Stress and the Cardiovascular System
The research reported in this thesis is primarily concerned with systemic arterial pulse transit time (PTT) which is of physiological significance because it is the most appropriate indicator of arterial compliance, the primary determinant of cardiac load, PTT acceleration or deceleration being associated with an increase or decrease in cardiac load respectively. Initially, PTT was investigated in the context of active/passive coping. Obrist, et al., (1978) proposed that active coping is generated by tasks of moderate difficulty and is characterized by large, sustained cardiac accelerations. Interbeat interval (IBI) was measured as the reference response for active/passive coping. The assumption is that active coping is the behavioural state which provides the link between psychological stress and hypertension. However, contemporary cardiovascular physiologists put more emphasis on arterial compliance than IBI because it is not only significant in essential hypertension, but is also a critical determinant of circulation efficiency in health and disease. In Experiment 1 men and women completed mental arithmetic, problem solving, reaction time and personal tempo (voluntary button pressing). Acceleratory and deceleratory PTT changes were found during all tasks; IBI changes were predominantly acceleratory and their magnitude was determined by task difficulty. In Experiment 2 subjects completed problem solving tasks at two levels of difficulty. The results confirmed that unexplained directional variability characterized PTT changes and acceleration characterized IBI changes. It was hypothesized that subject state could be a determinant of PTT response direction. In Experiment 3 the State-Trait Anxiety Inventory, the Stress Arousal Checklist, the Eysenck Personality Inventory and the Jenkins Activity Survey were administered before problem solving at three levels of difficulty and personal tempo were completed. PTT directional variability was not accounted for by any of the measures used. IBI changes were again acceleratory and their magnitude was determined by task difficulty. Two extensions of the active/passive coping hypothesis were proposed: 1) that task difficulty and magnitude of IBI change are related along a continuum of behavioural coping; 2) that task type, not difficulty, determines the frequency of IBI acceleration. The effect of task contingency on IBI change was identified as requiring more detailed investigation. It was further hypothesized that relative subject state as measured by a change in resting IBI (Malmo, 1959) could predict the direction of PTT change during task. In Experiment 4 subjects completed all tasks from Experiment 3 in both parts of an extended experimental session. Half the subjects ingested caffeine. IBI decelerated during baseline from part 1 to part 2 and deceleratory PTT changes dominated task responding in part 2. However, a caffeine-induced deceleration in IBI across baselines was not associated with deceleratory PTT change, and directional variability continued to characterize PTT changes. In order to eliminate the directional variability of PTT changes during tasks a further experiment was undertaken which manipulated relative behavioural state prior to task by informing subjects on their first attendance at the laboratory that they would be required to perform a demanding problem solving task on their fifth attendance. From sessions 1 to 4 subjects attended the laboratory for short rest periods only during which cardiovascular activity was recorded. On the fifth session subjects also completed a task. In that experiment (Experiment 5), IBI acceleration over multiple session baselines in anticipation of a task was associated with uniformly acceleratory PTT changes during tasks, supporting the hypothesis that subject initial state is an important determinant of the direction of PTT change, and hence in whether cardiac load increases or decreases under stress.