Effect of Continuous Positive Airway Pressure on Blood Pressure

Methods

Patients were located by public advertisements and word-of-mouth referral. All patients signed written informed consent approved by the University of California at San Diego Institutional Review Board. Patients were eligible if they were between the ages of 30 and 65 years and were 100% to 170% of ideal body weight as determined by Metropolitan norms.²⁰ Patients were ineligible if they had any major ongoing illness other than sleep apnea and hypertension.

Patients receiving antihypertensive medication had their medication slowly tapered and their BP status confirmed after a 3-week washout. Patients’ BP was screened repeatedly (3 times on 2 occasions) after they had been seated resting for at least 5 minutes. Individuals whose BP was <140/90 mm Hg were considered normotensive. Patients whose BP was >140/90 but <180/110 mm Hg were considered hypertensive and eligible to participate in this protocol.

Sleep was screened at home with a Nightwatch system, and individuals whose respiratory disturbance index (RDI) at home was >20 were considered provisionally to have OSA.

Patients wore a Spacelabs ambulatory blood pressure monitor (model 90207) for a 24-hour period on 3 occasions: before CPAP/placebo CPAP randomization, after 1 day of treatment, and after 7 days of treatment (indicated on graphs as day 0, day 1, and day 7). The cuff was positioned carefully with a “Y” adaptor, and placement was validated against a mercury sphygmomanometer. The cuff position was outlined in ink on the patient’s arm so that the cuff could be repositioned easily if it shifted over the course of the day. BP was programmed to be taken every 15 minutes between 6 am and 10 pm and every 30 minutes between 10 pm and 6 am. Standard criteria for artifact rejection were used in evaluating the BP measures.

Patients were admitted to the University of California at San Diego Clinical Research Center for a confirmatory overnight polysomnography (Nihon Koden, model 4412p). We recorded the following parameters: central and occipital electroencephalogram, bilateral electro-oculogram, submental and tibialis anterior electromyogram, ECG, nasal/oral airflow using thermistor, and respiratory effort using chest and abdominal inductance belts. Oxyhemoglobin saturation was recorded with a pulse oximeter (Biox 3740). Individuals whose RDI was >15 were considered to have OSA. On the next night, patients were randomized to receive either CPAP or placebo CPAP. Patients receiving CPAP underwent standard CPAP titration with the use of a Devilbis CPAP machine and a comfortably fitting mask. Pressure in the mask started at 2 cm H₂O and was increased over the night by 2-cm H₂O increments until apneic episodes were obliterated or until a pressure of 8 to 10 cm was reached. Further pressure titration was then done in increments of 1 cm H₂O on the basis of the presence of apnea, hypopnea, or snoring associated with arousals. The titration was considered ended when most respiratory events were controlled with CPAP while the patient was in the supine position and in the second or third rapid eye movement sleep period or until a pressure of 20 cm had been reached. All patients had their apnea treated within this pressure range.

Individuals randomized to receive placebo CPAP underwent a mock titration night. They wore a special mask that had 5 one-quarter-inch drill holes to create a large leak through the mask. Their CPAP pressure was set at 2 cm H₂O and was not advanced. All CPAP units had a hidden compliance clock that allowed measurement of the amount of time the machine was switched on.

One day after CPAP or placebo CPAP treatment was initiated, patients completed a second day of ABPM. They were then discharged from the hospital for 1 week of continuing CPAP or placebo treatment at home. The research staff was in frequent contact with patients to answer questions about mask placement and to encourage compliance with the treatment. After 7 days of home treatment, patients’ ambulatory BP was again studied with the use of the Spacelabs ambulatory monitor.

Data were analyzed with repeated-measures ANOVA with the use of SPSS software. If a significant effect was found for time alone, it would imply that CPAP did not have a specific effect on BP. If there was a significant interaction of time and treatment, it would imply that the group receiving a particular treatment (presumably CPAP) had a differential response to that treatment over time.

Results

Table 2⇓ summarizes the sample characteristics. Thirty-nine individuals were studied. Individuals randomized to the 2 treatments were comparable in age, pretreatment RDI, and screened BP; however, patients randomized to receive CPAP were heavier (P<0.05). The mean mask pressure required among the CPAP group was 10.1 cm H₂O; placebo CPAP patients all received CPAP at a pressure of 2 cm H₂O administered through a mask with numerous air holes to produce a large air leak. Both patient groups complied equivalently with the CPAP treatment over the 1-week interval of home treatment (>5 hours per night for each group). Patients receiving CPAP demonstrated a significantly greater drop in RDI than was observed in patients assigned to the placebo CPAP group (time×CPAP interaction, P=0.001) (Table 2⇓).

ABPM revealed a main effect for time on daytime mean arterial pressure (F=7.96, df=2, P=0.001) (Figure 1⇓). The daytime BP of both groups declined equivalently over the 1-week trial (ie, there was no effect specific to the CPAP group per se). At nighttime, there was a time-by-group interaction such that the BP of patients receiving CPAP fell considerably more over time than the BP of patients assigned to placebo CPAP (F=3.62, df=2, P=0.032) (Figure 2⇓).

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Figure 1.

Effect of CPAP on daytime mean arterial pressure. Mean arterial pressure was measured during daytime with ABPM before treatment (day 0), after 1 day, and after 7 days of treatment with either CPAP or placebo CPAP. There was a nonspecific time effect for lowering BP across both treatments (P=0.001).

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Figure 2.

Effect of CPAP on nocturnal mean arterial pressure. Mean arterial pressure was measured during nighttime with ABPM before treatment (day 0), after 1 day, and after 7 days of treatment with either CPAP or placebo CPAP. CPAP leads to a specific time-by-treatment effect on blood pressure (P=0.032).

Discussion

The treatment of sleep apnea is rapidly evolving. Medications have had limited value compared with weight loss, various surgical procedures, and CPAP.²¹ For the vast majority of patients, CPAP is an effective treatment of the apnea per se if the patient complies with the treatment. Our results suggest, however, that the effects of CPAP on BP are not so straightforward.

Had we not administered a placebo version of CPAP, we would have concluded that CPAP has a robust BP-lowering effect. That conclusion is not strongly supported by our data. Whereas CPAP lowered daytime mean arterial pressure, so did placebo CPAP. There is very little acclimatization effect to wearing an ABPM cuff²² ; that is, BP does not noticeably fall on a second 24-hour ABPM study. We interpret the uniform decrease of BP in our study to a nonspecific effect of treatment, ie, a placebo effect. Patients were observed closely by our research staff, and all patients used impressive-looking CPAP machinery while sleeping. The combination of professional attention and expectations from the machinery may well have lowered daytime BP. In this sense, the placebo effects of CPAP on BP join a long list of other treatments that have beneficial although nonspecific effects.

However, it is not accurate to attribute all of the effects of CPAP on BP to a placebo effect. We did observe a differential effect of CPAP on nocturnal BP. In the absence of the waking conscious awareness of the treatment (in which treatment had a beneficial effect on both groups), only the patients receiving effective CPAP lowered their nighttime BP.

We specifically examined weight and hypertension to assess whether our findings would materially change after controlling for these variables. Because of the difference in weight between the treatment and placebo groups, we repeated the analyses using body mass index as a covariate. The study still failed to reveal a specific time-by-treatment effect on BP lowering after controlling for BMI. We also reanalyzed the data using screening blood pressure (hypertension versus normotension) as a grouping factor to determine whether hypertension affected the response of BP to CPAP. We found no evidence to suggest that CPAP had a greater BP-lowering effect in hypertensive patients.

The study has a number of limitations. We examined CPAP effects for only a 1-week interval because we were uneasy about imposing a lengthy placebo CPAP treatment on patients who had OSA. It is possible that the BP-lowering effects of CPAP become even more apparent over a longer time interval; indeed, Figure 1⇑ suggests this, but we have no data beyond a 1-week trial. Our patients all had substantial levels of apnea (average RDI was 48.1), but in other ways they were relatively healthy. We excluded patients with other major illnesses requiring treatment, such as those with congestive heart failure, a history of myocardial infarction, or morbid obesity. Perhaps such latter patients may obtain substantially greater benefits with CPAP treatment.

Nonetheless, our study provides a cautionary note, demonstrating the value and importance of a placebo arm in medical research. Even though CPAP is effective in treating apnea, whether it benefits daytime BP is not clear.