Association Between Arterial Oxygen Saturation And Lung Ultrasound B Lines After Competitive Deep Breath Hold Diving
Authors: Alexander Patrician, Frank Pernett and Erika Schagatay
DOI / Source: https://doi.org/10.3389/fphys.2021.711798
Date: 4 August 2021
Reading level: Intermediate
Why This Matters for Freedivers
This paper gives divers and safety teams a cheap, portable tool that can flag possible lung stress after deep dives: delayed or reduced SpO₂ recovery tends to track with ultrasound signs of extra lung fluid. It supports a conservative practice: if your SpO₂ drops (especially below ~95% in the post-dive window) or you have symptoms, treat it as a stop sign—rest, reassess, and don’t rush back to depth.
Synopsis
Freedivers usually think of “lung squeeze” as a dramatic event: coughing, chest tightness, maybe even blood. But this paper shows something more subtle—and more useful for safety. The researchers looked at whether a simple finger pulse oximeter (SpO₂) could help spot divers who might have extra fluid in the lungs after deep competition dives, even when the diver might be tempted to “just do another dive.” To check this, they compared SpO₂ readings to lung ultrasound B-lines, a well-accepted ultrasound sign that extra fluid is present in lung tissue. 
They studied 40 freedivers at an international depth competition (dives across disciplines, roughly 40–122 m, average around the mid-70s). SpO₂ was recorded within about 20 minutes after surfacing (for 2 minutes in 15-second intervals), and lung ultrasound was done later (around 45 minutes post-dive). They also recorded symptoms like cough, chest tightness/irritation, and blood in sputum. The results were clear: more B-lines (more lung fluid) was linked with lower SpO₂, both the minimum and average SpO₂ during the post-dive window. B-lines also increased with greater depth, supporting the idea that deeper dives increase lung stress and fluid leakage risk. 
Most importantly for real-world use, the paper suggests a practical screening rule: if a diver’s minimum SpO₂ stayed at or above ~95% in the early recovery period, that was very reassuring (they report a very high “negative predictive value” for symptoms). If SpO₂ dipped below that, it doesn’t automatically diagnose edema—pulse oximetry isn’t perfect—but it’s a strong signal the diver should pause, recover, and potentially get evaluated rather than returning to deep diving immediately. The whole message is not “use SpO₂ to diagnose,” but “use SpO₂ to decide who needs extra caution.”
Abstract
Breath-hold diving (freediving) is an underwater sport that is associated with elevated hydrostatic pressure, which has a compressive effect on the lungs that can lead to the development of pulmonary edema. Pulmonary edema reduces oxygen uptake and thereby the recovery from the hypoxia developed during freediving, and increases the risk of hypoxic syncope. We aimed to examine the efficacy of SpO , via pulse-oximetry, as a tool to detect pulmonary edema by comparing it to lung ultrasound B-line measurements after deep diving. SpO and B-lines were collected in 40 freedivers participating in an international deep freediving competition. SpO was measured within 17 ± 6 min and lung B-lines using ultrasound within 44 ± 15 min after surfacing. A specific symptoms questionnaire was used during SpO measurements. We found a negative correlation between B-line score and minimum SpO 2 (r = −0.491; p = 0.002) and mean SpO (r = −0.335; p = 0.046). B-line scores were positively correlated with depth (r s= 0.408; p = 0.013), confirming that extra-vascular lung water is increased with deeper dives. Compared to dives that were asymptomatic, symptomatic dives had a 27% greater B-line score, and both a lower mean and minimum SpO2 (all p < 0.05). Indeed, a minimum SpO ≤ 95% after a deep dive has a positive predictive value of 29% and a negative predictive value of 100% regarding symptoms. We concluded that elevated B-line scores are associated with reduced SpO 2 after dives, suggesting that SpO via pulse oximetry could be a useful screening tool to detect increased extra-vascular lung water. The practical application is not to diagnose pulmonary edema based on SpO 2 – as pulse oximetry is inexact – rather, to utilize it as a tool to determine which divers require further evaluation before returning to deep freediving.