1. The breathing patterns of ten habitual smokers were monitored in the semi-recumbent position with respiratory inductive plethysmography before and after smoking cigarettes. The subjects smoked a high-tar-content (HTC) and a low-tar-content (LTC) cigarette. The mean (±sd) values and frequency histograms of minute ventilation , tidal volume (VT), frequency (fR), inspiratory time (TI), fractional inspiratory time (TI/TTOT) and mean inspiratory flow (VT/TI) during baseline were compared with the values during and after smoking.

2. On a separate occasion, specific airway conductance (sGaw) and multiple-breath nitrogen washout were measured before and after smoking in six of the subjects.

3. One group of smokers (n = 4) had a greatly increased mean (±sd) baseline VT/TI (390 ± 39 ml/s) compared with normal non-smokers and another group (n = 6) a near normal VT/TI (246 ± 36 ml/s). The first group (‘deep inhalers’) had a significantly higher mean inhalation fraction (volume of inhaled smoke and air/vital capacity; 0·25 ± 0·05). The second group had a lower mean inhalation fraction of 0·14 ± 003 (‘moderate inhalers’).

4. VT/TI decreased to 323 ± 33 ml/s in the post-smoking period in the deep inhalers, whereas it increased to 345 ± 65 ml/s in the moderate inhalers during smoking. No systematic difference in VT/TI was noted between smoking high- and low-tar cigarettes, although the high-tar brand tended to have a greater effect on VT/TI. Deep inhalers showed a fall in VT during and after smoking. Moderate inhalers showed a decrease in TI/TTOT during and after smoking.

5. sGaw, measured in four moderate inhalers and two deep inhalers, fell in all subjects after smoking HTC (19 ± 8·7%; P < 0·01) and LTC (13·9 ± 10%) cigarettes.

6. ‘Sham’ smoking with an unlit cigarette produced no change in breathing pattern.

7. In moderate inhalers, the increase in respiratory output (as reflected by increased VT/TI) combined with a reduction in TI/TTOT appears to reflect the respiratory-centre stimulant effect of nicotine directly or indirectly through broncho-constriction. This contrasts with the reduced neural drive in deep inhalers, which may relate to some overriding satiating effect of the smoking.

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