Fig. 1.

Examples of Spike2 traces showing the changes in P-P and P-R intervals following glutamate microinjections at 5 levels of the right nucleus ambiguus in one preparation. Recordings of P-R intervals were performed during cardiac pacing. Note that changes in heart rate (P-P interval) had relatively little effect on the perfusion pressure which was largely determined by the flow rate set by the perfusion pump less the flow through the bypass circuit. The pulse pressure arises through the addition to the perfusate of the contents of the left ventricle, the filling of which was not controlled for in these experiments i.e. no attempt was made to eliminate return from pulmonary veins. There was a relatively small effect on pulse pressure presumably attributable to greater diastolic filling and therefore greater ejection through the Frank–Starling mechanism.