Attempts to assess psychological stress rely heavily upon subjective techniques which measure changes in perceived mental loading and situational awareness (Hart and Staveland 1988, Reid and Nygren 1988, Lemyre and Tessier 2003, 1998). Although quantitative methodologies do exist, for example monitoring changes in the cardiopulmonary system (Gelfand et al. 2004, Harada et al. 2006), such parameters are subject to influence by factors other than stress. Psychological stress is known to influence the effectiveness of the innate immune system, leading to an increased risk of infection and immune-related disease (Dhabhar et al. 1996, Boscarino et al. 1999, Altemus et al. 2006). Leukocytes, primarily neutrophils have been identified as an essential component of this mechanism - periods of increased psychological stress have been shown to stimulate neutrophils to release reactive oxygen species into surrounding healthy tissues (Mian et al. 2003). The exact biochemical pathways by which this occurs have not yet been fully elucidated. However, this mechanism has become the basis for a novel in vitro technique (McLaren et al. 2003) which has the potential and sensitivity to rapidly quantify and discriminate between changes in psychological stress, resulting from exposure to short-term low-level everyday life-stressors.
The overall aim of this research was to further explore the relationship between short-term psychological stress and altered immune responsiveness. Leukocyte coping capacity (LCC) is a luminol-dependent chemiluminescent technique for the assay of reactive oxygen species production in whole blood samples. The feasibility of applying this test as an objective, quantitative, diagnostic measure of altered mental workload (mental stress), in the assessment of ergonomics within automotive research and development was examined.
Leukocyte activity was determined from whole blood, using a luminol-dependent, in vitro, chemiluminescent technique referred to as Leukocyte Coping Capacity (LCC). 2 The technique measures reactive oxygen species production following phorbol 12-myristate 13-acetate (PMA) stimulation.
Subjective psychological measures, including likert scales and the NASA task load index were employed to assess perceived stress and altered mental workload. Other traditional physiological parameters including heart rate, systolic and diastolic blood pressure and core body temperature were also measured. The ability of each parameter to detect and discriminate between related short-term stressors was investigated, and results were correlated with post-test changes in leukocyte activity.
To investigate the mechanism of stress induced leukocyte activation, standard ELISA was used to assess post-stressor plasma concentration changes in nine mediators including Adrenaline, Noradrenaline, Cortisol, E-Selectin, L-Selectin, Interleukin-1β, Interleukin-6, Endothelin-1, and Tumour Necrosis Factor-α.
All 5 studies involved the use of mental stressors that were associated with either driving or the ergonomics of driving. Participants were moderately fit and healthy, aged between 20 and 65 years. Study one assessed the ability of the LCC technique to objectively discriminate between two closely related stressors (performing a simple manoeuvre in two different vehicles). Study two investigated leukocyte sensitivity, by testing whether a quantifiable response was elicited following exposure to a low-level stressor lasting seconds. The third study was used to explore the mechanism of leukocyte activation following short-term low-level stress. In addition to testing the viability of leukocyte responsiveness as an objective quantitative ergonomic assay for use within the motor industry, study four investigated how the magnitude of leukocyte responsiveness changed following repeated exposure to the same stressor. The final study used leukocyte reactivity to investigate how mental loading was affected during the interaction with three different motor vehicle control interfaces, whilst simultaneously maintaining lane discipline within a simulated driving environment.
This research has shown that leukocyte reactivity is an effective means for objectively quantifying and discriminating between changes in stress levels, resulting from exposure to short-term low level stressors, encountered as part of daily life. In chapter 3 a significant difference in the magnitude of leukocyte reactivity (F1,38 = 5.94, P = 0.02) was observed after the same basic driving manoeuvre was performed in two different motor vehicles (Car A -738.7 ± 185.4 RLUadj; Car B -284.5 ± 96.6 RLUadj). Chapter 4 showed how a mild stressor lasting only seconds resulted in a quantifiable decrease in leukocyte responsiveness, in vitro. Subjects adjusted a car electric window, retrofitted with the means to covertly alter its normal operation, to four designated positions. Where modification of the window controls occurred, the decrease in the ability of leukocytes to respond to in vitro challenge, via the release of reactive oxygen species, was significantly greater compared to control (Intervention -240.0 ± 56.1 RLUadj, Control -46.2 ± 38.2 RLUadj) (F1,19 = 8.23, P = 0.001). The potential of LCC to discriminate between two closely related stressors was tested in chapter 6. The same basic tasks were performed using two different touch screen interfaces from the same motor manufacturer. The magnitude of change in post-test leukocyte responsiveness was significantly different between each interface (Interface A -429.6 ± 177.3 RLUadj, Interface B -86.9 ± 91.9 RLUadj) (F1,39 = 2.98, P = 0.04). In chapter 7 the ability of LCC to discriminate between changes in leukocyte responsiveness following interaction with 3 different interface design formats, while simultaneously maintaining lane discipline within a simulated driving environment, was demonstrated. The post-test change in leukocyte activity following the use of Interface A was significantly greater compared to the use of Interface B (P = 0.01 Tukey’s post hoc procedure) and also Interface C (P ≤ 0.001 Tukey’s post hoc procedure). Whereas the use of both Interfaces B and C resulted in post-test changes in leukocyte activity of similar magnitude (P = 0.47 Tukey’s post hoc procedure).
Studies have shown that repeated exposure to the same stressor results in a habituated response within the cardiovascular system (Veit et al. 1997, Bhatnagar et al. 2006, Barnum et al. 2007). Although evidence exists demonstrating the modification of the immune response during the presence of chronic stress (Altemus et al. 2006), there is a paucity of evidence that investigates the effects of short-term mental stress on leukocyte responsiveness. As part of the study described in chapter 6, 4 of the original 15 subjects returned for weekly re-testing on each interface on 2 further occasions. Repeated use of both interfaces resulted in a progressive decrease in the magnitude of post-test leukocyte activity. For Interface A post-test differences proved significant between week 1 (primary testing) and week 2 (P = 0.03 Tukey’s post hoc procedure), whereas the magnitude of change was non-significant (P = 0.36 Tukey’s post hoc procedure) for Interface B. For both Interfaces A and B post-test differences proved non-significant for tests in week 3 (Interface A P = 0.52 and Interface B P = 0.74 Tukey’s post hoc procedure), suggesting that leukocyte reactivity does exhibit habituation as familiarity to a situation increases. The overall findings of this research have led to the proposal of a novel pathway for eliciting systemic leukocyte activation in response to short-term low-level mental stress (See chapter 8.2 and figure 8.1).
The results presented in this thesis show there is a consistent and significant association between mental stress and leukocyte activity. The findings provide an initial insight into the diagnostic potential of altered leukocyte responsiveness as a quantitative, objective, physiological assay of mental workload. This research applied the technique for assessing ergonomic design within the car industry, however the potential areas for investigation are far reaching.
|Date of Award||2009|
|Sponsors||Jaguar Land Rover|
|Supervisor||Rubina Mian (Supervisor)|
- psychological stress
- physiological stress
- immune system
- immune responsiveness
- automotive design
- automotive control panels
- motor industry