Pressure breathing, also known as positive pressure ventilation, is a mechanical method that applies air pressure above the surrounding atmospheric pressure to the respiratory system. This technique deviates from normal, spontaneous breathing, which relies on the body creating a negative pressure inside the chest cavity to draw air in. Pressure breathing forces air into the lungs, serving as a powerful tool to overcome environmental or physiological limitations that compromise the body’s ability to take in sufficient oxygen.
The Physiological Need for External Pressure
The human body’s natural breathing mechanism operates effectively only within a certain range of external conditions. A major threat is hypoxia, a lack of adequate oxygen supply at the tissue level, which becomes a severe risk at high altitudes. Although the percentage of oxygen remains constant at about 21%, the total atmospheric pressure drops significantly as altitude increases. This decrease in total pressure means the partial pressure of oxygen also falls, reducing the driving force for oxygen to move from the lungs into the bloodstream.
At extreme altitudes, this reduced partial pressure leads to dangerously low oxygen levels in the alveoli, the tiny air sacs where gas exchange occurs. Beyond environmental challenges, physiological issues like respiratory muscle failure or airway collapse also necessitate external pressure. For instance, conditions that lead to the closing of small airways or the collapse of alveoli, known as atelectasis, compromise the surface area available for gas exchange. In these scenarios, the body’s own muscular effort is insufficient to maintain open airways or generate the necessary pressure gradient for ventilation.
How Positive Pressure Ventilation Works
Pressure breathing fundamentally reverses the natural respiratory process by applying positive pressure to the airways. Normal inspiration is a muscular process that expands the chest cavity, creating a negative pressure that passively draws air in. Positive pressure ventilation uses a machine to actively push air and oxygen into the lungs, increasing the pressure inside the alveoli above the outside pressure. This forced inflation achieves two primary physiological benefits: addressing hypoxia and preventing airway collapse.
Pneumatic Splinting
The positive pressure acts as a pneumatic splint, physically holding open smaller airways and recruiting collapsed alveoli. This action increases the total surface area available for gas exchange, improving lung compliance.
Increasing Partial Pressure
By increasing the pressure within the alveoli, the technique increases the partial pressure of oxygen at the blood-air barrier. This higher partial pressure gradient promotes a more rapid diffusion of oxygen into the pulmonary capillaries, correcting hypoxemia that occurs when ambient pressure is too low or lung function is compromised. Expiration then occurs passively as the pressure built up in the lungs is released.
Specialized Applications and Scenarios
The need for positive pressure breathing arises in diverse, specialized environments, ranging from extreme altitude to medical care. In high-altitude aviation, particularly for military pilots operating above 40,000 feet, the ambient atmospheric pressure is so low that breathing 100% oxygen is insufficient to prevent severe hypoxia. Specialized pressure breathing systems are integrated into the pilot’s mask, forcing air into the lungs against the low external pressure to maintain consciousness and function.
In the medical field, positive pressure ventilation is a common therapeutic intervention, often delivered non-invasively through a mask. Continuous Positive Airway Pressure (CPAP) devices apply a single, steady pressure throughout the entire breathing cycle, commonly used to treat obstructive sleep apnea by preventing upper airway collapse. Bilevel Positive Airway Pressure (BiPAP) delivers a higher pressure during inhalation and a lower pressure during exhalation, offering additional inspiratory support. These devices are also used in acute respiratory distress, such as in patients with Chronic Obstructive Pulmonary Disease (COPD) flare-ups or acute heart-related lung fluid buildup, to reduce the work of breathing.
Managing the Side Effects of Pressure Breathing
While life-saving, introducing positive pressure into the chest cavity is unphysiologic and can introduce significant side effects that require careful management. One major risk is barotrauma, which is physical damage to the lung tissue caused by excessive pressure. This can lead to the rupture of delicate alveoli, potentially causing air to leak into the space around the lung, a dangerous condition called pneumothorax.
The sustained positive pressure within the chest also has consequences for the cardiovascular system. Since the lungs and heart share the same confined space, the increased pressure impedes the return of venous blood to the heart’s right atrium. This reduced venous return decreases the filling volume of the heart, leading to a drop in cardiac output and potentially hypotension. Clinicians must carefully monitor the patient’s blood pressure and fluid status to counteract this effect. Patients may also experience discomfort or difficulty during exhalation, as they must push air out against the residual positive pressure held in the airways.