DYNAMICS OF THE HUMAN CEREBRAL BLOOD FLOW RESPONSE TO STEP DECREASES IN END-TIDAL PCO2

Hypocapnia is known to decrease brain blood flow, but this effect may wane over time. Poulin et al. (p. 388) have examined the dynamics of the middle cerebral artery blood flow response to hypocapnia in humans by using transcranial Doppler ultrasound. Beat-by-beat values of velocity, power, and their product were used to calculate the on and off time constants. The results indicate that the responses to onset and relief of hypocapnia were asymmetrical, having time constants of 6.8 and 14.3 s, respectively. The cerebral blood flow response to hypocapnia was characterized by significant slow adaptation, which persisted throughout the hypocapnic period. The paper is discussed in an Invited Editorial by Lambertsen (p. 386).

	ARCHITECTURE OF HUMAN TRICEPS SURAE MUSCLES DURING CONTRACTION

Kawakami et al. (p. 398) used a recently developed application of ultrasonography to study soleus and medial and lateral gastrocnemius muscle architectural changes at different ankle and knee joint angles in healthy male subjects. Specifically, plantar flexion torque was examined as a function of muscle fascicular angles and lengths. Among the interesting findings of this study are that 1) contraction-induced dissimilarities between fascicle angles and lengths between muscles likely contribute to the force-producing capabilities and elastic characteristics of triceps surae and 2) the behavior of muscle fibers may not be the same as that of muscle fascicles.

	WHAT IS THE SOURCE OF EXHALED NITRIC OXIDE?

The origins of exhaled nitric oxide (NO) have recently become a matter of considerable interest. Air exhaled from the nose contains much higher concentrations of NO than that exhaled orally when nasal gas flow is prevented. Lewandowski et al. (p. 405) made use of the unique anatomy of the baboon (absence of paranasal sinuses) to test the hypothesis that such sinuses are a major contributor to exhaled NO in other mammals. This hypothesis is, indeed, supported, with orally exhaled levels of NO of 1-5 parts/billion in baboons (compared with >20 parts/billion in normal humans) and nasal levels of NO in humans that are two orders of magnitude higher than those in the baboon.

	CARBOHYDRATE DEPENDENCY DURING HARD EXERCISE REGARDLESS OF PREEXERCISE MEAL COMPOSITION

Whitley and associates (p. 418) used indirect calorimetry to assess effects of preexercise meal composition on substrate utilization and exercise performance in highly trained male cyclists. After an overnight fast, the subjects took either an isoenergetic high-fat or high-carbohydrate meal or no meal. Four hours later, they cycled at 70% of maximal oxygen uptake for 90 min, followed by a 10-km time trial. Preexercise eating had large and predictable effects on blood insulin and free fatty acid levels. However, in all trials, respiratory gas exchange ratios were in the range of 0.9-0.92 (indicating a predominance of carbohydrate oxidation), and no effect on time trial performance was noted. In highly trained men, the preexercise diet can markedly affect hormonal status and free fatty acid availability, but carbohydrate-derived fuels predominate during prolonged hard exercise.

	INTERSTITIAL FLUID PRESSURE IN THE NASAL MUCOSA

The pressure of interstitital fluid (Pif) is an important variable influencing transcapillary fluid exchange according to the Starling principle. Berg et al. (p. 465) have developed a micropuncture technique for measuring Pif in the nasal mucosa of anesthetized rats and have applied this method to analyze fluid movements in acute inflammation. The results are consistent with the hypothesis that Pif is actively modulated during inflammatory responses through changes in mechanical properties of the interstitial matrix.

	THERMOREGULATION IN THE COLD: INFLUENCE OF THE MENSTRUAL CYCLE

The effects of the menstrual cycle on the metabolic and thermal responses to progressive cold exposure were studied by Gonzalez and Blanchard (p. 543) in six normal women. Repeated experiments were done in the follicular and luteal phases of the cycle, with the subjects wearing clothing of two different thermal resistances. Core, skin, and finger temperatures and area-weighted heat flux were recorded continuously, and integrated mean body temperature was calculated. The results were fitted to an analytic model to reveal the roles of hormone levels, core and skin temperatures, vascular responses, and variations in body heat balance in the overall thermoregulatory response to cold.

	DISTRIBUTION OF TRANSIT TIMES IN PULMONARY VESSELS

Clough et al. (p. 565) investigated the dispersion of radiographic contrast medium within the pulmonary vasculature of isolated canine lower lobes perfused with a steady flow. On average, intrapathway dispersion, constituting the velocity profiles along an arterial or venous pathway, was negligible. Similarly, interpathway dispersion, consisting of transit time variance across arterial pathways, was negligible. The authors concluded that most of the variation in transit time in the pulmonary vasculature occurs in the capillary bed rather than in the conducting arteries and veins.

	BLOOD FLOW TO RESPIRATORY MUSCLES IN MAXIMAL EXERCISE

Harms et al. (p. 609) examined the effects of changes in the work of breathing on cardiac output during maximal exercise in humans. Inspiratory muscle work was either at control levels, reduced by a proportional-assist ventilator, or increased by a resistive load. Stroke volume and cardiac output were not different between control and loaded trials but were lower than in control trials with inspiratory muscle unloading. Based on present and previous findings, the authors concluded that respiratory muscle work expended during maximal exercise has two major circulatory effects: 1) up to 14-16% of the cardiac output is directed to the respiratory muscles and 2) reflex vasoconstriction compromises blood flow to locomotor muscles.

	A MODEL OF NITRIC OXIDE EXCHANGE

NO transport and exchange in the respiratory system has been found to be extremely complex. Thus NO can be derived from alveolar tissue as well as from the conducting airways and paranasal sinuses. NO can be absorbed by bronchial and pulmonary circulations and presumably locally metabolized in lung tissue as well. Two papers by Tsoukias and associates (p. 642 and p. 653) provide a theoretical model that incorporates most of these features and shows that measured expired gas profiles can be reasonably predicted from the model. Although this does not prove the model correct, it does provide a means of generating hypotheses about pulmonary NO handling that may be experimentally testable in the future.

	EFFECTS OF EXOGENOUS SURFACTANT DISTRIBUTION AND BREATHING PATTERN IN INJURED RABBIT LUNGS

Kerr et al. (p. 676) studied the effects of the pattern of ventilation on adult rabbit lungs injured by saline lavage and given tracheal instillations of 50 mg/kg surfactant in either 2 or 4 ml/kg volumes. The rabbits were ventilated for 3 h using various combinations of tidal volume and positive end-expiratory pressure. Surfactant distribution was more uniform when delivered in the 4 ml/kg volume. In the animals treated with 2 ml/kg surfactant, arterial PO₂ was greatest in groups with small tidal volume, and mortality was highest in groups with high positive end-expiratory pressure. Thus different modes of mechanical ventilation in adult respiratory distress syndrome patients given exogenous surfactant might influence mortality.