Functional capacity and heart rate response: associations with nocturnal hypertension

In this study our objective was to understand the relationship between dipping status
in systolic blood pressure (SBP) and functional capacity indices, notably the maximum
heart rates (MHRs) and metabolic equivalents (METs) achieved during standard stress
tests with patients enrolled in a cardiac rehabilitation program. In addition, to
control for sex or individual differences in MHR, we used HRR to account for individual
or sex-related differences in resting heart rate. The control of nocturnal BP is an
important cardiac risk target. Although current data are insufficient to demarcate
the pathways of effective reduction, our results indicated cardiac performance (reflected
by the MHR) and functional capacity (reflected in METs) during stress testing, was
significantly but modestly associated with systolic blood pressure dipping.

Since the variables measured, reflecting basic heart function and fitness, do not
account for an appreciable variance in nighttime BP, the nocturnal hypertension phenomena
appears to be multi-faceted and complex. This raises the question of the impact of
factors relating more broadly to autonomic sympathetic hyperarousal, both behavioural
and physiological, and possibly modifiable via medication and behavior modification.
Along these lines, the interventions thus far most frequently applied employ exercise
and use of continuous positive airway pressure (CPAP) in managing nocturnal hypertension
29]. Wuerzner et al. 30] suggest higher physical activity levels were significantly related to reduced nocturnal
hypertension levels, but when physical training intervention was specifically assessed,
31] aerobic training had no effect on most subjects (N?=?14), and worsened the status of more subjects than it improved (5 subjects who
were dippers became non-dippers and 4 subjects who were non-dippers became dippers).
These results again suggest the the complexity of nocturnal BP which may not be linearly
influenced by exercise training. Training may require adjustments not typically undertaken
in exercise programs targeted at generic fitness. When non-dippers are found to have
Obstructive Sleep Apnea (OSA), the use of continuous positive airway pressure (CPAP)
has been identified, in some cases, as effective 29]. In our data, only 3 patients had been diagnosed and treated for OSA. Of the 3 patients,
1 had a nocturnal hypertension problem while 2 others did not. These results, in the
patients assessed, do not indicate that OSA diagnosis and CPAP are a definitive treatment
for nocturnal hypertension and the changes observed did not impact the model calculations
derived.

In past research, multiple studies implicate disrupted sleep and autonomic hyperarousal
as important factors that must be considered in the treatment of nocturnal hypertension.
Population-based studies reveal an increased prevalence of sleep disorder likely attributable
to increased daily stress. For example, the Sao Paulo Sleep Study 32] revealed a 32 % prevalence for objective insomnia (N?=?1042), and the New Zealand blood donor study (N?=?22,389) reported a 45 % prevalence of insomnia symptoms on at least one occasion
per week 33]. A Canadian study (N?=?2000) identified 40 % of individuals reporting 1 or more insomnia symptoms???3
times per week and a low incidence of help-seeking, reflecting an under-treatment
of sleep disruption 34]. Along these lines, encouraging results have been associated with the behavioural
treatment of sleep disorders, like insomnia, where cognitive behavioral approaches,
in randomized controlled trials, demonstrate efficacy 35], raising the question of how such treatments could be applied to nocturnal hypertension.
Caution must be taken, however, as the behavioural methods effectively applied in
reducing blood pressure, diurnally, have not necessarily affected nocturnal pressures,
suggesting specific tailoring for influencing autonomic arousal during sleep may be
needed 36].

Although our study introduced novel findings, we must acknowledge limitations that
indicate interpretive cautions. The sample was assessed while undergoing a Cardiac
Rehabilitation Program, which bases referrals on recent cardiac events resulting in
hospitalization. Accordingly, this group may represent a sample of patients more impaired
with respect to cardiac function, compared to samples being followed by primary care
physicians. Furthermore, the 24-h ambulatory monitoring assessment was voluntary and
thus patients less able to tolerate ABPM were self-selected for exclusion. An additional
point is that patients tolerate 24-h ABPM differently and thus some apparent nocturnal
BP readings may be related to the variability of reactions to nighttime assessment.
Until multiple serial ABPMs are undertaken, it is difficult to exclude the possibility
that when patients better adjust to ambulatory assessments, their nocturnal BP reading
may be reduced.