Resistant/Refractory Hypertension and Sleep Apnoea: Current Knowledge and Future Challenges

Hypertension is one of the most frequent cardiovascular risk factors. The population of hypertensive patients includes some phenotypes whose blood pressure levels are particularly difficult to control, thus putting them at greater cardiovascular risk. This is especially true of so-called resistant hypertension (RH) and refractory hypertension (RfH). Recent findings suggest that the former may be due to an alteration in the renin-angiotensin-aldosterone axis, while the latter seems to be more closely related to sympathetic hyper-activation. Both these pathophysiological mechanisms are also activated in patients with obstructive sleep apnoea (OSA). It is not surprising, therefore, that the prevalence of OSA in RH and RfH patients is very high (as reflected in several studies) and that treatment with continuous positive airway pressure (CPAP) manages to reduce blood pressure levels in a clinically significant way in both these groups of hypertensive patients. It is therefore necessary to incorporate into the multidimensional treatment of patients with RH and RfH (changes in lifestyle, control of obesity and drug treatment) a study of the possible existence of OSA, as this is a potentially treatable disease. There are many questions that remain to be answered, especially regarding the ideal combination of treatment in patients with RH/RfH and OSA (drugs, renal denervation, CPAP treatment) and patients' varying response to CPAP treatment.

Can pulse transit time be useful for detecting hypertension in patients in a sleep unit?

INTRODUCTION

Pulse transit time (PTT) is the time that a pulse wave takes to travel between two different arterial points, and may be useful in estimating blood pressure. This noninvasive technique, which does not add any cost to the procedure, offers the advantage of avoiding 'arousals' during sleep measurement as occurs with ambulatory blood pressure monitoring (ABPM). We aim to confirm the usefulness of PTT for the detection of hypertension, and to study the correlation between both measurements.

METHODS

Prospective observational study in a multidisciplinary sleep unit. We recruited 30consecutive patients attending a sleep clinic and ran a baseline polysomnography followed by an ABPM the following day. Average systolic and diastolic blood pressure (SBP, DBP) by PTT were calculated and compared with ABMP results. In accordance with international guidelines, patients with mean nocturnal ABMP ≥ 120/70 mmHg were diagnosed as having arterial hypertension.

RESULTS

Mean age of 60years; 66% male, 80% suffered from sleep apnoea (OSAS). Taking the ABPM as the reference technique, we found that the diagnostic sensitivity of PTT is 85% with a specificity of 88% in the case of SBP, with a positive predictive value of 85% and negative predictive value of 88%. By studying the relationship between mean SBP measured by ABPM and PTT, we found a linear correlation coefficient (R) of 0.88, showing a distribution of all subjects with a difference of between ±15mmHg between tests. There is also a positive correlation between mean DBP measured for the two tests, with a weaker linear correlation.

CONCLUSIONS

Pulse transit time shows a strong correlation with blood pressure (measured by ABPM). PTT provides continuous, non-invasive, cuffless blood pressure monitoring free of additional cost and could be an alternative for screening hypertension.

Resistant hypertension, obstructive sleep apnoea and aldosterone

Obstructive sleep apnoea (OSA) and hypertension commonly coexist. Observational studies indicate that untreated OSA is strongly associated with an increased risk of prevalent hypertension, whereas prospective studies of normotensive cohorts suggest that OSA may increase the risk of incident hypertension. Randomized evaluations of continuous positive airway pressure (CPAP) indicate an overall modest effect on blood pressure (BP). Determining why OSA is so strongly linked to having hypertension in cross-sectional studies, but yet CPAP therapy has limited BP benefit needs further exploration. The CPAP studies do, however, indicate a wide variation in the BP effects of CPAP, with some patients manifesting a large antihypertensive benefit such that a meaningful BP effect can be anticipated in some individuals. OSA is particularly common in patients with resistant hypertension (RHTN). The reason for this high prevalence of OSA is not fully explained, but data suggest that it may be related to the high occurrence of hyperaldosteronism in patients with RHTN. In patients with RHTN, it has been shown that aldosterone levels correlate with severity of OSA and that blockade of aldosterone reduces the severity of OSA. Overall, these findings are consistent with aldosterone excess contributing to worsening of underlying OSA. We hypothesize that aldosterone excess worsens OSA by promoting accumulation of fluid within the neck, which then contributes to increased upper airway resistance.

Obstructive sleep apnoea and 24-h blood pressure in patients with resistant hypertension

Obstructive sleep apnoea (OSA) is common in patients with resistant hypertension, but understanding of the pathogenic mechanisms linking both conditions is limited. This study assessed the prevalence of OSA and the relationships between OSA and 24-h blood pressure (BP) in 62 consecutive patients with resistant hypertension, defined as clinic BP values ≥ 140/90 despite the prescription of at least three drugs at adequate doses, including a diuretic. In order to exclude a 'white coat effect', only patients with ambulatory 24-h BP values ≥ 125/80 were recruited. Patients underwent polysomnography, 24-h ambulatory BP monitoring and completed the Epworth sleepiness scale (ESS). OSA was defined as an apnoea-hypopnoea index (AHI) ≥ 5 and excessive daytime sleepiness (EDS) by an ESS ≥ 10. A multiple linear regression analysis was used to assess the association of anthropometric data, OSA severity measures and ESS with 24-h systolic and diastolic BP. Mean 24-h BP values were 139.14/80.98 mmHg. Ninety per cent of patients had an AHI ≥ 5 and 70% had an AHI ≥ 30. Only the ESS was associated with 24-h diastolic BP [slope 0.775, 95% confidence interval (CI) 0.120-1.390, P < 0.02); age was associated negatively with 24-h diastolic BP (slope -0.64, 95% CI -0.874 to -0.411, P < 0.001). Compared with those without EDS, patients with EDS showed a significantly higher frequency of diastolic non-dipping pattern (69.2% versus 34.7%, P < 0.032). Our results demonstrate a high prevalence of severe OSA in patients with resistant hypertension and suggest that EDS could be a marker of a pathogenetic mechanism linking OSA and hypertension.

Resistant hypertension and obstructive sleep apnea: the sparring partners

Enhanced target organ damage and cardiovascular morbidity represent common issues observed in both resistant hypertension and obstructive sleep apnea. Common pathophysiological features and risk factors justify their coexistence, especially in individuals with increased upper-body adiposity. Impaired sodium handling, sympathetic activation, accelerated arterial stiffening, and impaired cardiorenal hemodynamics contribute to drug-resistant hypertension development in obstructive sleep apnea. Effective CPAP therapy qualifies as an effective "add-on" to the underlying antihypertensive pharmacological therapy, and emerging evidence underlines the favorable effect of mineralocorticoid antagonists on both resistant hypertension and obstructive sleep apnea treatment.