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This factsheet is a critical appraisal of the journal article "The Impact of High-Intensity Interval Training (HIIT) Vs Moderate-Intensity Continuous Training (MICT) on Vascular function: A Systematic Review and Meta-analysis".



What is the background to this research?
Cardiovascular diseases (CVD) are the leading cause of death in the world and nearly 17.5 million people died from CVD in 2012. Lack of physical activity is considered as the major risk factor for CVDs and recent studies suggest that only 57% of adults Australians achieve their adequate physical activity requirements. Cardio-protective roles of physical activities are explored through various potential mechanisms in recent researches such as endothelial dysfunction and vascular homeostasis. Brachial Flow-mediated dilation (FMD) is the standard noninvasive method used to assess sheer stress and endothelial nitric oxide (NO) bioavailability.

Recent randomized control trials (RCTs) integrating HIIT as the intervention for CVD patients discovered significant benefits on vascular functions, compared to MICT. Therefore, this study is aimed to compare the effectiveness of two types of aerobic exercises (HIIT vs MICT) on vascular functions and associated novel biomarkers.

Where is the research from?
The systematic review was conducted by researchers from three prominent research institutes: University of Queensland from Australia, Colorado university from USA and Norwegian University of Technology from Norway. This article was published in Sports Medicine Journal of New Zealand last year. No potential conflicts of interests were declared by the authors.

What kind of research was this?
This study was conducted as a systematic review and meta-analysis of RCTs regarding the impact of HIIT and MICT on vascular function. Considering the research quality of evidence hierarchy, it is reflected as a Level 1 evidence. A PEDro scale was used by researchers to evaluate the methodological quality of clinical trials. PEDro Score above 6 is required to limit any analytical bias in systematic reviews, whereas this study obtained a mean score of 7.29 out of 10.

What did the research involve?
Initially, potential studies were searched in PubMed, Embase and MEDLINE electronic databases from May 2014. Inclusion criteria of the review were
 * RCTs conducted among humans and published in English
 * Outcome vascular function was measured before and after intervention – HIIT, MICT
 * Duration of intervention was at least 2 weeks
 * only FMD was used to measure vascular function

Changes in mean differences of FMD before and after intervention between HIIT and MICT groups were analyzed with Review Manager software. Critical analysis of RCTs is considered as the best methodological approach with higher accuracy, validity, and efficacy.

What were the basic results?
Six out of seven trials with 182 participants, revealed a significant increase in FMD in both HIIT and MICT groups. RCTs that used 4×4 HIIT protocols [4 intervals for 4 minutes at 85-95% of maximum heart rate] with 3 minutes active recovery (50-70% HRmax) for 12-16 weeks, exposed a substantial progress in endothelial functions, compared to MICT. However, HIIT with shorter duration and greater bouts (4-10 intervals at 80-85% peak oxygen intake [VO2peak] with 4 minutes recovery at 50-60% VO2peak, found no significant difference in FMD related to MICT. Further, the meta-analysis discovered that mean FMD increased from 5.14 to 9.45% in HIIT, whereas FMD increased from 5.12 to 7.27% in MICT. The researchers interpreted that continuous 12 weeks of HIIT improved cardiorespiratory fitness (CRF), blood pressure, blood lipid profile, insulin sensitivity, antioxidant status and body fat percentage exceptionally, compared to MICT.



What conclusions can we take from this research?
HIIT is considered as an effective stimulus for improving vascular functions by 2.26% increase in brachial FMD, more than MICT. Possible mechanisms suggested for the exclusive benefits of HIIT are induced flow among blood vessels by activation of endothelial NO synthase, enhanced insulin sensitivity, faster clearance of low-density lipoprotein and improved antioxidant status. Findings of this study are consistent with previous researches in terms of CVD risk, CRF and HIIT. For example, another systematic review reported 9.1% increase in Vo2peak after HIIT compared to MICT among cardiometabolic patients. But few RCTs outlined that there is no convincing evidence to support the superiority of HIIT over MICT. Novel CVD risk assessment methods could be developed for public health interventions in future such as NO bioavailability.

Practical Advises
This review suggests 4*4 HIIT (weekly three times) for at least 12 weeks could improve endothelial functions and remodel blood vessels, compared to other exercise protocols of HIIT or MICT. Promoting HIIT among general public could increase their adherence to exercise intervention due to lack of time constraints. However, high-risk CVD patients must consider their co-morbidities, exercise threshold levels and medications to avoid adverse health risks.

Limitations
Effects of confounders such as drug interactions, abstinence from exercise, diverse exercise intensity threshold levels of participants and timing of FMD assessment were considered as the potential bias in this study.

Reference
Author: u3136445 (Post-graduate student of University of Canberra).