Normal Values for the Hypercapnic Ventilation Response: Effects of Age and the Ability to Ventilate

Normal values reported for the hypercapnic ventilation response (HCVR) vary considerably, but the reported normal values have come from studies containing small sample sizes and/or the subjects were young or of unidentified age. We speculated that age has a major effect on HCVR due to the generally lower maximal ventilation (VEmax) of elderly subjects. Therefore, we performed a large study to more accurately define the normal range and to reveal any effects which age and sex might have on HCVR. We studied 181 normal subjects (69 males, 112 females) between the ages of 20 and 93 years. Prior to measuring HCVR we measured forced expired volume in 1 s (FEV₁ and forced vital capacity to establish whether lung function was normal and to obtain an estimate of VEmax. Results for the entire group revealed a significant correlation between FEV₁ and HCVR [HCVR = 0.51 + (0.33 FEV₁, r = 0.43, p < 0.001], so it is clear that the ability to ventilate can influence HCVR. We also found a significant correlation between age and HCVR [HCVR = 2.08-(0.01•age), r = 0.34, p < 0.001]. The mean HCVR for our male group (1.86 ± 0.541/min•mm Hg) was significantly higher (p < 0.001) than that for the females (1.37 ± 0.60) even though their mean ages were similar (41.3 vs. 41.7 years, respectively). We expect that this difference in HCVR was due to the higher FΕV₁ in males compared to females (3.82 vs. 2.83 liter, respectively). To adjust HCVR for the reduced ability of smaller and older subjects to ventilate we corrected for HCVR (CHCVR) to the subject’s predicted maximal ventilation (35 × FEV₁) and expressed CHCVR as %VEmax/mm Hg. CHCVR for males (1.44 ± 0.45 %VEmax/mm Hg) was not different from that for females (1.39 ± 0.54). We found that the lower limit of normal for CHCVR is approximately 0.57 %VEmax/mm Hg, regardless of age or gender. On the other hand the lower limit of normal for the HCVR decreased with age, falling from approximately 0.90 l/min•mm Hg for 20 year olds to 0 1/min mm Hg for 87 years olds. Therefore, CHCVR is an index of CO₂ sensitivity which is not age or gender dependent but it still has a large scatter for the normal population.