Acute apnea and white blood cell count: A biphasic response formal comment on 'Hematologic changes after short term hypoxia in non-elite apnea divers under voluntary dry apnea conditions'.

Acute breath-holding poses a challenge to the human body as it deprives the body from its oxygen supply.In order to prevent the body against hypoxia, a series of physiological responses is elicited, called the diving response.This response is characterized by bradycardia, peripheral vasoconstriction and an increase in mean arterial pressure [1][2][3][4] and accompanied by a spleen contraction releasing red blood cells into circulation [5][6][7].Although the impact of apnea induced splenic contraction on red blood cell related parameters (erythrocyte count, hemoglobin (Hb), hematocrit (Hct), reticulocytes) in the time span up to 20 minutes post apnea has been studied quite extensively, the effect on circulating white blood cells (WBC) [6] and the evolution of these parameters in the following hours is less known.In this context, the study 'Hematologic changes after short term hypoxia in non-elite apnea divers under voluntary dry apnea conditions' provides new insights revealing exhaustive data on all three blood cell lines up to four hours after acute static apnea.Surprisingly however, the results of this study do not show an acute increase in Hct, [Hb] nor erythrocyte count, contradicting a well-established and documented apnea response.Based on experience with blood analysis in our lab, we expect that body position during [8], as well as activity before blood sampling might play a role [9].In this context, it has been shown that walking as little as 50m already significantly increases [Hb] and Hct due to an acute decrease in plasma volume [9].The methods section does not clarify how blood collection was performed nor mentions the activity and standardization in the time span before blood sample collection.It does however state that the apnea was performed in a horizontal position.In this case, the increase in [Hb] might very well be masked by an increase in plasma volume due to the prolonged time spent in a horizontal position while performing the breath-hold protocol.Indeed, changing from a standing to supine position has been shown to lead to an increase of around 418 mL of plasma volume (individually ranging from 149 to 717 mL) [10].Supine body position resulted in lower values, i.e., a decrease of 2.3 and 7.1 g/L for [Hb] and 1.7 and 6.9% for Hct, compared to respective sitting and standing posture [8].These findings emphasize the need for a standardized protocol prior to venous blood sampling in research in order to minimize pre-analytical variation.Therefore, the World Anti-Doping Agency (WADA) uses a very strict blood collection procedure to set up the Athlete Biological Passport [11].First, sampling should not occur within two hours after training or

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