Mechanical Ventilation and Noninvasive Ventilatory Support

Physiology of Positive-Pressure Breathing

A typical respiratory cycle in spontaneously breathing patients begins with negative intrathoracic pressure created by contraction and relaxation of the diaphragm in concert with the intercostal muscles. This elevates the lateral ribs in a bucket-handle fashion, increasing the intrathoracic volume and resulting in a pressure gradient between intra- and extra-thoracic airspaces and net airflow into the lungs. Relaxation of the diaphragm and recoil of the chest wall decreases intrathoracic volume, increases pressure in the chest cavity, and results in passive exhalation. The amount of force required to generate adequate inspiration is influenced by the work of breathing. When the work of breathing increases, patients may be unable to generate enough negative force to sustain successful respiration and require ventilatory support.

Unlike spontaneous breathing, invasive and noninvasive mechanical ventilation are based on the delivery of humidified air with positive pressure. The amount of positive pressure required for adequate ventilation is dependent on the patient’s respiratory effort, ranging from mild assistance to full support. Inhalation occurs by driving air into the lungs under positive pressure; air is passively exhaled when the chest wall recoils.

Transition from negative-pressure breathing to positive-pressure breathing affects cardiovascular and pulmonary physiology and can have significant clinical consequences. Pressure changes in the thoracic cavity directly affect pressures in the chambers of the heart. During spontaneous inspiration, decreased intrathoracic pressure augments venous return and preload. Cardiac output is increased, and there is an increased pressure gradient between the left ventricle and aorta. With the initiation of positive-pressure ventilation (PPV), the opposite occurs: venous return is diminished, cardiac output falls, and there is a decreased pressure gradient between the left ventricle and aorta. Hypotension can occur after ventilatory support has been initiated and may be exaggerated in patients with clinical hypovolemia or vasodilatory states.

Invasive Mechanical Ventilation: Control Variable and Ventilator Mode

The primary considerations when initiating mechanical ventilation relate to how gas is delivered to the lungs. This includes the volume, duration, and frequency of each breath, and the degree of interaction the patient has with the ventilator.

How the ventilator delivers gas to the lungs is referred to as the control variable . The amount of air delivered in each breath is either set directly as a specific volume or indirectly as a specific amount of pressure. These are referred to as volume-controlled ventilation (VC) and pressure-controlled ventilation (PC), respectively. The amount of time over which the breath is delivered is defined as the inspiratory time, and the speed at which air travels through the circuit is defined as inspiratory flow rate. The term “cycle” refers to how the ventilator terminates delivery of a breath.

With VC, a breath is defined by delivery of a set tidal volume to the lungs. Inspiratory volume and flow rate are set by the clinician, and the ventilator cycles once a preset tidal volume has been delivered. The inspiratory time is a function of the set flow rate. Lung pressures—peak inspiratory pressures (PIPs) and end-inspiratory alveolar pressures—vary based on respiratory system resistance and compliance, as well as set tidal volume. The main benefit to the use of VC is the ability to control tidal volume and minute ventilation, but in scenarios of impaired respiratory system compliance, delivery of desired tidal volume may result in dangerously high airway pressures and barotrauma.

In PC, a set amount of pressure is applied to the airway to expand the lungs for a specified amount of time. During PC, the inspiratory pressure is set by the clinician, whereas the delivered tidal volume and inspiratory flow rate vary as functions of dynamic lung compliance and airway resistance. An inspiratory time is also set, after which the ventilator cycles by terminating delivery of the set inspiratory pressure. Ability to control the pressure delivered to the lungs is particularly useful to prevent barotrauma, which is described in more detail below. In addition, because inspiratory flow is not fixed, PC may improve ventilator synchrony in intubated patients with a high respiratory drive. A significant disadvantage of PC is that tidal volume can neither be guaranteed nor limited as it changes with acute changes in lung compliance.

The choice between volume-controlled ventilation and pressure-controlled ventilation is driven by the underlying physiology of the condition for which mechanical ventilation is needed, and for patients who do not require strict control of pressure or volume, safe and effective ventilation can be achieved with either strategy ( Table 2.1 ). Volume-controlled ventilation should be used when strict control of tidal volume is mandated. Specifically, this includes patients with known acute respiratory distress syndrome (ARDS), in whom low tidal volume strategies have been proven to reduce mortality. In addition, patients with markedly decreased chest wall compliance should be placed on VC to ensure that adequate tidal volume is delivered. This includes patients with morbid obesity or severe chest wall burns. Conversely, PC offers advantages over VC in clinical conditions in which control of airway pressure is strictly mandated. This includes patients with the potential to develop dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEP) such as those with severe asthma or chronic obstructive pulmonary disease (COPD).

Notably, while both VC and PC can have specific advantages, patients with acute derangements in pulmonary mechanics may be difficult to optimize regardless of the ventilator control variable used. Newer ventilators have attempted to address this dynamic interdependence by delivering breaths that combine volume and pressure strategies, referred to as dual-control ventilation. A common dual-control method of ventilation is pressure-regulated volume control (PRVC). A variation of volume control, PRVC is set to deliver a specific tidal volume while simultaneously minimizing airway pressure. In contrast to strict volume control, pressure is measured and modulated by the ventilator with each breath to ensure the delivery of the preset tidal volume. In addition, a pressure limit is set, and the ventilator sounds an alarm when that pressure has been exceeded. Theoretically, this combines the advantages of pressure and volume control to ensure the delivery of a specific tidal volume while the airway pressure is monitored. That said, because the ventilator is set to deliver a specific tidal volume, the disadvantages of volume-controlled ventilation persist. In addition, elevations in airway pressure are still possible and must be addressed if changes in respiratory system compliance occur. The efficacy of PRVC has not been compared to traditional PC or VC but likely does not offer significant advantage over traditional volume- or pressure-controlled ventilation, particularly if strict parameters for airway pressure are desired.

The term ventilator mode refers specifically to the amount of respiratory support provided by the ventilator and how often the ventilator initiates a breath for the patient. These can be divided broadly into assist-control mechanical ventilation (A/C), intermittent mechanical ventilation (IMV), and continuous spontaneous ventilation (CSV). The key difference is that A/C and IMV are intended to provide patients with a specific minimum number of preset breaths as defined by the ventilator and can be delivered via pressure or volume control methods. Conversely, in CSV, no mandatory breaths are delivered to a patient; the size and rate of the breaths are determined by the effort of the patient and are augmented with applied pressure or volume to the airway. These methods are compared in Table 2.2 . Other, more complex modes of ventilation include proportional assist ventilation (PAV) and airway pressure release ventilation (APRV), although these are rarely used in the emergency department (ED).

A/C is intended to provide full ventilatory support for patients with little or no spontaneous respiratory activity by continuous delivery of breaths at a preset rate. However, if a patient generates respiratory effort while on A/C, that breath will also be assisted by the ventilator. Patients can trigger a breath at any rate but will always receive at least the preset number of breaths, hence the nomenclature of “assist-control.” This is the most useful initial mode of mechanical ventilation in ED patients, because most patients are initially paralyzed and sedated and do not interact with the ventilator.

In A/C mode, breaths can be volume-controlled (AC/VC) or pressure-controlled (AC/PC) as detailed previously. For the promotion of ventilator synchrony, a spontaneous patient-initiated breath will take priority over a preset breath, meaning that if the ventilator is set to deliver 12 breaths/min, a breath is provided every 5 seconds in the absence of spontaneous inspiratory effort. When the patient makes a spontaneous effort, the ventilator provides an additional breath and the timer resets for another 5 seconds. One of the biggest challenges with A/C ventilation, however, is that patient-initiated breaths are not proportional to patient effort; when inspiratory effort is detected, a full-sized breath is delivered. Clinically, this requires adequate sedation of patients when ventilated in the A/C mode to prevent spontaneous respiratory efforts that will result in hyperventilation, air trapping, hypotension, and poor ventilator synchrony.

Intermittent mandatory ventilation provides intermittent ventilatory support to patients by delivering both mandatory and spontaneous breaths. In this mode, mandatory breaths are given at a preset rate, but the breath is synchronized as much as possible with spontaneous patient effort. For this reason, it is most commonly known as synchronized IMV or SIMV. Similar to A/C, the patient will receive at least the minimum number of preset mandatory breaths; if the patient provides no effort, the preset number of breaths will be given. If a patient has a rate of spontaneous respirations lower than the set rate, the ventilator will provide the preset number of full breaths but will deliver as many as possible in synchrony with patient effort. In these scenarios, there is little difference between A/C and SIMV. If a patient has a higher spontaneous respiratory rate than the preset rate, the patient receives all preset full breaths at the set rate. In contrast to A/C, additional breaths generated by the patient will be volume or pressure-supported breaths commensurate with the patient’s respiratory effort. This attenuates the effects of air trapping and hyperventilation potentially seen with A/C and is one advantage of SIMV in less sedated patients.

CSV, in contrast to A/C or SIMV, provides no mandatory breaths and only augments a patient’s spontaneous respiratory effort. On a ventilator, the most common way to eliminate mandatory delivery of preset breaths is via pressure-supported ventilation (PSV) and is designed to support patients’ spontaneous respiratory effort by delivering an applied pressure to the airway on the trigger of a breath. The amount of pressure required to support a full breath is variable and depends on the patient’s ability to overcome the work of breathing. When inspiratory flow slows to a prespecified fraction of maximal inspiratory flow (often 25%), this signals the end of inhalation. At this point, pressure support ceases and exhalation is allowed to proceed spontaneously. The level of pressure support is the only set parameter in PSV; inspiratory flow, inspiratory time, and tidal volume are determined by patient effort. This mode of ventilation most closely resembles normal spontaneous breathing and, for this reason, promotes patient control and comfort. In the ED, PSV is rarely used for intubated patients because most patients who require intubation are unable to breathe spontaneously and effectively and may have failed noninvasive support before intubation. PSV may prove to be most useful in awake and interactive patients who have been intubated for temporary airway protection rather than for respiratory failure. If PSV is used, careful monitoring and ventilatory alarms are needed to ensure against undetected hypoventilation or apnea.

Noninvasive Techniques

Noninvasive positive-pressure ventilation (NPPV) is the delivery of CSV via sealed mask rather than endotracheal tube. As with PSV, the ventilator is set to provide a defined level of pressure when a patient takes a breath; inspiratory flow and inspiratory time are completely patient-mediated. The most common types of noninvasive ventilation in the ED are continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BL-PAP). BiPAP, a term commonly used for BL-PAP, is the proprietary name of a portable device that uses this method of noninvasive ventilation rather than a term for the ventilation itself (Philips Respironics, Murrysville, PA). CPAP provides constant positive pressure throughout the respiratory cycle, whereas BL-PAP alternates between higher pressure during inspiration (IPAP) and lower pressure during expiration (EPAP). Although, strictly speaking, CPAP applies positive pressure to the airway during inspiration, the amount of inspiratory assistance is minimal. Conversely, just as with invasive mechanical ventilation, IPAP augments patient respiratory effort by decreasing the work of breathing during inspiration, whereas EPAP acts as PEEP to maintain FRC and alveolar recruitment. Notably, although PEEP, CPAP, and EPAP all represent positive airway pressure at the end of expiration, PEEP, by convention, refers to pressure applied during invasive mechanical ventilation, whereas CPAP is the application of positive pressure (invasively or noninvasively) during spontaneous breathing. The terms are occasionally used interchangeably.

In addition to NPPV, noninvasive oxygen supplementation can also by achieved with high-flow nasal cannula (HFNC). HFNC and other high-flow oxygen delivery devices are specially designed to use high-pressure oxygen and air, a gas blender, humidifier, and large-diameter tubing to deliver oxygen at flow rates often exceeding 60 L/min. The theoretical benefits are as follows: (1) high flow rates more closely match patients’ inspiratory flow and volume demands, so more inspired gas comes from the device than ambient air increasing the fraction of inspired oxygen (Fi o ₂ ); (2) the high flow washes out anatomic dead space and replaces it with oxygen; (3) Fi o ₂ and flow rate can be titrated independently (classically, Fi o ₂ is titrated for hypoxemia and flow rate for dyspnea); (4) many devices deliver a small amount of PEEP (1 to 3 cm H ₂ O); (5) gas is humidified and heated, which makes the high flow rate more tolerable; and (6) the large nasal prongs on the cannula devices often occlude the entire nares, reducing entrainment of ambient air during closed-mouth breathing. HFNC supplementation is indicated for acute hypoxemic respiratory failure without significant hypercarbia, and in patients for whom supplementary intrathoracic pressure would not be necessary. HFNC cannot be used in patients without a patent upper airway. Other relative contraindications include depressed mental status, facial injury, inability to manage secretions, or respiratory arrest.

Decision Making: Noninvasive Versus Invasive Ventilation

The decision to intubate carries significant implications for patients, including complications related to invasive airway management, the use of neuromuscular blocking agents (NMBAs), and the risk of prolonged mechanical ventilation in the intensive care unit (ICU). NPPV is an appealing option for patients requiring ventilatory assistance with rapidly reversible conditions or for those with “do-not-intubate” directives. Relative contraindications include decreased level of consciousness, lack of respiratory drive, increased secretions, hemodynamic instability, and conditions such as facial trauma that would prevent an adequate mask seal. Although the need for emergent intubation is generally a contraindication to treatment with noninvasive ventilation, noninvasive ventilation may improve preoxygenation prior to intubation when compared to standard methods of oxygen delivery. If NPPV is initiated, patients should be reassessed frequently for progress of therapy, tolerance of the mode of support, and any signs of clinical deterioration that would indicate a need for intubation.

Patients most likely to respond to NPPV in the ED are those with more readily reversible causes of respiratory distress such as COPD exacerbation or cardiogenic pulmonary edema in which fatigue is a significant factor. Robust evidence has supported the use of NPPV for both conditions. In patients with acute COPD exacerbations, NPPV decreases the need for intubation by 65%, decreases hospital length of stay, and improves mortality with NNT of 12 when compared with standard therapy. Treatment failure, defined as a subsequent need for intubation, is predicted by a Glasgow Coma Scale score of less than 11, sustained arterial pH less than 7.25, and tachypnea greater than 35 breaths/min. Several studies highlighted the need for appropriate patient selection in that a failed trial of NPPV was associated with higher mortality when compared to those who received immediate intubation.

In patients with acute cardiogenic pulmonary edema (ACPE), NPPV reduces the work of breathing while simultaneously improving cardiac output. The application of NPPV causes elevations in intrathoracic pressure that decrease left ventricular (LV) ejection pressure and LV transmural pressure, which results in afterload reduction. In addition, decreases in right ventricle (RV) preload may improve LV compliance via ventricular interdependence. Compared with standard therapy, NPPV has been shown to reduce mortality with number needed to treat (NNT) 17 and reduce rates of endotracheal intubation by 50% for patients with ACPE. Benefits were found to be independent of whether patients received CPAP or BL-PAP and, despite suggestions from early clinical data, no increased rate of acute myocardial infarction occurred in patients receiving any form of NPPV. Specific predictors of failure of NPPV in those with congestive heart failure (CHF) have not been systematically examined.

Evidence regarding the use of NPPV in other patients with respiratory compromise, including asthma and pneumonia, is limited. Several small studies have suggested that NPPV may be beneficial for patients with acute asthma exacerbations by improving lung function, decreasing bronchodilator requirements, and shortening overall hospital length of stay, suggesting a potential role for NPPV in these patients (see Chapter 59). Studies have failed to establish a definitive role for NPPV in pneumonia, and the presence of pneumonia has been shown to be an independent risk factor for failure of noninvasive ventilation. In addition, the duration of NPPV prior to intubation has been associated with in-hospital mortality, suggesting that early intubation is preferable for patients who do not rapidly improve on noninvasive therapy.

A recent randomized, controlled trial compared HFNC, standard oxygen therapy, and NPPV in patients with acute hypoxemic respiratory failure. While the trial found no difference in the proportion of patients intubated at day 28 (the primary outcome), there was a mortality benefit favoring HFNC. HFNC has also been trialed post-extubation and after cardiothoracic surgery with favorable results. Another trial in immunocompromised patients showed no difference compared with standard oxygen delivery devices. In light of potential mortality benefit, HFNC should be considered early in the treatment of hypoxemic respiratory failure.