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Department of Medicine, Charing Cross Hospital Medical School, Fulham Palace Road, London W6 8RF

1. Reflex respiratory responses to unilateral and bilateral changes of lung volume have been studied in anaesthetized paralysed, open-chest dogs.

2. The two lungs were separately ventilated by two phrenic-driven respirators via a specially designed double-lumen tube. Respiratory motor output was measured from the phrenic motoneurone activity recorded from the C₅ root. Expiratory lung volumes were set by the use of expiratory threshold loads (e.t.l.s).

3. The reflex changes of expiratory time (t_e) were used to study the summation of left and right expiratory lung volume information. Changes in the peak amplitude of the phrenic `integral' (Phr) and inspiratory time (t_i) were used to assess summation of left and right tidal volume information.

4. Summation in the reflex responses to bilateral lung volume changes was estimated by comparing the measured responses to these manoeuvres with the sum of the component unilaterally evoked responses. If simple addition were present, response (measured)/response (predicted) would equal 1.0 Mutual facilitation would give a value higher than this; mutual inhibition, a lower value.

5. The responses of t_e to changes of e.t.l. on the right side were always greater than for changes confined to the left, and in each animal the response of t_e to bilateral changes of e.t.l. were greater than for either of the unilaterally evoked responses. In six out of eight animals this was shown to be due to simple addition of the responses evoked from the two lungs individually. In the remaining two animals, slight mutual inhibition was seen.

6. The tidal volume V_T was changed in one or both lungs. During the bilateral V_T changes, the volumes were changed simultaneously, either in the same direction (`same' V<sub>T</sub> changes) or in one direction in one lung and in the opposite direction in the other (`opposite' V_T changes).

7. In the bilateral `same' V_T changes, mutual facilitation was seen in the response of Phr; Phr (measured)/Phr (predicted) = 1.60 ± 0.42 (S.D.), n = 8. There was only slight facilitation in the response of t_i; t_i (measured)/t_i (predicted) = 1.18 ± 0.17, n = 8.

8. With the bilateral `opposite' V<sub>T</sub> changes, responses of t<sub>i</sub> and Phr were markedly and significantly reduced compared to those for the bilateral `same' V_T changes. During these manoeuvres significant mutual inhibition was seen in the response of t_i, and the predicted responses of Phr in general could not be correlated with the measured response.

9. The responses of the phrenic `integrals' were the same in both right and left phrenic nerves.

10. Unilateral vagotomy abolished both the responses of Phr and t_i to ipsilateral V_T changes in the range ± 100% of control V_T, and also the response of t_e to ipsilateral expiratory volume changes.