A. Guz

Effects of Alterations in Aortic Impedance on the Performance of the Ventricles

We have investigated the effects of sudden changes in the load against which the left ventricle contracts in healthy conscious dogs. During a steady state, initial responses were assessed by comparing the beat before with the beat after such a change. Flow was measured by electromagnetic flowmeters and pressure usually by a catheter-tip transducer. Results show that under the conditions of these experiments the curves relating external work and power to load are consistent with the parabolic relationship shown for isolated hearts and muscle strips and that in each case the left ventricle normally functions near the peak of the curve. When an increased load was maintained until a new steady state was reached the left ventricle still functioned on the descending limbs of what presumably were different work-load and power-load curves. It was concluded from this that either the increase in load was too great or the time too short for reflex humoral and intrinsic mechanisms to readjust and permit the ventricle to function again at the peak of each curve, or that the circulatory system does not make relatively rapid adaptations in this way to an increase in systemic load. The immediate effects of changes in load on left ventricular stroke volume were similar to those seen in isolated preparations. Stroke volume varied inversely with the load. Stroke volume changes were accompanied by changes in the duration, rate, and velocity of ejection. The duration of the isovolumetric phase was affected little but the period from valve closure to the end of ventricular relaxation varied with the load so that the total duration of contraction and relaxation of the ventricle was relatively unchanged, though an increase in load did tend to prolong it. Before the onset of the baroreceptor response the right ventricle was not affected, the two ventricles acting independently in relation to all these changes.

Studies of the pulmonary vagal control of central respiratory rhythm in the absence of breathing movements

1. Breuers hypothesis that the vagus nerves exert a tonic control of respiratory rhythm, in addition to the phasic control, was examined.

The effect of carbon dioxide in the airways and alveoli on ventilation; a vagal reflex studied in the dog

1. The inhalation of CO2 produces a tachypnoea only in the presence of intact vagus nerves; the present study was designed to examine the mechanism of this phenomenon in the dog.

Putative cerebral cortical involvement in the ventilatory response to inhaled CO2 in conscious man.

1. The response of the diaphragm to both transcranial magnetic stimulation and electrical phrenic nerve stimulation was studied in thirteen normal subjects under conditions of either a ‘reflex’ drive to ventilation with inhaled CO2 or during volitional ventilation of similar magnitude. 2. The induced compound action potential in the diaphragm was recorded using an oesophageal electrode, and in some cases transdiaphragmatic pressure was recorded using oesophageal and gastric balloon catheters. 3. The response of the diaphragm to transcranial magnetic stimulation was invariably facilitated with volitional inspiration; there was either minimal or no response at functional residual capacity. 4. Facilitation with inspiration was also seen during a ‘reflex’ drive to ventilation with inhaled CO2 in the presumed absence of any volitional contribution to ventilation. A similar degree of facilitation was seen with voluntary ventilation of similar magnitude and pattern. 5. If the facilitation is predominantly a cortical phenomenon, then these results imply that there is a behavioural component in the previously supposed purely ‘reflex’ drive to ventilation with inhaled CO2. We also discuss the interpretation of these results if some of the facilitation occurs at the phrenic motoneurone.

Effect of Changes in Heart Rate on Left Ventricular Performance in Conscious Dogs

An increase in heart rate produced in 12 conscious dogs by right atrial pacing resulted in a fall of end-diastolic volume, stroke volume, stroke work and end-diastolic pressure. Stroke power and the maximum rate of increase of left ventricular pressure (LV dP/dt max) were affected only slightly. Sudden large changes of rate produced transient changes in LV dP/dt max typical of the positive and negative inotropic effects of activation, but steady-state LV dP/dt max was not affected; these effects persisted after autonomic blockade with propranolol and atropine. Postural changes produced no change in LV dP/dt max even after autonomic blockade. We conclude that in this preparation, the interval-strength relationship has a plateau at the physiologic range of heart rate, i.e., myocardial contractility is unaffected by change in heart rate over the range 90 to 190 beats/min.

The role of non-myelinated vagal afferent fibres from the lungs in the genesis of tachypnoea in the rabbit.

1. The use of a direct current (d.c.) to produce a differential block of conduction in the cervical vagus nerves of rabbits is described; the myelinated fibres are blocked, while the non‐myelinated ‘C’ fibres conduct normally. The method produces reproducible and reversible results.

Pulmonary stretch receptor activity in man: a comparison with dog and cat

1. The ‘collision’ technique has been shown to be suitable for recording orthodromic ‘A’ fibre activity in a whole, functioning cervical vagus nerve. This activity predominantly arises from pulmonary stretch receptors.

Is the voluntary control of exercise in man necessary for the ventilatory response

The ventilatory response to electrically induced exercise (EEL) was studied in eighteen normal subjects and compared with the response to performing the same exercise voluntarily (EV). EEL was produced by surface electrode stimulation of the quadriceps and hamstring muscles so as to cause a pushing movement at 1 HZ against a spring load; this produced no pain or discomfort. Matching of EV to EEL was achieved by subjects copying a tension signal recorded during EEL and displayed on a storage oscilloscope. There were no differences between the resting states measured before either form of exercise. The ventilatory response (change in ventilation as a ratio of the change in CO2 elimination) was similar in the two types of exercise. The increases in ventilation and CO2 elimination were greater with EEL. Small but significant increases in the gas exchange ratio and serum lactate were found for EEL but not for EV, suggesting an increase in anaerobic metabolism in EEL. End‐tidal PCO2 showed little change in either form of exercise. In some runs end‐tidal PCO2 rose, but insufficiently to account for the ventilatory response as judged by the response to inhaled CO2. In two subjects arterial blood samples showed small and inconsistent changes in both Pa,CO2 and PaO2 for EV and EEL. pH and base excess changes also were consistent with more anaerobiosis with EEL compared to EV. The first ten breaths of exercise were used to study the on transient. In EV, expiratory duration shortened and ventilation increased significantly on the first breath but CO2 elimination did not increase until the second breath; in EEL, these variables did not change significantly until the second breath. For the remainder of the on transient the pattern of the ventilatory response was similar for EV and EEL. By the end of the on transient both EV and EEL had reached approximately 80% of their final steady‐state values. These results suggest that a normal ventilatory response can occur in the absence of a drive to exercise from the cortex.

The role of spinal cord transmission in the ventilatory response to electrically induced exercise in the anaesthetized dog

1. 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 transection at T8/9 (dermatome level T6/7) on fifteen occasions.