Rochester, MN- Patients with nocturnal onset of MI have a high likelihood of having obstructive sleep apnea, which may be a trigger for MI, a new study suggests [1].

Senior author Dr Virend Somers (Mayo Clinic, Rochester, MN) commented to heartwire: "A number of observational studies have implicated sleep apnea as being associated with an increased risk of MI and CHF, but whether sleep apnea is a definitive cause of MI we still need to prove. We believe that it is sleep apnea that is causing these patients to have an MI at night, but whether it is just changing the timing of an MI that is going to happen anyway or is fundamentally increasing the risk of MI independent of other factors, we cannot yet say for sure. But I would advise that patients who have suffered an MI at night be investigated for sleep apnea, and if they are found to have the condition, that it be treated, and this may lower their risk of having another MI in the future."

The current study, published in the July 29, 2008 issue of the Journal of the American College of Cardiology (available online on July 21), was conducted by a team led by Dr Fatima Sert Kuniyoshi (Mayo Clinic).

They explain that obstructive sleep apnea is an increasingly prevalent condition that remains underdiagnosed and undertreated. They note that in the general population, MI and sudden cardiac death occur most frequently between 6 AM and noon. As obstructive sleep apnea causes hypoxemia, sympathetic activation, and surges in blood pressure, they suggest that this may lead to plaque rupture, coronary thrombosis, and MI, and they thus conducted the current study to investigate whether patients with sleep apnea are more likely to have a nocturnal MI.

The researchers prospectively studied 92 patients admitted with incident MI for which the time of onset of chest pain was clearly identified. The presence of obstructive sleep apnea was determined by overnight polysomnography. Results showed that patients with and without sleep apnea had similar prevalence of comorbidities. MI occurred between midnight and 6 AM in 32% of sleep-apnea patients and 7% of non-sleep-apnea patients (p=0.01). The odds of having sleep apnea in those patients whose MI occurred between midnight and 6 AM was sixfold higher than those whose MI occurred in the remaining 18 hours of the day. Of all patients having an MI between midnight and 6 AM, 91% had sleep apnea.

Sert Kuniyoshi et al say their data suggest that obstructive sleep apnea may be a trigger for MI, with a striking reversal in the expected diurnal timing of MI onset. Conversely, non-sleep-apnea patients had a nadir of MI onset at night and a peak in the morning, similar to the diurnal distribution of MI seen in the general population. "Our findings identify obstructive sleep apnea as the first disease condition recognized to actually reverse the usual day-night variation in the incidence of MI," they write. "Our findings suggest that the pathophysiology of obstructive sleep apnea leads to an increased risk of MI during the night," they add.

Somers commented to heartwire: "What is very consistent in all studies is that patients with sleep apnea experience significant physiologic stress at night—low oxygen levels activate the sympathetic nervous system, which thus causes a significant rise in blood pressure, and the patient is therefore exposed to a significant level of simultaneous problems. In addition, patients with sleep apnea show nighttime changes on their ECG that suggest the heart is not getting enough blood. This is what made us look at the possibility that these people may be having MIs at night. I would say that it is likely that sleep apnea could be causing these nighttime MIs, but we have not proven that it actually is the cause. But I would still advise anyone who has had an MI during the night to get checked out for sleep apnea." He added: "The biggest drawback is that there are not enough data out there at present. We are trying to identify the mechanisms of MI at night—to find out exactly what happens to these patients. But we also need more studies to investigate whether sleep apnea therapy such as continuous positive airway pressure can prevent nocturnal cardiac events."