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. 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. Expiration of air happens passively secondary to the build-up of pressure in the alveoli that escapes into the less pressurized conductive airways. Historically, the 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. The overwhelming polio epidemic of Copenhagen in 1952 and insufficient availability of “iron lungs” led to the implementation of PPV into clinical practice.
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). Each form of ventilation has its own benefits, risks, indications, and contraindications. NIPPV can be used in acute hypercapnic respiratory failure so long as the patient’s condition is responsive to this form of therapy. Conditions that respond the most to NIPPV include exacerbations of chronic obstructive pulmonary disease (COPD) and acute cardiogenic pulmonary edema. 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.
A 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. The upper airway is the first part of the respiratory system that has an important role in conducting air towards the lungs. It begins with the nasal cavity and continues onto the nasopharynx and oropharynx, down to the larynx and trachea. The mucosa, cartilages, neural, and lymphatic system of upper airways affect the flow and physical qualities of the air that is inhaled. Likewise, they have an impact on the delivery of positive pressure ventilation. 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).
The lower airway continues down below the vocal folds to trachea to form the right and left mainstem bronchi, that further divide into smaller segmental bronchi. These subdivide into even smaller bronchioles that eventually end with the terminal portion of the airway, the alveoli, in which gas exchange occurs.
When endotracheal intubation is performed, the tube is positioned in the trachea with the cuff inflated below the vocal cords.
In the most technical sense, a breath during PPV can be characterized by changes in pressure, volume, and flow during inspiration and expiration. These principles can be mathematically defined by a simple equation:
Paw = P1 + (R + flow) + (Vt + Ee)
Where Paw = airway pressure, P1=initial alveolar pressure, R = resistance to flow, Vt = tidal volume, Ee = inherent elastance of the pulmonary system.
P1 is the alveolar pressure at the beginning of inspiration. In an individual that is not mechanically ventilated, this number can be atmospheric pressure, or in the case of PPV, greater than atmospheric pressure. 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. 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 accumulation of air and pressure eventually lead to barotrauma and hypotension secondary to increased intrathoracic pressure and reduction in preload to the heart. Ventilation can become more difficult.
R represents the resistance to airflow. In mathematical terms, it is defined by the difference between peak pressure and plateau pressure divided by flow. Peak pressure is the maximum pressure measured at end inspiration and is affected by airflow in the circuit. Any airway resistance down to the lungs (e.g., mucus plugging, endotracheal tube kink, endotracheal tube diameter, bronchospasm, etc.) can affect the peak pressure. Plateau pressure represents alveolar pressure and is represented by the compliance of the lungs and can only be measured during a breath-holding maneuver during volume-controlled ventilation on a ventilator. Factors that affect the compliance of the lungs, such as acute respiratory distress syndrome (ARDS), pneumothorax, pneumonia, etc. can affect the plateau pressure.
Ee is the elastance of the pulmonary system. Stated another way it is the inverse compliance or 1/C of the lungs.
The indications to initiate positive pressure ventilation and more specifically, mechanical ventilation, are many, but this article will focus on some of the major indications:
Contraindications to NIPPV include:
Contraindications to invasive positive pressure ventilation include:
NIPPV is administered most commonly by a CPAP (continuous positive airway pressure) or BiPAP (bilevel positive airway pressure) machine. There are other less conventional modes for NIPPV, especially in pediatrics, but this is out of the scope of this article. A CPAP machine usually consists of a pump with a tube that is attached to a mask that goes over the patient’s face. A CPAP machine essentially delivers constant PEEP (normally adjusted between 5-12 cm H2O during sleep and is useful for at-home treatment of obstructive sleep apnea (OSA). By providing constant positive pressure to the airway, it splints open its upper and lower portions, preventing the collapse of tissues that may occur at or after exhalation.
A BiPAP machine is the most commonly used noninvasive form of NIPPV. The machine works similarly to a CPAP machine in that it delivers a constant level of PEEP (usually 3 to 12 mm Hg), but it also provides positive inspiratory pressure (usually 5 to 25 mm Hg) when the patient initiates a breath. A mask that goes over the patient’s face is also required, and a tight seal must be formed. Otherwise, the positive pressure being delivered can escape from around the mask, thereby lowering the inspiratory pressure and the effectiveness of the therapy. BiPAP provides additional inspiratory support, helping the patient to actively inhale. It is often used to treat sleep apnea, asthma exacerbations, and chronic obstructive pulmonary disease.
In order to initiate invasive positive pressure ventilation, a patient must either be intubated with an endotracheal tube or ventilated through a tracheostomy tube. Once an artificial airway is acquired, a circuit is connected to a ventilator machine, and the patient’s work of breathing is either supported (by a mode of ventilation called pressure support ventilation) or completely taken over. There are about 47 different mode names and multiple ventilator models that are discussed in separate articles. However, all of these ventilators work on the same principles in that they provide oxygenation and ventilation to a patient by insufflating air or other gas mixture into the lungs.
The application of positive pressure ventilation, and more specifically, mechanical ventilation requires a trained group of healthcare providers open to communication that have a fundamental understanding of ventilators and positive pressure devices. The therapeutic index for mechanical ventilation is very narrow, as multiple studies have elucidated the impact of tidal volumes and high airway pressures on ventilator-induced lung injury. Thus, an interprofessional group of healthcare professionals is required to take care of critically ill patients who require ventilator support. This may include, but is not limited to, anesthesiologists, intensivists, emergency department physicians, critical care nurses, respiratory therapists, pulmonologists, and others.
Any changes made to the settings of a ventilator or BiPAP machine need to be clearly communicated between the physician, respiratory therapist, and nurse so that the patient is not harmed by improper use of the machine. Alarms that sound on the ventilator should not be ignored or silenced without first paying attention to its underlying cause.
Ventilators providing both non-invasive and invasive positive pressure ventilation are complex mechanisms with numerous sensors, valves, drives, and conducting tubing in need of constant maintenance and testing prior to connecting it to a new patient. Most modern ventilators have the capacity to self-test.
NIPPV is generally provided via a face mask, with a tight seal created over the nose and mouth. Occasionally, however, a smaller mask can be utilized that covers only the nose. Patients using CPAP due to OSA related nocturnal hypoxemia on a nightly basis may prefer nasal prongs. The positive pressure of gas delivered through these provides a pneumatic splint that keeps the upper airway open. Every NIPPV machine is specific in terms of operation, yet as a rule, the patient controls the CPAP or BiPAP levels.
IPPV requires a skilled medical provider to obtain an artificial airway (most commonly, to perform endotracheal intubation). Successful intubation, with rare exceptions, requires patients to be deeply sedated and often pharmacologically paralyzed. Once the endotracheal tube is in place, it’s cuff is inflated, effectively sealing the tube inside the trachea. The ETT is then connected to a ventilator, and ventilation mode is adjusted by the medical provider.
The complications associated with positive pressure ventilation are many and can be quite serious. This is why a thorough understanding of its use is required by an interprofessional group of medical providers. This article will list some of the most recognized complications and explain how or why they occur. Most complications will focus specifically on classic mechanical ventilation as its exact nature is more invasive, and it is important to understand its consequences. However, although rarer, complications can and do occur, with non-invasive positive pressure ventilation.
Ventilator-associated Lung Injury and Barotrauma
Barotrauma occurs when there is alveolar damage due to high pressures entering the lungs. Specifically, the transalveolar pressure, which is the difference in pressure between the alveolus and the surrounding interstitial space, is increased to such an extent that the epithelial lining of the alveoli is damaged. With repeated breaths and inappropriate ventilation, the damage often occurs on a microscopic level until it is severe enough to cause an overt pneumothorax, subcutaneous emphysema, or pneumomediastinum, which are all conditions associated with high mortality rates.
Excessively high tidal volumes or inspiratory pressures are some of the underlying culprits in the cause of barotrauma. One of the landmark studies on acute respiratory distress syndrome, known as ARDSNet, demonstrated that using lung-protective strategies with lower tidal volumes (between 4-8 mm Hg/kg based on ideal body weight) have improved mortality benefits and are associated with more ventilator-free days for patients. Additional studies have examined lung protective strategies for patients not suffering from acute lung injury and have also found reduced morbidity and mortality benefits, although additional studies are needed for conclusive evidence.
Positive pressure ventilation results in physiologic alterations in intrathoracic pressure and cardiac output. One of the most common hemodynamic effects from positive pressure ventilation is a reduction in preload to the heart secondary to increased intrathoracic pressure that impressive the inferior vena cava and right atrium. This effect is more prominent in individuals that may already have reductions in intravascular volume. Likewise, changes in intrathoracic pressure or cardiac output can precipitate an arrhythmia or myocardial ischemia.
Even though most arrhythmias are related to structural changes in the heart, it is common for them to occur secondary to positive pressure ventilation. This is because ventilated patients often have respiratory and electrolyte derangements, including hypoxemia, hypercapnia, acidemia, alkalemia, hypokalemia, hypomagnesemia, or hypocalcemia. Serial arterial blood gasses are often obtained to identify and correct underlying derangements and prevent arrhythmias. Likewise, mechanical ventilation and critical illness lead to hormonal and catecholamine changes in the body that can precipitate myocardial ischemia. This process can occur at any time while a patient requires mechanical ventilation but is also likely during weaning trials when myocardial oxygen demand is increased, and a patient’s sedation is decreased or turned off to assess their ability to come off the ventilator.
The most common infection among mechanically ventilated patients receiving positive pressure ventilation is ventilator-associated pneumonia (VAP). The mortality rate is high and can range between 20% to 50%. An endotracheal tube serves the purpose of protecting a patient’s airway from aspiration and providing a path for the flow of air between the patient and ventilator. However, it also serves as a conduit between the world and the normally sterile lower respiratory tract. In the setting of critical illness and reduced immune system function, bacteria often invade and colonize the lower respiratory tract of patients causing VAP.
Common pathogens include gram-negative bacteria such as Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, and Acinetobacter species, and gram-positive bacteria such as Staphylococcus aureus. Powerful broad-spectrum antibiotics exist to counteract these organisms, but antibiotic resistance among bacteria is a growing concern in intensive care units across the globe. Thus, it is important to be vigilant with patients that require mechanical ventilation and to frequently follow up on complete blood counts, bacterial cultures, and chest radiographs to identify any source of infection.
Critically ill patients that require positive pressure ventilation or mechanical ventilation are often hypoxemic and require high levels of inspired oxygen (FIO2). However, not all conditions that require a patient to be mechanically ventilated require high levels of oxygen therapy. In fact, high levels of oxygen can be toxic to the human body. Areas of the lungs with ventilation-perfusion mismatches under high FIO2 can actually lead to a phenomenon known as re-absorption atelectasis that will further aggravate any ventilation-perfusion mismatches. High oxygen levels are also associated with increased vasoconstrictive effects on coronary circulation and increased mortality from cardiac causes.
Patients that require mechanical ventilation often need sedation due to the stimulating nature of the endotracheal tube. Likewise, the act of being on a ventilator can be a traumatic experience. Some patients that are in desynchrony with the ventilator, colloquially known as “fighting the vent,” may require paralytic medications such as rocuronium, vecuronium, or cisatracurium so that they can be safely ventilated to treat their underlying condition. However, important neurochemical changes occur within patients’ bodies, such as the depletion of bioenergetic neuron reserves, altered sodium channel activation, and increased inflammatory cytokines, which lead to changes such as atrophy of the diaphragm or critical illness myopathy.
These changes make it difficult to wean patients off the ventilator and may require the patient to undergo a separate surgical procedure known as a tracheostomy to create an opening in the neck to place a tracheostomy tube for further ventilatory support based on the patient’s underlying critical illness. These patients often require prolonged mechanical ventilation.
Miscellaneous Injuries Related to Non-invasive Ventilation
Non-invasive ventilation, by its very nature, is associated with fewer risks than invasive mechanical ventilation, but not all patients can benefit from its use. Nevertheless, it is important to be aware of some of the common complications. Facial ulcers and lacerations occur in approximately 13% of patients.
A tight seal is required to properly ventilate patients, but it is important to be aware that the seal is not too tight as to cause injury to the face. Other issues that arise include eye irritation, dry airway passages, and gastric distention. Thus, if there is any concern for aspiration, it is safer to intubate a patient to protect their airway and to avoid gastric distention.
Modern medicine has made significant strides in the treatment of complex cardiovascular and respiratory illnesses. With the advent of ventilators in the past century, and their continued development and improvement, remarkable progress has been made in treating patients that normally would have succumbed to their illness.
Some disease processes would most certainly lead to death if it were not for the ability to ventilate and oxygenate a patient. However, the therapeutic window for providing positive pressure ventilation is very narrow, and a thorough understanding is required of its use in order to garner its benefits. An interprofessional team of healthcare providers is required to take take care of critically ill patients who require positive pressure ventilation.
Open and frequent communication is a must between providers, in addition to enhanced vigilance for these patients, as respiratory derangements can quickly deteriorate a patient’s condition. The initiation of positive pressure ventilation requires a strong understanding of the indications, contraindications, equipment, and most certainly, the complications related to its use so that effective and safe care is provided to patients.
The application of positive pressure ventilation most often occurs in intensive care units where patients require a higher acuity of care. Thus, some degree of critical care training and understanding of the physiologic principles behind NIPPV delivery systems and mechanical ventilation are required to take care of patients who require such treatment modalities adequately.
The initial introduction to the ICU and the use of PPV is an overwhelming experience for many junior providers. Mechanical ventilation is an essential topic in the world of medicine, but rarely any information exists as to how its taught among medical and nursing schools. Furthermore, about half of the trainees are dissatisfied with the level of education they receive on mechanical ventilation. The use of positive pressure ventilation requires a team of healthcare providers, and intensive care nurses are essential in taking care of patients that are mechanically ventilated. Often times, they are the first person to notice an acute problem. Yet, limited continuing education programs exist to solidify existing knowledge on PPV.
Thus, some level of standardized training should exist among medical curriculum for blossoming medical professionals both in the pre-clinical and clinical years that move beyond basic multiple-choice questions. These healthcare providers should be introduced directly to PPV systems early in their training and be taught to identify and understand basic controls in addition to their physiology education. Continued medical education should continue into the postgraduate years as studies have demonstrated that healthcare providers feel that they receive inadequate continued training and have some degree of discomfort in managing these systems. Boot camp curriculums that span over several days have been implemented and studied, which have demonstrated increased provider competency and confidence.
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