In mathematical terms, it is defined by the difference between peak pressure and plateau pressure divided by flow. This accumulation of air and pressure eventually lead to barotrauma and hypotension secondary to increased intrathoracic pressure and reduction in preload to the heart. This air-trapping can lead to a phenomenon known as auto-PEEP, or auto-positive end-expiratory pressure, in which the intra-alveolar pressure is higher at the end of expiration than a pre-determined PEEP amount. This is an important concept because, in patients with any form of airway obstruction (e.g., COPD or status asthmaticus), there may be insufficient time for the air to be exhaled.
In an individual that is not mechanically ventilated, this number can be atmospheric pressure, or in the case of PPV, greater than atmospheric pressure. P1 is the alveolar pressure at the beginning of inspiration. Where Paw = airway pressure, P1=initial alveolar pressure, R = resistance to flow, Vt = tidal volume, Ee = inherent elastance of the pulmonary system. These principles can be mathematically defined by a simple equation: In the most technical sense, a breath during PPV can be characterized by changes in pressure, volume, and flow during inspiration and expiration. When endotracheal intubation is performed, the tube is positioned in the trachea with the cuff inflated below the vocal cords. These subdivide into even smaller bronchioles that eventually end with the terminal portion of the airway, the alveoli, in which gas exchange occurs. The lower airway continues down below the vocal folds to the trachea to form the right and left mainstem bronchi, which further divide into smaller segmental bronchi. It may be difficult to administer PPV via mask ventilation in patients with congenital facial defects, defects secondary to trauma, or in obese patients with excess soft tissue who often suffer from obstructive sleep apnea (OSA). Likewise, they have an impact on the delivery of positive pressure ventilation. The mucosa, cartilages, neural, and lymphatic system of upper airways affect the flow and physical qualities of the air that is inhaled. It begins with the nasal cavity and continues onto the nasopharynx and oropharynx, down to the larynx and trachea. The upper airway is the first part of the respiratory system that has an important role in conducting air towards the lungs. However, the need for emergent endotracheal intubation is a contraindication to NIPPV and may be required for conditions such as cardiac arrest, hemodynamic instability, the inability of the patient to cooperate, or maintain an open and protected by reflexes airway, as well as many others to be discussed further.Ī basic understanding of the anatomy of the upper and lower airway is essential to comprehend respiratory physiology and the interplay between inspiratory and expiratory forces that work together to form a breath. Conditions that respond the most to NIPPV include exacerbations of chronic obstructive pulmonary disease (COPD) and acute cardiogenic pulmonary edema. NIPPV can be used in acute hypercapnic respiratory failure so long as the patient’s condition is responsive to this form of therapy.
Each form of ventilation has its own benefits, risks, indications, and contraindications. Positive pressure ventilation can be delivered in two forms: non-invasive positive pressure ventilation (NIPPV), which is delivered through a special face mask with a tight seal (air travels through anatomical airways), or invasive positive pressure ventilation (IPPV), which involves the delivery of positive pressure to the lungs through an endotracheal tube or tracheostomy (or any other device that delivers gas bypassing parts of the anatomical airway). The overwhelming polio epidemic of Copenhagen in 1952 and insufficient availability of “iron lungs” led to the implementation of PPV into clinical practice.
Historically, artificial lung ventilation based on the opposite to PPV physical principle – the negative pressure ventilation – was used widely to treat acute respiratory failure in poliomyelitis patients via hermetically sealing the patient’s torso inside the vacuum-producing “iron lung” machine. Expiration of air happens passively secondary to the build-up of pressure in the alveoli that escapes into the less pressurized conductive airways. As gas enters the lungs, the interalveolar pressure increases until a change in flow or pressure are detected by the machine delivering the mixture, or the set volume of gas was delivered to signal the end of a breath.
Positive pressure ventilation is a form of respiratory therapy that involves the delivery of air or a mixture of oxygen combined with other gases by positive pressure into the lungs.
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