Dead Space in Helmets Is a Fair Point, and It Has a Fix
Aurika Savickaite
10/19/2026 Chicago
A new review of non-invasive ventilation, or NIV, after cardiac surgery is a welcome addition to the helmet conversation. Its authors note that helmets are being used more because they protect the face and patients tolerate them better, and they raise a fair and important point: helmets "may increase dead space and require careful monitoring."
That issue deserves an open discussion, and it has a fix. Carbon dioxide (CO2) rebreathing in a helmet usually comes from the wrong setup, not from the helmet itself. With good training and user-friendly equipment, teams can keep the benefits and avoid the risk.
What the review said
The narrative review by Mahamaden A. Kery and Hao Cao of Tongji University in Shanghai appears in the October 2026 issue of The Egyptian Journal of Chest Diseases and Tuberculosis. It looks at oxygen therapy, high-flow nasal cannula and NIV for patients whose breathing fails after extubation following heart surgery.
"The use of helmet interfaces has expanded in recent years because they reduce facial pressure injuries and improve patient tolerance, although they may increase dead space and require careful monitoring," the authors wrote.
Why the helmet has dead space
The helmet is a clear hood that covers the whole head. Nothing touches the face, and a soft neck collar creates the seal. That large inside volume is what makes it comfortable. It is also where exhaled CO2 can build up if not enough fresh gas flows through.
In our online course, CO2 rebreathing is listed next to armpit skin breakdown as one of the two main helmet challenges. Some helmets have an inner inflatable cushion that adds comfort and reduces the internal volume.
The study that explains it
A study by P. Taccone and colleagues at Massachusetts General Hospital, published in Critical Care Medicine in 2004, tested helmet CPAP in a lung model and in eight healthy volunteers.
When CPAP was delivered with a ventilator in CPAP mode, inspired CO2 was high: 12.4 ± 3.2 torr.
When CPAP was delivered with a continuous high-flow system, inspired CO2 was low: 2.5 ± 1.2 torr.
The authors concluded that the helmet "should not be used to deliver CPAP with a ventilator," that continuous high flow minimizes rebreathing, and that minute ventilation and PCO2 should be monitored.
How to prevent CO2 rebreathing
Give enough fresh gas flow. For helmet CPAP in hypoxemic respiratory failure, Dr. John P. Kress of the University of Chicago told us the key is a fresh gas flow of at least 50 liters per minute. His team uses a high-flow nasal cannula setup at 60 liters per minute with a PEEP valve on the expiratory limb.
Never use ventilator CPAP mode with a helmet. If you connect the helmet to a ventilator, use pressure support, as explained in our FAQ.
Don't copy face mask settings. Helmet NIV and helmet CPAP are different therapies, and face mask CPAP or BiPAP settings can lead to rebreathing and NIV failure.
Monitor closely. Watch PCO2, breathing rate and minute ventilation. Our course stresses close monitoring, especially in the first 2 hours of NIV.
Learning is the key to success
The course names three main challenges of NIV: choosing the right patient, the clinician's level of training and the interface. Dead space falls under the last two. When staff learn the setup, the problem becomes manageable.
Equipment matters too. Savickaite's course recommends a simple NIV setup and an easy-to-fit interface so staff can start NIV on time. Kress said the helmet used in the 2016 University of Chicago trial "was not easy to operate." Newer helmets are easier to use, but every team still needs training and clear helmet-specific instructions.
The payoff is real. In that 2016 JAMA trial by Dr. Bhakti Patel and colleagues, 18% of patients in the helmet group were intubated, compared with 62% in the face mask group.
"Dead space is not a reason to give up on the helmet. It's a reason to train. When nurses, respiratory therapists and doctors understand flow, the PEEP valve and the right ventilator mode, CO2 rebreathing stops being a problem and patients get all the comfort the helmet offers," said Aurika Savickaite, MSN, RN, founder of helmetbasedventilation.com.
Limitations
The review is a narrative review, not a systematic review, and it focuses on cardiac surgery patients. It does not give helmet-specific flow numbers. The Taccone study was small and used healthy volunteers, not patients. More research on helmet settings in real patients is needed.
Learn more
Module 4 of our online course, "Helmet NIV Issues and Their Solutions," opens with a lesson on carbon dioxide rebreathing. Module 3 covers titration of PEEP, FiO2 and flow rate.
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Related reading: Helmet NIV Success: Combining Knowledge and Experience, Helmets Used to Reduce Clinicians' Workload and Improve NIV, our FAQ and our Research list.
References
Kery MA, Cao H. Noninvasive ventilation for postextubation acute respiratory failure after cardiac surgery: a review of clinical outcomes. Egypt J Chest Dis Tuberc. 2026;75(4):500-508. doi:10.4103/ecdt.ecdt_123_25
Taccone P, Hess D, Caironi P, Bigatello LM. Continuous positive airway pressure delivered with a "helmet": effects on carbon dioxide rebreathing. Crit Care Med. 2004;32(10):2090-2096. doi:10.1097/01.ccm.0000142577.63316.c0
Patel BK, Wolfe KS, Pohlman AS, Hall JB, Kress JP. Effect of noninvasive ventilation delivered by helmet vs face mask on the rate of endotracheal intubation in patients with acute respiratory distress syndrome. JAMA. 2016;315(22):2435-2441. doi:10.1001/jama.2016.6338
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