated for all I t500 and 2,000 ml/min dry air challenges published to date (15, 20, 23 t 25). After a 1,500-ml/min challenge, airways rewarmed over the first 60-s postchallenge period, from 34.1 ± 0.35 to 37.7 ± 0.180 Cat a rate of 0.06 ± O.oo/s (n = 52). When 2,000-ml/min challenges were used, airways rewarmed during the first minute of recovery, from 33.9 ± 0.92 to 37.5 ± 0.49 0 C at a rate of 0.06 ± O.01/s(n = 9). Thus, after correction for the difference between Tpa and Taw, rewarming after challenge with sublobar cooling (0.12 ± O.OI/s, n = 6) occurs twice as fast (p < 0.001) as rewarming after normal dry-air challenge (0.06 ± O.OO/s, n = 61).
Effect of Transient Sub/obar Cooling on Responses to Hypertonic NaC/ Exposing wedged sublobar segments to 60 s of 14.46,10 NaCl aerosol, a concentration used in human studies (12), produced a 33 ± 16070 (n = 6) increase in Pb 30 s postchallenge when the LLL was maintained at normal temperature (38.5 ± 0.06 0 C). However, when the LLL was transiently cooled to 29.8 ± 0.23 0 C during hypertonic saline challenge (figure 2), Pb increased only 18 ± 4070 at 30 s, but continued to increase to a peak of 23 ± 7% 2 min postchallenge. HR (173 ± 13
FREED, FULLER, AND STREAM
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Time (min) Fig. 2. The effectof transientcooling on sublobarpressure (Pb) during a 60-s aerosol challenge with 14.4% Nacl. A. Pb before and after aerosol challenges (vertical bar). B. Temperature of blood (Tpa) perfusing the lobe before.during. and after challenge (n .. 6. mean ± SE). Double asterisks indicate p = 0.01. Open circles = warm; closed circles = cold.
versus 172 ± 8 beats/min, p = 0.822), Pa (98 ± 5 versus 95 ± 6 mm Hg, p = 0.375), mean pulmonary artery pressure (20 ± 1 versus 20 ± 2 mm Hg, p = 0.720), and Tpa (38.5 ± 0.1 versus 38.5 ± 0.1, p = 1.000) recorded just prior to either challenge did not differ significantly (n = 6).
Effect of Transient Sub/obar Cooling on Responses to Hypocapnia When warm (1)Ja = 38.4 ± 0.07° C), a 2-min exposure to hypocapnia produced a 85 ± 18070 (n = 10) increase in Pb 30 s after challenge. When the LLL was transiently cooled during challenge to 29.1 ± 0.20° C, Pb increased 65 ± 21070 30 s postchallenge. Transient cooling did not significantly affect (P = 0.423) hypocapnia-induced bronchoconstriction (figure 3). HR (182 ± 12versus 180 ± 10 beats/min, p = 0.716), Pi (91 ± 6 versus 95 ± 5 mm Hg, p = 0.115), mean pulmonary artery pressure (17 ± 1 versus 17 ± 2 mm Hg, p = 0.452), and Tpa (38.4 ± 0.1 versus 38.4 ± 0.1° C, p = 0.847) were not significantly different directly preceding warm and cold challenges, respectively (n = 10). Effect ofTransientSub/obar Cooling on Responses to Aerosolized Histamine When warm (1)Ja = 38.3 ± 0.08° C), a 6O-s exposure to histamine aerosol produced a 43 ± 9070 (n = 5) increase in
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