Correction for mechanical dead space in the calculation of physiological dead space

External dead space explains sex-differences in the ventilatory response to submaximal exercise in children with and without obesity

We compared the exercise ventilatory response (slope of the ventilation, V̇_E and carbon dioxide production, V̇CO₂ relationship) in boys and girls with and without obesity. 46 children with obesity (BMI percentile: 97.7 ± 1.4) and 27 children without obesity (BMI percentile: 55.1 ± 22.2) were included and divided into groups by sex (with obesity: 17 girls and 29 boys; without obesity: 13 girls and 14 boys). A 6 min constant load cycling test at 45 % of peak work rate was performed. The V̇_E/V̇CO₂ slope was similar (p = 0.67) between children with (32.7 ± 4.3) and without (32.2 ± 6.1) obesity; however, it was higher (p = 0.02) in girls (35.4 ± 5.6) than boys (32.6 ± 4.9). We also examined a corrected V̇_E/V̇CO₂ slope for the effects of mechanical dead space (V_DM), by subtracting V̇_DM from V̇_E (V̇_Ecorr/V̇CO₂ slope). The V̇_Ecorr/V̇CO₂ slope remained similar (p = 0.37) between children with (26.8 ± 3.2) and without obesity (26.1 ± 3.1); however, no sex differences were observed (p = 0.13). Therefore, V_DM should be accounted for before evaluating the V̇_E/V̇CO₂ slope, particularly when making between-sex comparisons.

Dynamic responses to tube breathing during the first 10 days of life

To assess whether there is a maturation of the response to tube breathing during the first 10 days of life, we have measured the dynamic ventilatory responses of nine term infants (34 studies) to added tubes equivalent to two anatomical dead spaces. Such a method allows an assessment of the baby's ability to increase dead space ventilation in response to an added stress. There was a significant improvement with age over the first 10 days in the babies' ability to reach "expected" values of minute ventilation when the tubes were added (P = 0.003). The rates at which 63% and maximum adaptation were achieved also increased significantly with age (P less than 0.005). The rate to 63% adaptation appeared to be faster in active compared with quiet sleep (P less than 0.05). The dynamic responses to tube breathing improve over the first 10 days of life. This is likely to be due to a maturation of peripheral chemoreceptor control, although improving lung mechanics may also be important.

Alterations in ventilatory pattern and ratio of dead-space to tidal volume

The effects of alterations in ventilatory pattern on the simultaneously measured physiologic and anatomic dead-spaces (VDphys and VDan, respectively) and the dead-space to tidal volume ratio (VD-VT) were studied in 17 healthy normal subjects (13 men, four women, ages 21 to 36 years). There were no significant changes in VDan with increases in respiratory frequency (f) or tidal volume (VT). The VDphys increased (mean change +0.153 L, p less than 0.05) with increase in VT (mean increase +0.84 L, p less than 0.01), but did not alter significantly with a twofold increase in f, at control VT. Increase in VT significantly reduced VD/VT (mean change -10.4 percent, p less than 0.05), but increase in f, at control VT, did not significantly alter VD/VT. These results indicate that in normal subjects, increase in VT alters ventilation/perfusion matching in the lungs, whereas an increase in f, at constant VT, has no effect on ventilation/perfusion matching. Increases in VD/VT cannot, therefore, be ascribed to alterations in ventilatory pattern where either VT, or f, or both are increased.

Physiologic effects of oral bronchodilators during rest and exercise in chronic obstructive pulmonary disease

At rest and during exercise, noninvasive studies of cardiopulmonary physiology in patients with chronic obstructive pulmonary disease (COPD) were carried out to determine the objective benefits of commonly used oral bronchodilator drugs in 15 stable patients without cardiovascular disease or reversible obstruction of airflow. Theophylline, terbutaline, a combination of theophylline and terbutaline, and placebo were given for ten days each in a randomly sequenced double-blind protocol for outpatients. Spirometric values, the ratio of physiologic dead space to tidal volume (VDp/VT), and the alveolar-arterial oxygen pressure difference (P[A-a]O2) were studied at rest on each regimen. During steady-state exercise the changes in VDp/VT and P(A-a)O2, as well as the ventilatory equivalent for oxygen and oxygen pulse, were measured. When compared with placebo, no significant change was noted in the previously mentioned measurements with any regimen, with the exception of a small improvement in the forced expiratory volume in one second, which was significant for all regimens. These findings suggest that commonly used oral bronchodilator drugs in usual doses may have small effects on airflow even in "irreversible" COPD but that the objective effect of these agents on gas exchange during rest and exercise is not significant.

Interaction of series and parallel dead space in the lung

The volume of ventilation delivered to unperfused zones of the respiratory system (respiratory dead space) can be divided into the volume occupied by the conducting airways (series dead space) and the volume of unperfused alveolar space (parallel dead space). The effect of the interaction between these two components of dead space on steady-state gas exchange was first evaluated with a mathematical model. The presence of both parallel and series dead space was predicted to underestimate the dead space measured by the inert gas elimination technique (VDIG). This error was largest when the volumes of parallel and series dead space were equal. The size of the parallel dead space in the model could be calculated from measurements of VDIG made before and after adding a series dead space of known volume. In 16 anesthetized dogs series and parallel dead space were quantitated using the multiple inert gas elimination technique with addition of known volumes of series dead space. In five normal dogs, the series and parallel dead space averaged 20% and 13% of the tidal volume, respectively. In eleven dogs with the left pulmonary artery occluded the parallel dead space averaged 26%. This method represents the first means of quantitating these two anatomically separate components of wasted ventilation.

Breathlessness and mouth occlusion pressure in patients with chronic obstruction of the airways

In order to evaluate the hypothesis that the sensation of dyspnea is due to an "inappropriate relationship between length and tension" in the respiratory musculature, studies were performed in 24 patients with chronic obstruction of the airways. They were divided into two groups solely on the basis of subjective and objective presence or absence of breathlessness and dyspnea at rest. Spirometric tests and body plethysmography were performed in each patient. The ventilatory pattern at rest was recorded and repeated measurements of mouth occlusion pressure were made. It is concluded that breathlessness in patients with airways obstruction is not associated with major differences in arterial blood gases or in VO2, VE, f or the pattern of individual breaths; there is a greater degree of airway obstruction and increased inspiratory neuromuscular drive. The minute ventilation achieved for a given inspiratory neuromuscular output, which may be considered analogous to the length-tension relationship of the respiratory musculature, is significantly decreased. These results are in accordance with the theory of mechanical inappropriateness as a cause of dyspnea.