SUMMARY1. The ventilatory response to electrically induced 'exercise' was studied in six chloralose-anaesthetized dogs. The on-transient and steady-state responses to ' exercise' were compared in the same dogs before and after spinal cord transaction at T8/9 (dermatome level T6/7) on fifteen occasions.2. Phasic hind limb 'exercise' was induced for periods of 4 min by passing current (2 Hz modulated 50 Hz sine wave) between two needles inserted through the hamstring muscles. The maximum current used was 30 mA. This was below the level previously found to produce an artifactual stimulation of breathing with the cord intact.3. Cord transaction produced no significant change in either the resting values of ventilation (PI) and CO2 production (fco2) or the ventilatory equivalent for CO2 during 'exercise' (A Vj/A TCo2) 4. During the steady state ofexercise Pa Co2 was on average significantly lower than at rest with the cord intact (mean AIa, co2, -2-1 mmHg; range-57 to +1), and higher, though not significantly, with the cord cut (mean Pa Co,, + 1-2 mmHg; range -1-5 to + 4-3). However, even in the absence of spinal cord transmission, the ventilatory response to exercise could not be accounted for on the basis ofCO2 sensitivity; the A P/API,CO2 obtained with exercise (apparent sensitivity) was significantly greater than that obtained with CO2 inhalation (true sensitivity) both before and after cord section.5. V, and PcoC increased more slowly with the cord cut than with the cord intact.This was thought to be due to a slower increase in venous return in the absence of sympathetic innervation of the lower half of the body following cord transaction.6. Similar experiments were performed during muscle paralysis (following gallamine triethiodide). Ventilation was maintained with a respirator controlled by phrenic nerve activity. These experiments showed an increase in ventilation, independent of muscle contraction, which was only present when the cord was intact and which was confined to the on-transient. Only in the absence of spinal cord transmission could

SUMMARY1. The effect of electrically induced 'exercise' on the respiratory oscillation of arterial pH was studied in chloralose-anaesthetized dogs with spinal cord transaction at T8/9 (dermatome level T6/7).2. Respiratory oscillations of arterial pH (presumed to be due to oscillations of arterial Pco2) were sensed with a fast-responding electrode in one carotid artery.Breath-by-breath estimates of the maximum rate of change of pH of the downstroke of the pH oscillation (dpH/dtjmax) were obtained by differentiating the pH signal.3. Consistent with the findings of the previous paper (Cross et al. 1982), the ventilatory response to exercise could not be explained on the basis of sensitivity to C02; the A T4/APaco, was significantly greater for 'exercise' than for CO2 inhalation.4. On average, the amplitude of the pH oscillations decreased during 'exercise'.The change in the phase relationship (0) between respiratory and pH cycles, although significant from the second breath onwards, was not thought to be responsible for the increased ventilation V,; the direction of the change was opposite to that previously found to increase VI. 14 and dpH/dtjmax were also linearly related during the on-transient, although the same relationship did not hold true throughout 'exercise'.6. The dpH/dtjmax was related to CO2 production (1rco,) lending support to the prediction that the slope of the downstroke of the pH oscillation is a function of TCo2.7. It was concluded that the dpH/dtimax (dpCO2/dttmax) is a potential humoral signal in 'exercise' and could account totally for the shortening of te. Since there was a late rise in VI (due to an increase in tidal volume VT) in the absence of a change in dpH/dtjmax, it was considered unlikely that the dpH/dtjmax was the only humoral signal present during 'exercise'.

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SummaryA tracheal tube with a facility for delivering a jet of gas near its t@ (Portex Profile tube incorporating gas delivery line) has been used in high frequency jet ventilation. It appears to be more efficient at pulmonary gas exchange than the system using a short catheter jet placed co-a.xiully in the tracheal tube. It also provides a facility ,for tracheal toilet without interruption of ventilation.

Key wordsVentilation; artificial, high frequency jet ventilation High frequency jet ventilation (IIFJV) is a relatively new approach to artificial ventilation. The concept of ventilation using small tidal volumes and fast respiratory rates with specially designed ventilators has many advantages over conventional IPPV. These include: lower mean intrapulmonary pressures' (due to less lung distension) with minimal effect on cardiac output; an implied reduced incidence of barotrauma to emphysematous bullae and recently repaired bronchial stumps; more efficient pulmonary gas distribution with better Paol for the same F I O~;~ adequate ventilation of patients with large airway leaks3 (i.e. bronchopleural fistulae); evidence of lower antidiuretic hormone levels during prolonged ~e n t i l a t i o n ;~ and, better patient acceptibility. Rapid rates produce a neurogenically induced apnoca at n o r m o~a p n i a .~,~ This greatly reduces the need for sedatives, narcotics and muscle relaxants to settle patients on this typc of mechanical ventilation.6These features are of value to patients with low compliant lungs due to adult respiratory distress syndrome (ARDS).