Functional capacity and heart rate response: associations with nocturnal hypertension

BACKGROUND Absences of normative, 10-20 % declines in blood pressure (BP) at night, termed nocturnal non-dipping, are linked to increased cardiovascular mortality risks. Current literature has linked these absences to psychological states, hormonal imbalance, and disorders involving hyper-arousal. This study focuses on evaluating associations between nocturnal non-dipping and indices of functional cardiac capacity and fitness. METHODS The current study was a cross-sectional evaluation of the associations between physical capacity variables e.g. Metabolic Equivalent (MET) and Maximum Heart Rate (MHR), Heart rate reserve (HRR), and degree of reduction in nocturnal systolic blood pressure (SBP) or diastolic blood pressure (DBP), also known as 'dipping'. The study sample included 96 cardiac patient participants assessed for physical capacity and ambulatory blood pressure monitoring. In addition to evaluating differences between groups on nocturnal BP 'dipping', physical capacity, diagnoses, and medications, linear regression analyses were used to evaluate potential associations between nocturnal SBP and DBP 'dipping', and physical capacity indices. RESULTS 45 males and 14 females or 61.5 % of 96 consented participants met criteria as non-dippers (<10 % drop in nocturnal BP). Although non-dippers were older (p = .01) and had a lower maximum heart rate during the Bruce stress test (p = .05), dipping was only significantly associated with Type 2 Diabetes co-morbidity and was not associated with type of medication. Within separate linear regression models controlling for participant sex, MHR (β = 0.26, p = .01, R(2) = .06), HRR (β = 0. 19, p = .05, R(2) = .05), and METs (β = 0.21, p = .04, R(2) = .04) emerged as significant but small predictors of degree of nighttime SBP dipping. Similar relationships were not observed for DBP. CONCLUSIONS Since the variables reflecting basic heart function and fitness (MHR and METs), did not account for appreciable variances in nighttime BP, nocturnal hypertension appears to be a complex, multi-faceted phenomena.