Ani Aydin

Modes of Invasive Mechanical Ventilation

The ventilator mode refers to the way the ventilator interacts with the patient. Assist control (AC) is one of the most commonly used modes of ventilation, set to either target a desired pressure or volume. Pressure support ventilation is a partially supported, or spontaneous, pressure-controlled mode of ventilation with no set respiratory rate or tidal volume. The peak inspiratory pressure (PIP) represents pressures in the entire airway system and is a measure of both the resistance and compliance. The plateau pressure (Pplat), which is measured during an inspiratory hold, is the reflection of the pressure delivered to the alveoli and the compliance of the system. When air is trapped in the alveoli at the end of exhalation, it exerts a pressure above and beyond the set PEEP. This pressure can actually be quantified on the ventilator by pressing the expiratory pause button and is known as AutoPEEP or intrinsic PEEP (iPEEP).

Specific Circumstances: Asthma and COPD

Patients with asthma and chronic obstructive pulmonary disease, or COPD, are a challenging patient population to ventilate in the ED. These patients are at risk for air trapping, manifesting as increased autoPEEP, which can lead to decreased venous return and hemodynamic instability. These patients require sufficient time to exhale, which can be achieved with low respiratory rates, low inspiratory to expiratory ratios (I:E ratios), low inspiratory times, and high inspiratory flow rates. AutoPEEP should be monitored periodically or after any ventilator change with an expiratory hold maneuver.

Troubleshooting the Ventilated Patient

Patients in the emergency department are at high risk for deterioration after intubation. Using a systematic approach to assess these patients is key. The differential for the low pressure alarm includes a break in the circuity, anywhere from the ventilator to the lungs. The high pressure alarm similarly can arise from an issue anywhere from the patient to the ventilator. To quickly recall the differential diagnosis for deterioration on the ventilator, clinicians should recall the DOPES mnemonic. A separate, but related mnemonic, DOTTS, reminds clinicians of steps in assessing and treating the deterioration of an intubated patient.

Specific Circumstances: Neurologic Injury

Numerous studies have shown that early proper management of patients with neurologic injury can improve outcomes, by reducing the risk of secondary injury. The goals for patients with neurologic injury are to maintain normoxia and eucapnia, with particular emphasis on avoiding hypocapnia, hypoxemia, and hyperoxia. Guidelines recommend targeting a normal PaCO2 of 35–40, and monitoring frequent ABGs and capnography is very important in this patient population.

Terminology and Definitions

Normally, a glossary is found at the end of educational materials. This chapter, however, will start with a review (or an introduction!) to key terms that we will use throughout the rest of the text. Understanding the definition of each term now will assist in appreciating its use in context later.

Conclusions and Key Concepts

Many terms are used interchangeably in mechanical ventilation, and this leads to confusion. Select appropriate terms, and use them consistently. Key concepts include tidal volume, respiratory rate, minute ventilation, PEEP, resistance, compliance, peak inspiratory pressure, plateau pressure, autoPEEP, and derecruitment. Modes of ventilation are assist control (including volume control and pressure control, as well as pressure-regulated volume control), pressure support, and synchronized intermittent mandatory ventilation. Tidal volume should be selected for 6–8 mL/kg of predicted body weight, based upon height and sex. The respiratory rate should be selected to target a reasonable minute ventilation. PEEP should be set at minimum of 5 cmH2O, and titrated higher as needed to correct for hypoxemia and derecruitment. Once the ventilator settings are selected the patient must be continuously reassessed, settings such be titrated based on ABG results, and peak inspiratory pressures and plateau pressures monitored to reduce harm. Patients with acute respiratory distress syndrome (ARDS), asthma, COPD, and neurologic injury are especially vulnerable populations of ventilated patients who require additional attention and careful ventilator management. While mechanically ventilated patients are among the most critically ill patients cared for in the ED, appropriate, evidence-based care can positively impact the patients’ trajectories and improve outcomes.

Understanding the Ventilator Screen

Interpreting ventilator screens may be intimidating for beginners, but the data are actually fairly simple despite being presented slightly differently on the screens of each mechanical ventilator brand. There are a few key points to remember when reviewing ventilator screens. (1) The values set by clinicians are found on the bottom of the screen. The patient’s response is located at the top of the screen. (2) Data are provided in both numerical and graphical contexts on the screen. (3) Ventilators provide three types of tracings: flow, pressure, and volume. All values are readily labeled directly on the ventilator screens.

Placing the Patient on the Ventilator

Initiation of mechanical ventilation can lead to a decrease in the cardiac output and hypotension, especially in the intravascularly depleted patient. The mode, tidal volume, respiratory rate, PEEP, inspiratory time/I:E ratio, and fraction of inspired oxygen should all be selected. Low tidal volume ventilation and targeting low pressures on the ventilator have been shown to improve outcomes for patients with and without acute respiratory distress syndrome (ARDS) alike. Once initial settings have been selected, the patient requires close monitoring for hemodynamic deterioration and to ensure that the selected settings are appropriate.

Noninvasive Respiratory Support

Determining whether the patient has an oxygenation problem, ventilation problem, or both is the first step in addressing respiratory failure. Many causes of respiratory failure can be well-managed noninvasively. High flow nasal cannula (HFNC) is an excellent means of supporting hypoxemic patients. Noninvasive positive pressure ventilation (NIPPV) is one of the most important advances in the care of patients in respiratory failure, especially for patients with respiratory failure from COPD and congestive heart failure. Continuous positive airway pressure (CPAP) and bilevel positive airway pressure are the two forms of NIPPV.

Specific Circumstances: Acute Respiratory Distress Syndrome (ARDS)

Acute respiratory distress syndrome, (ARDS) is a condition of diffuse alveolar damage and inflammation, secondary to numerous possible etiologies. Few interventions in critical care have been as reproducibly beneficial to patients as low tidal volume ventilation. Prevention of ventilator-induced lung injury is a key benefit of active ventilator management in the ED. In ARDS, as well as other patients, maintaining a plateau pressure of less than 30cmH2O is essential to preventing ventilator-induced lung injury. Recruitment maneuvers, neuromuscular blockade, and prone positioning should be used for persistent or severe hypoxemia. Inhaled pulmonary vasodilators can improve oxygenation, and ECMO should be considered for severe ARDS.