Mouth opening/breathing is common in sleep apnea and linked to more nocturnal water loss.

Background: Mouth opening/breathing during sleep is common in patients with obstructive sleep apnea (OSA), which is probably associated with more water loss and higher risk for nocturnal ischemic heart attack. This study aimed to evaluate nocturnal changes in hematocrit/hemoglobin levels and estimated plasma volume loss in OSA patients and its relation to their OSA severity and mouth open/breathing.

Methods: Sixty OSA patients and fifteen healthy controls were enrolled and underwent overnight polysomnography. Mouth status was evaluated via an infrared camera and nasal/mouth airflow. Hematocrit and hemoglobin levels in peripheral venous blood were measured before and after sleep to estimate the change of plasma volume.

Results: Compared to controls, OSA patients had a greater nocturnal increase in hematocrit (1.35% vs. 1.0%, p = 0.013), hemoglobin (0.50% vs. 0.30%, p = 0.002) and more estimated water loss (5.5% vs 3.7% of plasma volume, p < 0.013). The extent of increase was correlated to apnea-hypopnea index (AHI)_the marker of OSA severity (Spearman's ρ = 0.332, p = 0.004; ρ = 0.367, p = 0.001 for hematocrit, hemoglobin, respectively), which remained significant after serial multivariate adjustment. OSA patients had more sleep time with mouth open (96.7% vs 26.7% of total sleep time, p < 0.001) and time with complete mouth breathing (14.1% vs 2.7%, p < 0.001). The extent of mouth breathing was correlated to AHI (ρ=0.487, p < 0.001), nocturnal increase in hematocrit/hemoglobin levels (ρ = 0.236, p = 0.042; ρ = 0.304, p = 0.008, respectively) and estimated plasma volume loss (ρ = 0.262, p = 0.023).

Conclusion: OSA patients had a greater increase in hematocrit/hemoglobin levels after sleep, which is probably linked to more water loss and more sleep time with mouth open/breathing.