Modes of Mechanical Ventilation
Modes of Mechanical Ventilation
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First Things First (assess & treat for the following)
- Ventilation: Ventilation requires the creation of airflow through the upper and lower respiratory tract via the generation of either negative or positive intrathoracic pressure.
- Negative-Pressure Ventilation: can occur through both natural and artificial (mechanical) methods
- Natural (spontaneous) ventilation: the principal mode of ventilation that generate an inspiratory flow of air through the creation of a negative intrathoracic pressure through the contraction of the diaphragm and accessory muscles
- Mechanical (artificial) ventilation (MV): the first widespread application of MV used a tank ventilator ("iron lungs"), which created inspiration through the application of negative pressure to the tank. Other forms of mechanical ventilation were developed (eg, cuirass wrap) but had limited application and are essentially no longer available.
- Positive-Pressure Ventilation: Essentially all current mechanical ventilation approaches (invasive and non-invasive) use positive pressure methods. Some consider mouth-to-mouth resuscitation an exception. There are multiple modes of positive-pressure ventilation and multiple principles that guide the appropriate application of each mode. The goal of the remainder of this section will be to summarize those modes and principles. This topic will also focus only on invasive forms of positive-pressure ventilation (via endotracheal tube or tracheostomy). Non-Invasive Ventilation focuses on non-invasive forms of positive pressure ventilation (CPAP and BiPAP).
- Respiratory Failure: The principal respiratory functions of the lungs are oxygenation and ventilation. Failure of the lungs can come in the form of hypercarbia (ventilation failure) or hypoxia (oxygenation failure). Although patients may often present with both hypoxia and hypercarbia during an episode of respiratory failure, the majority of patients who require mechanical ventilation have hypercarbic respiratory failure. The ability to provide supplemental oxygen at concentrations near 100% makes the need for ventilator support in the setting of pure hypoxic failure uncommon. As a result, the majority of this topic will be committed to modes of ventilation to support hypercarbic respiratory failure. At the end of this topic, some aspects of ventilation related to optimizing oxygenation will be addressed.
- Physiologic Principles: In all modes of mechanical ventilation, the following principles are involved and must be considered:
- However, modes of mechanical ventilation differ primarily by how the clinician can or cannot control those variables (see below for volume-cycled modes versus pressure-cycled modes).
- Compliance: In general, compliance is defined by the change in volume (ΔV) versus the associated change in pressure (ΔP), or ΔV/ΔP. During mechanical ventilation, compliance can be influenced by 3 key physiologic factors:
- Lung compliance
- Chest wall compliance
- Airway resistance
- There are disease states and ICU-based issues that could alter all 3 of these variables and lead to changes in compliance that can vary daily, hourly or even breath to breath in some circumstances. Lung compliance is influenced by a variety of primary abnormalities of lung parenchyma, both chronic (eg, COPD, ILD) and acute (eg, edema, pneumonia). Airway resistance is typically increased by bronchospasm and airway secretions. Chest wall compliance can be decreased by fixed abnormalities (eg, kyphoscoliosis, morbid obesity) or more variable problems driven by patient agitation while intubated in the ICU.
- Calculating Compliance on MV: ΔV is always defined by tidal volume (Vt), but ΔP is different for the measurement of dynamic vs. static compliance.
- Dynamic Compliance (Cdyn): Calculated by Vt / (PIP - PEEP), where PIP = peak inspiratory pressure (the maximum pressure during inspiration). Alterations in airway resistance, lung compliance and chest wall compliance influence Cdyn.
- Static Compliance (Cstat): Vt / (Pplat - PEEP), where Pplat = plateau pressure. Pplat is measured at the end of inhalation and prior to exhalation using an inspiratory hold maneuver. During this maneuver, airflow is transiently (~0.5 sec) discontinued, which eliminates the effects of airway resistance. Pplat is never > PIP and is typically < 3-5 cmH2O lower than PIP when airway resistance is not elevated.
- Airway Resistance: See static compliance above. In the setting of elevated airway resistance, Pplat is often ≥10 cmH2O lower than PIP.
- Minute Ventilation (Ve): Calculated by Vt x respiratory rate (RR). This is a commonly overlooked and important variable in understanding the etiology of hypercarbic respiratory failure and the effective delivery of mechanical ventilation. Normal resting Ve (ICU patients are typically still and/or sedated) is 4-5 lpm. Hypercarbic failure is common and weaning is typically unsuccessful when Ve >15 lpm. In the setting of underlying lung disease (eg, COPD, ILD), a smaller increase in Ve can often lead to hypercarbic respiratory failure, as these patients are typically less able to increase Vt.
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