Another important confounder affecting the trials selected for inclusion in this review is that they included patients with different types of hypertension. As noted previously, half of the trials included patients with essential hypertension only (Yin et al., 2007; Zheng et al., 2016; Zheng et al., 2018), while the others also included patients with secondary hypertension (Macklin et al., 2006; Flachskampf et al., 2007; Kim et al., 2012). Secondary hypertension in adults may have a variety of causes, including – but not limited to – atherosclerotic renal arterial stenosis, renal failure and hypothyroidism (Charles et al., 2017). It is reasonable to assume that if acupuncture is to be effective at managing patients with secondary hypertension, the underlying cause of the hypertension must be addressed, either through acupuncture of another treatment. Indeed, the three trials which included patients with secondary hypertension returned less-positive results about the use of acupuncture for the management of hypertension than the other three trials.
The problems associated with the inclusion of pre-hypertensive patients in the trial by Yin et al. (2007) must also be noted. In Western medicine, pre-hypertension is typically controlled through non-pharmacological interventions such as a low sodium diet, weight reduction and increased physical activity, etc. (Zhang & Li, 2011). These interventions also benefit patients with hypertension, but in these patients anti-hypertensive medication is also usually prescribed (Zhang & Li, 2011). It is unclear if there are also differences in the activity and effectiveness of acupuncture in patients who are pre-hypertensive, versus those with hypertension.
As noted, there are a variety of modifiable ‘lifestyle’ factors which impact hypertension – including the consumption of sodium, body mass index (BMI) and level of physical activity (Zhang & Li, 2011). Most of the trials (n=5, 83.3%) reported patients’ baseline BMI (Macklin et al., 2006; Flachskampf et al., 2007; Yin et al., 2007; Kim et al., 2012; Zheng et al., 2018); however, only one of the studies controlled for this in the analysis (Macklin et al., 2006). None of the studies controlled for potential confounders such as sodium intake or level of physical activity. It is possible that patients enrolled in a trial about hypertension may become more aware of, and more willing to change, the modifiable factors associated with their condition. However, none of the trials selected for inclusion measured this as a potential confounder.
It is also important to note that two of the trials recorded significant rates of participant loss-to-follow-up: Flachskampf et al. (2007: 27/160 lost, 16.0%) and Kim et al. (2012: 5/33 lost, 15.2%). These losses may have occurred because participants chose to leave the study (e.g. because the time commitment to participate was too great), or because participants were eliminated from the study (e.g. because they developed significant illness) (Flachskampf et al., 2007; Kim et al., 2012). It is possible that the results of a study will be biased if participants who are lost to follow-up are systematically different to those who remain in the study (Polit & Beck, 2010). For example: it is possible that people who were lost to follow-up were more seriously unwell, perhaps because of more significant hypertension, than patients who remained; thus, those who remained had less-serious hypertension more responsive to acupuncture. However, it is impossible to determine this as neither Flachskampf e
