Respiratory and circulatory responses to CO2 breathing
It is widely known that the respiratory and the vasomotor centers are extremely sensitive to Pco^^2 of blood perfusing them. In particular, this mechanism constitutes one of the most important ways through which the alveolar ventilation is regulated. Hence, it is important from physiological point of view to know how far the sensitivity of the respiratory and the vasomotor centers to CO^^2 changes as one gets older. It has been known that, as one gets old, there develops a mild physiologic emphysema which would tend to depress the CO^^2 sensitivity of the respiratory center. On the other hand, it has also been reported that, as one gets old, the vasomotor response to cold pressor test becomes very much exaggerated.
These reports strongly indicate the possibility that the sensitivity of the above centers to an increase in Pco^^2 of blood may by modified during the aging process.
Hence attempts have been made in this investigation to study the ventilatory as well as the circulatory response to CO^^2 breathing in different age groups.
120 subjects (60 males and 60 females) consisting of various age groups were employed for the study. As a preliminary step, the normal minute volume(V^^E) as well as the normal minute alveolar ventilation (V^^A) was determined during air breathing. In determining the latter, the dead space was first computed by the use of Christian-Bohr equation to which measured values of V^^E and the fractional concentrations of CO^^2 in the expired and the alveolar gas samples were inserted.
The blood pressure and the pulse rate were also determined. Having finished these preliminary measurements, a CO^^2-O^^2 mixture was given to the subject to breathe for 15 minutes. Since it is known to take approximately 5 minute to reach a new steady state, the actual measurement of V^^E during CO^^2 breathing was begun 5 minutes after the start and lasted for 10 minutes thereafter. The blood pressure and the pulse rate were taken 10 minutes after the start of CO^^2 breathing. At the end of air or CO^^2 breathing, an alveolar gas sample was also obtained to estimate the Pco^^2 of blood. The fractional concentration of CO^^2 was determined by a Scholander micro gas analyzer. Two different concentrations of CO^^2 in O^^2 mixtures were used: 2.4% and 4% CO^^2 in males, and 2.9% and 4.5% CO^^2 in females.
Results and Discussion The normal minute volume was lowest in teen agers but it was quite comparable between middle and old groups, although middle age group showed a significantly
greater value than old age group in females. However, the normal minute volume per unit surface area was the same in all age groups in male while it was still low in old females. The dead space volume as well as the dead space ventilation increased
with age. Even when these were corrected for the difference in the body surface area, they still increased with advancing age. This increase in the dead space in the aged was more marked in males than females. As a result of such an increase in the dead space with advancing age, the alveolar ventilation per unit surface area was significantly lower in the aged as compared to other age groups. As it is well known, the blood pressure was lowest in teen agers and highest in old age group.
During CO^^2 breathing, both the tidal volume and the frequency of breathing increased in all age groups and consequently, the minute volume increased. Since the normal minute volume during air breathing was different in 3 age groups, the ventilatory response to CO^^2 breathing was expressed as per cent of the normal ventilation and designated as V^^ER. During 2.4% CO^^2 breathing. V^^ER in males was in the order of 170 in all age groups; however, during 4% CO^^2 breathing V^^ER was 284±13, 249±12 and 229±7 in teen-agers, middle and old age groups,
respectively, the differenct between teen agers and old age group being significant (P＜0.05). This clearly indicates that the ventilatory response to CO^^2 tends to decrease progressively with advancing age. Similar but more convincing results were
obtained from experiments on females. During 2.9% CO^^2 breathing, V^^ER was 188±7, 176±5 and 167±3 in teen agers, middle and old age groups, respectively, and the difference between teen agers and old age group was significant even at
this low CO^^2 comcentratiom. During 4.5% CO^^2 breathing, V^^ER declined from 286±8 in teen agers to 240±6 in middle age group and to 228±6 in old age group, the difference between 3 groups being highly significant (P＜0.01).
Although above results may seem to suggest that the respiratory center becomes less sensitive to CO^^2 stimulus with advancing age, an erronous conclusion could be drawn unless appropriate corrections are made for changes in the dead space ventilation during CO^^2 breathing. Hence, the dead space ventilation during each CO^^2 breathing was computed and was then subtracted from the respective V^^E to calculate the alveolar ventilation. The alveolar ventilation so calculated was again expressed as per cent of the normal alveolar ventilation and designated as
V^^AR. As a whole, the values of V^^AR were significantly greater than V^^ER in all groups and moreover, they were lowered progressively with advancing age. In particular, V^^AR of teen agers during 4-4.5% CO^^2 breathing was significantly greater than the other two groups in both males and females. There results again support the contention that the CO^^2 sensitivity of the respiratory center becomes depressed with advancing age in both sexes. As for the sex difference, no definitite conclusion could be drawn because of the difference in the concentrations of CO^^2 in the experimental gas mixtures given to males and females. In order to overcome this complication, values of V^^ER and V^^AR were plotted as a function of the concentration of CO^^2 for both sexes. When this was done, the tendency was that, for a given CO^^2 concentration, the ventilatory response tended to be lower in females as compared to males.
Blood pressures, both systolic and diastolic, increased during CO^^2 breathing. However, the magnitude of increase was in old age group and least in teen agers. For instance, the increase in systolic blood pressure during 4% CO^^2 breathing in males was by 5, 9 and 23 mmHg in teen agers, middle and old age groups,
respectively. Similar trend was also evident in females. The diastolic pressure increase during CO^^2 breathing. These results suggest that the CO^^2 sensitivity of the vasomotor center is lowest in teen agers and is greatest in old age group. No obvious difference between males and females were noted.
On the basis of these results, it is suggested that the CO^^2 sensitivity of the respiratory center tends to be depressed while the CO^^2 sensitivity of the vasomotor center tends to be augmented with advancing age.