Positive pressure ventilation (PPV) is a medical intervention that assists or completely takes over natural breathing when a patient’s lungs are failing. This mechanical support forces air into the lungs by applying pressure greater than the atmospheric pressure outside the body. PPV is used when a person is unable to move enough air to sustain life due to an underlying illness or injury. Specialized machines, known as ventilators, deliver a controlled mixture of air and oxygen directly to the patient’s airways. The goal is to sustain the patient through respiratory distress until their body can recover and resume adequate breathing.
Core Physiological Objectives
The primary purpose of positive pressure ventilation is to accomplish two life-sustaining physiological objectives: correcting gas exchange abnormalities and reducing the effort required for breathing. Gas exchange involves ensuring sufficient oxygen enters the bloodstream (oxygenation) and removing carbon dioxide, a waste product of metabolism (ventilation). If carbon dioxide is not effectively removed, it accumulates in the blood, leading to respiratory acidosis. PPV mechanically facilitates these processes, preventing organ damage that would otherwise occur from a lack of oxygen or an excess of acid in the blood.
The second objective is reducing the patient’s work of breathing (WOB), which is the physical effort expended by the respiratory muscles. When a patient experiences respiratory failure, their breathing muscles become severely fatigued from overwork. PPV takes over this mechanical load, allowing exhausted muscles to rest and recover, which is necessary for survival. This reduction in WOB also conserves the body’s energy reserves, which can then be redirected toward fighting the underlying disease or injury.
The Mechanics of Lung Support
Positive pressure ventilation achieves its goals by fundamentally changing the mechanical process of breathing. Normal breathing is a negative pressure process where the diaphragm contracts, creating a vacuum that draws air into the lungs. In contrast, PPV pushes air into the lungs until a set pressure or volume is reached, creating positive pressure within the airways.
This applied pressure helps prevent the tiny air sacs, called alveoli, from collapsing at the end of exhalation (alveolar recruitment). Keeping these alveoli open maximizes the surface area available for gas exchange. Furthermore, positive pressure increases the functional residual capacity (FRC), the volume of air remaining in the lungs after a normal breath out.
Maintaining a higher FRC improves the elasticity of the lungs, referred to as lung compliance. This mechanical improvement means less pressure is required to inflate the lungs with each subsequent breath, reducing stress on the delicate lung tissue. The pressure applied at the end of expiration, known as Positive End-Expiratory Pressure (PEEP), is responsible for maintaining this open lung state and stable FRC.
Clinical Situations Requiring Positive Pressure
Positive pressure ventilation is indicated when a patient’s spontaneous breathing is insufficient to maintain adequate oxygen and carbon dioxide levels. One severe condition requiring PPV is Acute Respiratory Distress Syndrome (ARDS), where inflammation causes fluid to leak into the alveoli, severely limiting gas exchange. PPV is also used for patients experiencing severe exacerbations of pneumonia, where infection and inflammation impair lung function.
Severe asthma exacerbations can cause significant airway obstruction, making PPV necessary to overcome resistance and move air effectively. Post-operative respiratory failure is another common indication, particularly after major surgery when the effects of anesthesia, pain, or muscle weakness impair the ability to take deep breaths. In all these situations, the underlying problem requires mechanical assistance to ensure continuous, life-sustaining breathing.
Non-Invasive Versus Invasive Delivery
Positive pressure ventilation can be delivered through two main methods, distinguished by how the pressurized air reaches the lungs. Non-Invasive Ventilation (NIV) uses a tight-fitting mask over the nose or the nose and mouth to create a seal and deliver the air. Methods like Continuous Positive Airway Pressure (CPAP) and Bilevel Positive Airway Pressure (BiPAP) fall under NIV, offering support without bypassing the patient’s upper airway.
Invasive ventilation, by contrast, requires placing a tube directly into the windpipe, typically an endotracheal tube, which is then connected to the ventilator. This method is necessary for patients with severe respiratory failure, those who are unconscious, or those who cannot protect their airway. While invasive ventilation provides the most reliable and precise control over breathing, NIV is often preferred as a first line of treatment because it avoids the risks associated with intubation, such as infection or airway damage.