Browsing by Author "Walker, Andrew"

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    Early childhood risk factors for later onset of pediatric chronic pain: a multi-method longitudinal study
    (2024-08-08) Pavlova, Maria; Noel, Melanie; Orr, Serena L.; Walker, Andrew; Madigan, Sheri; McDonald, Sheila W.; Tough, Suzanne C.; Birnie, Kathryn A.
    Abstract Background Pediatric chronic pain (i.e., pain lasting ≥ 3 months) is prevalent, disabling, and costly. It spikes in adolescence, interrupts psychosocial development and functioning, and often co-occurs with mental health problems. Chronic pain often begins spontaneously without prior injuries and/or other disorders. Prospective longitudinal cohort studies following children from early childhood, prior to chronic pain onset, are needed to examine contributing factors, such as early pain experiences and mental health. Using data from a longitudinal community pregnancy cohort (All Our Families; AOF), the present study examined the associations between early developmental risk factors, including early childhood pain experiences and mental health symptoms, and the onset of pediatric chronic pain at ages 8 and 11 years. Methods Available longitudinal AOF data from child age 4 months, as well as 1, 2, 3, 5, 8, and 11 years, were used. Mothers reported their child’s pain experiences (e.g., hospitalizations, vaccinations, gut problems) at each timepoint from 4 months to 8 years, child chronic pain at age 8, and child mental health symptoms at ages 5 and 8 years. Children reported their chronic pain frequency and interference at age 11. Adaptive least absolute shrinkage and selection operator (LASSO) regressions were used to select predictor variables. Complete case analyses were complemented by multiple imputation using chained equation (MICE) models. Results Gut problems, emergency room visits, frequent pain complaints, and headaches at age 5 or earlier, as well as female sex, were associated with increased risk of maternal reported child chronic pain at age 8. Maternal reported chronic pain at age 8 was associated with higher levels of child-reported pain frequency and pain interferences at age 11. Boys self-reported lower levels of pain interference at age 11. Conclusions Some, but not all, painful experiences (e.g., gut problems, ER visits, pain complaints) in early life contribute to pediatric chronic pain onset and should be considered for screening and early intervention.
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    The Characterization of Common Cardiovascular Flow Regimes Using Newtonian and Non-Newtonian Fluids
    (2013-07-15) Walker, Andrew; Rival, David; Johnston, Clifton
    This thesis consists of three projects with the enveloping theme devoted to the characterization of common cardiovascular flow regimes using Newtonian and non-Newtonian fluids. The viscous behaviour of hydroxyethyl starch (HES) fluids available in Canada to treat hypovolemia remains unknown. The first project characterized the viscosity of these fluids using capillary viscometry and pressure drop measurements. The viscosities of HES 130/0.4 (Voluven®, Volulyte®) and HES 260/0.45 (Pentaspan®) were less than and greater than blood respectively. HES 130/0.4 (Voluven®) at 100% concentration unexpectedly displayed shear thickening behaviour at high flow rates. HES fluids were subsequently diluted in a blood analog fluid of aqueous xanthan gum. As expected, both HES 130/0.4 fluids decreased the analog viscosity while HES 260/0.45 increased analog viscosity and no evidence of previous shear thickening was found. Variability in viscous behaviour suggests changes in the molecular composition between batches. The non-Newtonian behaviour of blood is often ignored in cardiovascular flow modelling. To address the importance of non-linear rheology, flow patterns were experimentally measured using Newtonian and non-Newtonian blood analog fluids in separated flow environments. Flow induced by a Gianturco Z-stent showed that Newtonian assumptions underestimated wall shear stress (WSS) while expanding recirculation and oscillatory shear. This suggested that linear viscous assumptions overestimated the risk of intimal hyperplasia. In transitional flow induced by a stenosis, the non-Newtonian fluid extended laminar flow behaviour while damping turbulent shear stress. Conversely, the Newtonian fluid displayed downstream shear layer break-up and a radial expansion in elevated turbulent shear stress at the distal end of the field of view at peak pulsatile flow. Translation to the measurement of separated flow in vivo requires imaging in opaque environments. Echo particle image velocimetry (Echo PIV) has presented itself as an attractive tool; however, application to stenotic flows has been minimal. Echo PIV measured centerline velocities showed a good fit to PIV; however, failure to accurately resolve near wall flow patterns suggested further refinement is necessary prior to its use as a reliable quantitative imaging tool in such environments.