Donald McKenzie


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
High-intensity exercise and safety concerns in breast cancer survivors (2013)

Prescribing exercise as an adjuvant therapy for cancer survivors is becoming anacknowledged rehabilitative tool to offset the numerous biological toxicities that candevelop with cancer treatments. Exercise interventions for breast cancer survivorstypically prescribe low-to- moderate intensities to counteract the treatment-induceddysfunctions. These prescriptions have elicited inconsistent results. Interval training iscurrently being explored in many clinical populations with positive outcomes; however,the use of higher intensity exercise in the cancer population is not yet encouraged.Through four different studies, this doctoral dissertation investigated the safety ofimplementing higher intensity exercise protocols into breast cancer survivor rehabilitativeprograms during chemotherapy (N=60) and post-primary therapy (N=33). The influenceof intensity on safety, aerobic capacity and anaerobic capacity was explored. Supervisedexercise programs and maximal aerobic stress tests were administered on a treadmill as itwas demonstrated to be the preferred mode of exercise; in spite of this, the measure ofanaerobic capacity was determined to be the most reliable on a cycle ergometer. Safetyissues concerning neutropenia, altered chemotherapy dose, and biomarkers associatedwith poor outcome, were investigated.During chemotherapy (FEC-D), 30 early-stage breast cancer patients exercised atintensities between 70- 90% V02peak, thrice weekly, for the duration of their treatment(18.5 weeks). Women were able to safely incorporate bouts of higher intensity exercise without increased hospitalization, immunosuppression, or negatively impacting theirchemotherapy dose (achieved RDI was 92%, achieved RDI in the matched, usual caregroup was 90%).Post-primary therapy, 33 postmenopausal breast cancer survivors were randomized into 3groups (supervised high-intensity interval training (HIT), supervised continuousmoderate-intensity exercise training (CMT) and an unsupervised control group). For 6weeks, women in the HIT group exercised at intensities between 70 to 100% V0₂peak,while the CMT group exercised between 60-70% V0₂peak. HIT and CMT led tosignificant improvements in health-associated outcomes, however the mechanisms thatled to these benefits may have differed between the groups.No adverse events occurred due to high-intensity exercise in either supervisedintervention. This dissertation provides evidence that breast cancer survivors canincorporate bouts of higher intensity exercise into their supervised rehabilitationprograms during, or post-primary therapy.

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The pulmonary profile of competitive swimmers (2013)

Purpose: The purpose of this thesis was to investigate the nature of the following conditions in competitive swimmers: the occurrence of exercise-induced arterial hypoxemia (EIAH), and the relationship to gender and exercise-induced bronchoconstriction (EIB) (Study 1), and the development of exercise-induced pulmonary edema (PE), and changes in oxyhemoglobin saturation (Study 2).Methods: Study 1: Twenty-one well-trained swimmers (10 male, 11 female) completed a eucapnic voluntary hyperpnea test and an incremental swim test to exhaustion with pre and post-exercise spirometry. Metabolic data (VO₂, VCO₂, ventilation and heart rate) along with oxyhemoglobin saturation (SpO₂) were collected throughout exercise. Study 2: Baseline lung density was obtained in eight well-trained male swimmers using computerized tomography after 24 hours rest. After a standard warm-up, subjects performed 6 x 50m maximal effort intervals on 90 s. Oxyhemoglobin saturation (SpO₂) and heart rate were collected across exercise intervals. Scans were repeated at 45-60 min post-exercise.Results: Study 1: Eleven of the twenty-one subjects tested positive for EIB. No subjects fulfilled our criteria for EIAH (SpO₂ ≤95% or ≥3% drop from resting values) despite a small mean drop in SpO₂ from rest to maximal exercise in both males and females. There was no correlation between EIAH and EIB (p=0.21). Study 2: For the duration of the intervals, mean saturation levels remained close to resting values. Mean lung density significantly increased (p
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Master's Student Supervision (2010-2017)
Exercise prescription in future medical practice (2014)

Physical inactivity is a major risk factor in chronic disease and Canadians are insufficiently active. Exercise prescription has been shown to be effective but few physicians prescribe it. The purpose of this study was to determine family medicine residents’ perceived importance of exercise prescription, and to assess the factors associated with residents who indicate the strongest conviction to prescribe exercise. All 396 family medicine residents registered in first or second year at the University of British Columbia, during June 2013 - August 2013, were eligible to complete the cross-sectional 49-item survey. The outcome measures were (1) the importance of exercise prescription in future practice (2) perception of their training in exercise medicine (3) change of the importance of exercise prescription over the course of residency. The data were analyzed using descriptive and inferential statistics to assess significant relationships between each independent variable, resident physical activity levels, attitudes/beliefs, current counselling/prescribing behaviours, awareness/knowledge of physical activity guidelines, self-perceived competence in exercise prescription, and perception of training received, to their perceived importance of exercise prescription in future practice. The data were analyzed as continuous or categorical variables primarily using bivariate analysis with statistical significance set at the level of 0.05. The response rate was 80.6% (319/396). 95.6% of residents indicated exercise prescription would be important in their future practice with 37.5% strongly agreeing (termed "prescribers"). Prescribers had stronger beliefs in the importance of physical activity in health (p
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The effect of exercise intensity on insulin levels in postmenopausal breast cancer survivors (2012)

Higher serum levels of glucose, C-peptide and insulin have been linked to poorer breast cancer prognosis and fluctuations of these substances resulting from acute bouts of exercise in breast cancer survivors have not been studied. In this pilot study, 28 postmenopausal women were in 3 groups: controls (n = 10), aromatase inhibitor users (AI, n = 8) and breast cancer survivors not using aromatase inhibitors (BCS, n = 10). Fasting glucose, C-peptide and insulin levels were compared and changes in these substances with acute exercise of different intensities were explored. V0₂peak values were determined and fasting blood samples were obtained. Blood was collected before, immediately after, and 45 minutes after 2 exercise bouts: moderate (45 minutes at 60% V0₂peak) and intense (10 minutes at 85% V0₂peak). Fasting glucose, C-peptide and insulin values, plus HOMA-IR values, were homogeneous between groups. Exercise intensity affected glucose, C-peptide and insulin levels similarly in all 3 groups. For the breast cancer survivor groups combined (n = 19), the pre-post changes and post-recovery changes of glucose and C-peptide were significantly different between the moderate and intense bouts (glucose: p = 0.01 and 0.01; C-peptide: p = 0.04 and 0.04, respectively) showing greater increases in glucose and C-peptide during intense exercise. The pre-post change in insulin approached a significant difference between intensities (p = 0.09) showing a greater increase during intense exercise. Of importance, glucose, C-peptide and insulin levels all transiently increased with intense exercise in breast cancer survivors. In conclusion, more research is warranted on the possible detrimental effects of transiently high glucose, C-peptide and insulin levels induced by intense exercise on breast cancer etiology and prognosis.

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The effect of exercise on the concentration of platelets in a platelet rich plasma preparation (2012)

Traumatic tendon injuries and tendinopathy are common problems in sports medicine practice. The active population seeks minimally invasive treatments that speed healing time. New strategies, such as platelet-rich plasma (PRP) therapies, may achieve this. The use of PRP in sports medicine has been stimulated by the advancing knowledge regarding the role of growth factors (GF) in tissue repair. GF concentration is thought to increase linearly with platelet concentration (Eppley et al., 2004). Several studies are emerging with favourable outcomes in injection of PRP into the area of injury (Kon et al., 2009, Mishra & Pavelko, 2006). Some postulate that the greater the concentration of platelets in a sample the greater the healing augmentation (Smith, 2009). There is a lack of literature addressing the clinically practical issue of how to best maximize the platelet-enhanced product drawn from the patient. The purpose of this study was to evaluate the effect of 2 exercise intensities on the concentration of platelets in a PRP preparation. The participants exercised on a cycle ergometer on three occasions. First, a V0₂max test was carried out. The participant then exercised, on two separate days, for 15 minutes at 50% (moderate exercise) or at 85% (intense exercise) of their predetermined V0₂max heart rate. Blood was drawn at baseline and within 3 minutes post exercise. The samples were prepared into a PRP preparation. The concentration of platelets was analyzed in the PRP. We found a significant increase in the concentration of platelets in the post-intense exercise PRP samples. No significant increase was seen in the moderate exercise condition. A significant effect was found for the mean differences between pre and post in the moderate versus intense exercise groups. These results indicate that intense exercise is a practical and safe way to increase the concentration of platelets in a PRP sample.

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Intrapulmonary shunting in asthmatic athletes (2011)

This study investigated the presence of intrapulmonary shunt in asthmatic athletes during exercise. The effects of airway obstruction on the onset of intrapulmonary shunt were examined by comparing two conditions of airway obstruction: induced airway obstruction and reversed airway obstruction. The study also examined the effect of body position on intrapulmonary shunt recruitment by comparing three positions at rest: supine, head-down tilt and upright. We hypothesized that induced airway obstruction would trigger an earlier onset of intrapulmonary shunt during exercise when compared to reversed airway obstruction; we also hypothesized that the head-down tilt position will recruit intrapulmonary shunt at rest. Ten asthmatic, highly aerobically trained (VO₂max = 62.6 ± 6.7 ml/kg/min) males completed 3 resting stages and 6 stages of incremental exercise under both conditions of airway obstruction. Agitated saline contrast echocardiography was used to determine the presence of intrapulmonary shunt. Ten of ten subjects demonstrated intrapulmonary shunt during exercise; three of ten subjects demonstrated shunt at rest. Nine of ten subjects demonstrated shunt onset at the same workload for both conditions; one individual showed a delay in shunt onset with the reversed airway obstruction condition. Mean shunt onset under induced airway obstruction and reversed airway obstruction was 41.4 ± 7.9 %VO2max and 42.5 ± 8.1 %VO₂max, respectively; no significant difference in mean shunt onset was found. Among the 3 subjects who shunted at rest, 2 subjects shunted at resting supine and 1 subject at both supine and head-down tilt. Conditions of airway obstruction did not have an influence at shunt onset at rest. Intrapulmonary shunting during exercise is evident in asthmatic athletes and appears to occur at low workloads but is not consistently influenced by acute conditions of airway obstruction or body position.

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Comparison of aerobic exercise intensity prescription methods in breast cancer patients and survivors (2010)

It is accepted that exercise plays a significant role in breast cancer rehabilitation, but there has been limited emphasis on control and measurement of the intensity of exercise in cancer research. It is unknown how intensities achieved by different methods of intensity prescription compare, which complicates the interpretation and comparison of studies. The accuracy of these methods in achieving the prescribed intensity is also unknown; and methods that are inaccurate could be unsafe or ineffective in this population. Therefore, a cross-sectional study was performed to compare the achieved intensity and accuracy of four common methods of intensity prescription within and between three post-menopausal groups: breast cancer patients recently finished chemotherapy, survivors finished treatment and healthy controls (N=30). In randomized order, the metabolic equation for walking (MET equation), heart rate reserve (HRR), direct heart rate (direct HR) and rating of perceived exertion (RPE) methods were used to prescribe an intensity of 60% of oxygen consumption reserve (VO₂R) in separate 10-minute bouts, with recovery between bouts. Expired gas analysis was used to measure the intensity achieved during each bout. Accuracy was defined as: [60%VO₂R-achieved intensity]. In ranked order, the average achieved intensity (%VO₂R) and accuracy (percentage points (+/-ppts)) of the methods in the patient group were: HRR: 61%, 3 ppts; MET equation: 56%, 4 ppts; direct HR: 60%, 8 ppts; RPE: 53%, 9 ppts. The HRR method is recommended in this population based on accuracy and feasibility (no expired gas analysis or re-testing required). The MET equation method is also recommended, with re-testing to account for changes in peak oxygen consumption. The direct HR method could be unsafe, as it achieved intensities much higher than intended (77%), and would be ineffective in research where the effect of exercise is measured, as there was a large range of achieved intensities (42%). In the survivor group results were: MET equation: 59%, 3 ppts; HRR: 63%, 5 ppts; direct HR: 64%, 5 ppts; RPE: 47%, 13 ppts. The top three methods were comparable in accuracy in this group, and appear to be safe and effective, while the RPE method was inaccurate and is not recommended.

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