COLD AIR AND VENTILATORY FUNCTION

Eucapnic Voluntary Hyperpnea: Gold Standard for Diagnosing Exercise-Induced Bronchoconstriction in Athletes?

In athletes, a secure diagnos is of exercise-induced bronchoconstriction (EIB) is dependent on objective testing. Evaluating spirometric indices of airflow before and following an exercise bout is intuitively the optimal means for the diagnosis; however, this approach is recognized as having several key limitations. Accordingly, alternative indirect bronchoprovocation tests have been recommended as surrogate means for obtaining a diagnosis of EIB. Of these tests, it is often argued that the eucapnic voluntary hyperpnea (EVH) challenge represents the ‘gold standard’. This article provides a state-of-the-art review of EVH, including an overview of the test methodology and its interpretation. We also address the performance of EVH against the other functional and clinical approaches commonly adopted for the diagnosis of EIB. The published evidence supports a key role for EVH in the diagnostic algorithm for EIB testing in athletes. However, its wide sensitivity and specificity and poor repeatability preclude EVH from being termed a ‘gold standard’ test for EIB.

Sleep apnea, respiratory cooling and thermoregulation

This article will suggest that moderate sleep apnea may have an adaptive benefit, the reduction of nocturnal, respiratory heat loss, that may have caused it to be selected for despite the associated disadvantages. Sleep apnea is caused by the collapse of the throat, specifically the pharyngeal airway, during sleep and can result in discomfort and hypoxia. The large size of the fleshy structures in the pharynx and hypotonicity of the pharyngeal musculature are what cause humans, and many other species of mammals, to be susceptible to apnea. There are many functional and anatomical parallels between sleep apnea and a well known, thermoregulatory response that minimizes respiratory cooling in the lower airways known as reflex bronchoconstriction. Bronchoconstriction involves the constriction of the inner airways, the bronchioles, in response to cold air, a reaction that acts to warm inspired air before it makes contact with the more sensitive lower airways and alveoli. Because air is colder at night and body temperature drops severely during sleep, sleep apnea may represent a similar protective, thermoregulatory adaptation. When the pharynx is collapsed, the diameter of the pharyngeal airway decreases, allowing increased intermolecular collisions between inspired air and the epithelial walls of the upper airway. An increase in the number of collisions facilitates the transference of both warmth and humidity to inspired air before it reaches the more sensitive lower airway resulting in the maintenance of internal, core temperature and alveolar heat retention. In fact, sleep apnea can be conceptualized as a functional reversal of the thermoregulatory process of panting, where the pharynx is highly dilated instead of collapsed. Given the disorder associated with sleep apnea, the absence of a disease state secondary to surgical correction, the fact that the disorder is exclusive to warm-blooded animals, the high proclivity for collapse documented in pregnant mothers, and the functional similarities between collapse and reflex bronchoconstriction, it appears that the phenomenon may represent an evolutionary trade-off that helped to minimize respiratory cooling.

Effects of the combination of skin cooling and hyperpnea of frigid air in asthmatic and normal subjects

To investigate whether reducing integumental temperature influences pulmonary mechanics and interacts with inhaling cold air, 10 normal and 10 asthmatic subjects participated in a three-part trial in which cooling the skin of the head and thorax and isocapnic hyperventilation of frigid air were undertaken as isolated challenges and then administered in combination. Integumental cooling for 30 min caused airway obstruction to develop in both populations [change in 1-s forced expiratory volume (delta FEV1) asthmatic subjects = 10% ; normal subjects = 6%)]. Hyperventilation, however, only affected the asthmatic subjects (delta FEV1 asthmatic subjects = 18%; normal subjects = 3%). In contrast to expectations, the combined challenge did not produce a summation effect (delta FEV1 asthmatic subjects = 21%; normal subjects = 7%). These data demonstrate that the skin of the trunk and head is cold sensitive and when stimulated causes similar degrees of bronchial narrowing in both normal subjects and patients with airway disease independent of any ventilatory effect. They also indicate that cooling of the skin does not add to the obstructive consequences of hyperpnea.

Role of cooling and drying in hyperventilation induced asthma

Respiratory heat loss has been proposed as a mechanism of exercise induced asthma. Whether the predominant stimulus is airway drying or cooling remains unclear. We have measured changes in FEV1 after isocapnic cold air hyperventilation (CAH) (-23.4 degrees (SD 0.43 degrees) C) and dry ambient air hyperventilation (AAH) (18.7 degrees (0.52 degrees)C) in seven asthmatic patients (mean age 31 (SD 9) years and baseline FEV1 3.2(0.9)1) and in seven normal subjects (age 28(6) years and FEV1 3.6(0.7)1). The inspired water content in both cases was 0.3 mg/l air. The rate of respiratory heat exchange per breath was calculated in watts (W) with microcomputer based equipment. Cold air hyperventilation caused a fall in FEV1 almost twice that of ambient air hyperventilation at each level of ventilation: CAH v AAH (% fall) 8.0 (5.1) v 3.9 (4.0) at 15 l/min, 11.6 (7.8) v 7.0 (4.4) at 30 l/min, and 20.7 (10.9) v 12.4 (6.3) at 60 l/min. Identical latent heat loss (evaporative drying) was imposed on the airway during the two challenges. Sensible heat loss (convective cooling) in cold air hyperventilation was 41 W at 15 l/min, 63 W at 30 l/min, and 114 W at 60 l/min; whereas in ambient air hyperventilation the loss was 6, 13, and 23 W respectively. It is concluded that the rate of cooling of the upper airway is the predominant stimulus in hyperventilation induced asthma.

Role of the oesophagus in asthma induced by the ingestion of ice and acid

Twelve Asian patients with a history of asthma exacerbated by ingestion of ice and acidic drinks were selected for study. To determine the site of response to ingested ice and acid they were challenged with ice or dilute hydrochloric acid, which was orally retained on one day and swallowed on another. On a third day a placebo was given. The airway response was assessed by measuring FEV1 and the provocative concentration of histamine that reduced the FEV1 by at least 20% (PC20). There was no significant change in FEV1 or histamine PC20 after placebo or the orally retained challenges for the group as a whole or for any individual. After the ice and hydrochloric acid had been swallowed there was a small but statistically significant mean fall in FEV1, increasing to a maximum 90 minutes after ingestion, together with a significant increase in bronchial responsiveness. As conditioning of the inspired air would have been similar after orally retained and after swallowed ice or acid, the response is likely to be due to oesophageal stimulation. The mechanism of the response to oesophageal stimulation is unclear, but the slow time course seems to preclude a simple neural reflex.

Enhancement of exercise-induced asthma by cold air

To study the possibility that the inhalation of cold air accentuates the bronchoconstrictor response to exercise in asthma, eight subjects exercised while breathing air at ambient or subfreezing temperatures. On a separate day, cold air was breathed at rest so as to isolate the effects of this stimulus. Pulmonary mechanics were measured before and after each experiment. In all subjects acute bronchoconstriction followed the control exercise challenge. With cold-air breathing, however, the magnitude of the response was markedly enhanced. Residual volume increased 158 per cent more than it did previously, and specific conductance and one-second forced expiratory volumes changed an additional 85 and 100 per cent, respectively. The effects of cold air at rest were very small. The results demonstrate a positive interaction of two common naturally occurring stimuli in the induction of asthmatic attacks, and constitute objective verification of a frequent clinical complaint.

Influence of temperature and decreased water content of inspired air on the ciliated bronchial epithelium. A physiological and electron microscopical study

Experiments have been carried out on the bronchi and intrapulmonary airways of rats in vitro, to determine the physiological effects of air with water content (WC) of: 2.19 mg/1 (5% RH (relative humidity) 37 degrees C), 8.77 mg/1 (20% RH, 37 degrees C), 24.13 mg/1 (50% RH, 37 degrees C) and 30.71 mg/1 (70% RH, 37 degrees C) on ciliary beat frequency; and also, the morphological effect of air at temperatures of 30, 40 and 48 degrees at a constant WC of 28.0 mg/l on the ciliated epithelium. The results showed that (i) ciliary beat frequency decreased with decreasing WC and irreversible ciliostasis was reached sooner, the lower the WC value; (ii) ciliary beat frequency was not significantly affected above 24 mg/l; (iii) 45 min and 90 min exposure at 30 degrees C caused an enormous increase in the depth of the mucus film. The cilia and the epithelial cells did not differ from the controls at 40 degrees C; (iv) in airways exposed for 45 min at 48 degrees C, the cilia appeared to be in total disarray. Some were clustered together in whirling arrangements, while others were detached from their bases. The substructure and the cytoplasm of the cells on the surface and in the deeper layers, appeared to have been destroyed; (v) after 90 min at 48 degrees C, the damage to all structures had progressed. Only isolated tissue or cytoplasmic elements were perceptible.

An annual cycle in the ventilatory capacity of men with pneumoconiosis and of normal subjects

The forced expiratory volume over 0·75 second (F.E.V._0·75) was measured monthly for 3 years in 28 working ex-miners with pneumoconiosis and for one year in 17 normal laboratory workers. A significant annual cyclical variation of 0·044 litre amplitude was found in the ex-miners, with minimum and maximum values on 2 February and 3 August respectively. In the normal subjects the amplitude of the change was about 0·10 litre. An attempt was made to relate the changes in ventilatory capacity in the ex-miners to those in environmental temperature, relative and absolute humidity, and barometric pressure. Dry-bulb temperature showed a cyclical pattern in phase with that of ventilatory capacity, but there was no strong correlation between the deviations from these cyclical components, which would have been desirable to support a causal relation between change of temperature and of ventilatory capacity. There was no definite evidence of a relationship between any other of the environmental parameters and ventilatory capacity. Fourteen of the ex-miners had purulent sputum at intervals but remained at work. Immediately after these attacks the ventilatory capacity was not lower than before, but those having attacks showed a greater annual decline in their F.E.V._0·75. We think that the cyclical variation in ventilatory capacity is not directly caused by the environmental conditions measured, or by sputum purulence, but could represent an intrinsic biological rhythm which is perhaps modified by the environmental temperature.