Journal of Applied Physiology
Vol. 83, No. 2, pp. 329-330, August 1997

This Month in the Journal

AIRWAY THERMAL VOLUME AND BODY SIZE

HICCUPS AND BLOOD PRESSURE IN PRETERM INFANTS

COCA CHEWING AND FLUID SHIFTS DURING EXERCISE

EVIDENCE FOR RESPIRATORY CHEMORECEPTOR FUNCTION IN THE RETROTRAPEZOID NUCLEUS

SMALL EXPIRATORY LOADS ENHANCE EXERCISE PERFORMANCE

CAPILLARY RECRUITMENT AND TRANSIT TIME IN THE RAT LUNG

ANIONS AFFECT PULMONARY ENDOTHELIAL PERMEABILITY

RESISTANCE TO ACIDOSIS DURING EXERCISE IN LATE PREGNANCY

AIRWAY THERMAL VOLUME AND BODY SIZE

In mechanically ventilated anesthetized humans, Serikov et al. (p. 668) used a step decrease in humidity of inspired gas to cool the lungs. The effects of ventilation and blood flow on the time constant of the decrease in expired gas temperature were measured. Airway thermal volume was defined as the product of blood flow and time constant. Time constant was inversely proportional to cardiac output but unrelated to ventilation. In subjects in whom blood flow increased, airway thermal volume was positively correlated with body height. The authors suggest that expired gas temperature might provide a noninvasive estimate of cardiac output. The paper is discussed in an Invited Editorial by McFadden (p. 331).

HICCUPS AND BLOOD PRESSURE IN PRETERM INFANTS

Spontaneous hiccups generate abrupt decreases in intrathoracic pressure. Mathew (p. 371) examined the effect of hiccups on arterial blood pressure in intubated preterm infants during both the systolic and diastolic phases. Early systolic hiccups decreased systolic but not diastolic blood pressure, whereas hiccups in late systole did not affect blood pressure. Both early and late diastolic hiccups decreased diastolic pressure, but systolic pressure on the subsequent cardiac cycle was decreased only if the hiccup occurred in late diastole. Thus transient decreases in intrathoracic pressure during specific phases of the cardiac cycle decrease blood pressure transiently, presumably by increasing the volume of the thoracic aorta and reducing stroke volume.

COCA CHEWING AND FLUID SHIFTS DURING EXERCISE

Favier et al. (p. 376) determined the effect of chewing coca leaves, a source of cocaine, on cardiovascular and hormonal responses to bicycle exercise. Coca chewing decreased blood and plasma volumes. These body fluid changes were accompanied by exaggerated heart rate and blood pressure responses to exercise. The coca-induced hemoconcentration attenuated the exercise-related shifts in body fluids. This attenuation was not linked to an altered catecholamine, renin, or arginine vasopressin response to exercise. However, plasma atrial natriuretic peptide was significantly reduced during exercise by prior coca chewing. The authors suggest that the reduction in body fluid volumes induced by coca chewing is a major contributing factor to the higher heart rate during exercise.

EVIDENCE FOR RESPIRATORY CHEMORECEPTOR FUNCTION IN THE RETROTRAPEZOID NUCLEUS

Many brain stem sites have been proposed to transduce local CO₂- and pH related variables to modulate ventilation. Identification is persuasive when small, physiologically relevant changes in these variables produce appropriate changes in breathing. Li and Nattie (p. 420) have developed a technique for inducing small, localized changes in CO₂. Their initial study focuses on the retrotrapezoid nucleus (RTN), a candidate site for medullary chemoreception. Using phrenic nerve discharge as a surrogate for ventilation, they found a consistent relationship between changes in local CO₂/pH and respiratory output, a result that confirms the proposed role of the RTN. This technique has promise for advancing the understanding of central chemoreception.

SMALL EXPIRATORY LOADS ENHANCE EXERCISE PERFORMANCE

Expiratory flow limitation occurs during heavy exercise in highly fit subjects because of their high ventilatory requirements. Fee et al. (p. 503) applied mild expiratory resistive loads in highly trained subjects during progressive exercise. Contrary to what has been previously shown at much higher expiratory loads, these investigators demonstrated that their small loads caused increased flow rates, tidal volume, and ventilation with small but significant increases in maximal oxygen uptake and peak exercise performance. The authors speculate that the accompanying increase in end-expiratory lung volume with expiratory loads may attenuate the limititations of airflow and blood flow limitations that normally occur in heavy exercise.

CAPILLARY RECRUITMENT AND TRANSIT TIME IN THE RAT LUNG

In a study of isolated, blood-perfused rat lungs, Presson et al. (p. 543) measured microvascular transit time video-microscopically at two blood flow rates (25 and 69 ml · min¹ · kg¹). The hypothesis that capillary volume recruitment would attenuate the expected fall in transit time with increased flow was confirmed, transit time going from 3.9 to 2.4 s, implying a 70% increase in capillary volume. However, unlike in larger animals, this reserve appeared limited, leading to speculation that small animals with high resting metabolic rates may be at risk of exercise limitation from too rapid pulmonary red cell transit, resulting in hypoxemia. This will need reconciling with Gonzalez et al. (J. Appl. Physiol. 75: 1608-1614, 1993), who reported some years ago that at maximal oxygen uptake the rat maintains arterial PO₂ above resting values and does so in the face of a cardiac output of 450 ml · min¹ · kg¹, six times higher than the higher blood flow in the present study.

ANIONS AFFECT PULMONARY ENDOTHELIAL PERMEABILITY

Griffin (p. 615) has examined the hypothesis that extracellular anions can influence pulmonary endothelial albumin permeability. The albumin permeability of monolayers of endothelial cells was measured when chloride, bromide, iodide, fluoride, acetate, gluconate, or propionate was used as the extracellular anion, with and without the addition of a phosphodiesterase inhibitor to decrease monolayer permeability. The permeability was lowest with acetate or propionate, but the decrease with phosphodiesterase inhibition was greatest with chloride. The permeability was affected by anion-channel blockers, but not by anion-transport inhibitors, leading to the conclusion that the anion effects were mediated by anion channels rather than anion transporters.

RESISTANCE TO ACIDOSIS DURING EXERCISE IN LATE PREGNANCY

Kemp et al. (p. 644) employed Stewart's physiochemical approach to evaluate the effects of graded exercise to maximum on acid-base balance in pregnant women (33 wk gestational age) and age-matched controls. Venous blood was sampled, and pulmonary gas exchange was monitored. At rest, H⁺ concentration, PCO₂, HCO₃ concentration, total protein, and strong ion difference were lower in pregnancy. Acid-base variables changed as expected during exercise, but a less acidic environment was maintained in the pregnant subjects. Decreased PCO₂ and weak acid concentration reduce H⁺ concentration in late pregnancy, and the effect is maintained in exercise. The results support the view that healthy and physically active pregnant women respond appropriately to the metabolic acidosis induced by exercise.