Isocapnic Clamping with Feedback and Feed-Forward Control

Stimulation of ventilation increases CO2elimination and reduces PaCO2. An acute fall in PaCO2 reduces the level of carotid chemoreceptor activity and inhibits ventilation (hypocapnic braking). Isocapnic clamping breaks this feedback loop and allows full expression of the ventilatory response to the initial stimulus. With a simple negative feedback system, control is limited by the need to sense an error before any correction can be made. If the negative feedback system has a low gain, several breaths will be needed before correction is achieved and the correction will always be incomplete. If the feedback gain is too high, the system will oscillate between over and under correction. Complete correction of errors can be achieved with negative feedback by incorporating an integrator. Adding the error to its time integral causes the controlled variable to approach and then reach its target. We have constructed a novel apparatus (capnostat) for clamping end tidal C02 at any desired level. The apparatus (figure 1) comprises an analogue feedback circuit and a gas mixer. FEEDBACK CIRCUIT (figure 2). Inputs to the feedback circuit are tidal volume, flow and airway CO2 concentration. A single output drives the gas mixer. The circuit incorporates both feedback (error plus integral of error) and feedforward. During inspiration the instantaneous volume of the current breath is continuously compared with the stored tidal volume of the previous breath. If the volume of the current breath exceeds 110% of the previous breath the inspired gas mixture is altered to have the same CO2 concentration as the set end tidal value. This feed-forward effectively prevents hypocapnia in the breath following a single large breath. GAS MIXER [1].