Deeper Dive: An inspirational approach to lowering blood pressure

Aging increases cardiovascular disease (CVD)^[1] risk, partly due to higher blood pressure (BP) and lower vascular function^[2]. These increases in BP are, in part, caused by cardiovascular aging^[3], which is, in turn, driven by decreases in nitric oxide (NO) bioavailability^[4] caused partially by increases in systemic inflammation^[5] and reactive oxygen species (ROS)^[6]. Since all these processes of vascular aging increase BP, most interventions (lifestyle and pharmaceutical) are aimed at lowering BP (and blood lipids) to reduce CVD risk^[7].

Besides dietary approaches and anti-hypertensive drugs, aerobic exercise is an effective and evidence-based strategy to counteract cardiovascular aging and decrease BP^[8]. However, adherence rates in midlife and older adults are often too low to make a meaningful difference. For instance, less than 40% of midlife/older adults meet the recommended 150 minutes of aerobic exercise per week^[9]. Most often, lack of time and mobility issues are reported as barriers to aerobic training. Alternatively, dietary sodium restriction^[10] and energy restriction-based weight loss^[11], as well as multi-component diet strategies (e.g., the DASH diet) can effectively lower BP and restore vascular function. Yet, these strategies are also ineffective if they are not adhered to. Consequently, more time-efficient and less-effortful strategies that are as safe and effective as conventional interventions are needed, especially for elderly people.

Inspiratory muscle strength training (IMST)^[12] is a potentially time- and effort-efficient form of resistance training and an attractive alternative to conventional physical exercise. As shown in Figure 1, during IMST, participants repeatedly inhale against resistance (e.g., 30% of maximal inspiratory pressure for 30 minutes) to train their diaphragm and accessory respiratory muscles using a handheld device. IMST strengthens the inspiratory/respiratory muscles and vascular function and has shown promising results in people with certain respiratory conditions, such as chronic obstructive pulmonary disease (COPD)^[13]. Recently, a more time-efficient high-resistance IMST strategy (30 breaths at 50–70% maximal inspiratory pressure for about 5 minutes) has been tested^[14]. In this study, which recruited normotensive adults with obstructive sleep apnea, high-resistance IMST effectively reduced systolic BP by about 10% (-12 mmHg). Two even more recent studies^[15][16] also found BP-lowering effects of IMTS. However, while most of these studies found a BP-lowering effect of IMST within the intervention group, the findings were most often not statistically significant compared to the control group. One possible explanation is that these studies had small sample sizes and thus insufficient statistical power to detect significant changes compared to the control group. Also, these studies were conducted in healthy younger adults, while evidence in favor of IMST’s efficacy in older populations (who would benefit most from new effective and time-efficient BP-lowering interventions) is scarce. To date, only one trial^[17], also with promising results, has been conducted with elderly participants.

Figure 1: The basics of IMST

Even though the evidence suggests IMST may be effective, the mechanism of action is not fully understood. There are, however, some hypotheses as to how inspiratory and respiratory muscle strength training may influence the vascular system^[18]. The simplest hypothesis assumes that IMST strengthens the diaphragm and the inspiratory/respiratory muscles, thus improving vascular function. A more complicated model suggests that repeated changes in lung volume induced by IMST lead to shear stress in the vascular system, which acts as a stimulus for endothelial cells to improve their capacity to open up and improve blood flow. This process is called vasodilation. See the FMD sidebar for more information. Even though these models sound appealing, mechanistic studies are needed to test these hypotheses.

Aging adults are at risk of developing CVD, partly due to hypertension and endothelial dysfunction. Since aerobic sports show low adherence rates, alternative, more time-efficient forms of resistance training, such as high-resistance IMST, are being researched. However, the mechanisms of IMST remain unknown, and the current body of evidence is mainly limited to healthy young adults. Thus, the question remains: Can high-resistance IMST reduce BP and improve vascular function in hypertensive adults at midlife or older age?