Acute effects of apnea bouts on hemoglobin concentration and hematocrit: a systematic review and meta-analysis.

Objective: This study aimed to systematically analyze the existing literature and conduct a meta-analysis on the acute effects of apnea on the hematological response by assessing changes in hemoglobin (Hb) concentration and hematocrit (Hct) values. Methods: Searches in Pubmed, The Cochrane Library, and Web of Science were carried out for studies in which the main intervention was voluntary hypoventilation, and Hb and Hct values were measured. Risk of bias and quality assessments were performed. Results: Nine studies with data from 160 participants were included, involving both subjects experienced in breath-hold sports and physically active subjects unrelated to breath-holding activities. The GRADE scale showed a "high" confidence for Hb concentration, with a mean absolute effect of 0.57 g/dL over control interventions. "Moderate" confidence appeared for Hct, where the mean absolute effect was 2.45% higher over control interventions. Hb concentration increased to a greater extent in the apnea group compared to the control group (MD = 0.57 g/dL [95% CI 0.28, 0.86], Z = 3.81, p = 0.0001) as occurred with Hct (MD = 2.45% [95% CI 0.98, 3.93], Z = 3.26, p = 0.001). Conclusions: Apnea bouts lead to a significant increase in the concentration of Hb and Hct with a high and moderate quality of evidence, respectively. Further trials on apnea and its application to different settings are needed.