Apneic patients, such as those who have suffered catastrophic central nervous system (CNS) damage.
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Asthma, chronic obstructive pulmonary disease (COPD), pneumonia, cardiogenic pulmonary edema, and acute respiratory distress syndrome (ARDS) are just a few of the many conditions that cause an increase in work of breathing and, with it, increased energy expenditure by the respiratory muscles.
The energy expenditure of the respiratory muscles can be quantified in terms of pressure-time product2—the time integral of the difference between the esophageal pressure tracing and the estimated recoil pressure of the chest wall3,4 (Fig. 4-1). The pressure-time product of patients in acute respiratory failure is about four times5–7 the normal value (100 cm H2O·s/min), and it can be increased sixfold in individual patients.5,6 The inspiratory pressure-time product can be partitioned into resistive, elastic, and intrinsic positive end-expiratory pressure (PEEP) components (Fig. 4-1).6 Patients in respiratory distress typically have a 30% to 50% greater inspiratory resistance,6 100% greater dynamic elastance,6 and 100% to 200% greater intrinsic PEEP5,6 than do similar patients who are not in acute respiratory failure. Inspiratory effort is almost equally divided in offsetting intrinsic PEEP, elastic recoil, and inspiratory resistance.6 The increase in respiratory effort means that the respiratory muscles account for a much larger fraction of the body’s oxygen consumption. In healthy subjects, this fraction is only 1% to 3% of total oxygen consumption. In patients with acute hypoxemic respiratory failure and shock who are undergoing cardiopulmonary resuscitation, the respiratory muscles account for approximately 20% of total oxygen consumption.8
The primary objectives of mechanical ventilation are to decrease the work of breathing, thus avoiding respiratory muscle fatigue, and to reverse life-threatening hypoxemia and progressive respiratory acidosis.
When respiratory failure is chronic, neither of the two types is obligatorily treated with mechanical ventilation, but when it is acute, mechanical ventilation may be lifesaving.
In some cases, mechanical ventilation is used as an adjunct to other forms of therapy. For example, it is used to reduce cerebral blood flow in patients with increased intracranial pressure.
Mechanical ventilation also is used frequently in conjunction with endotracheal intubation for airway protection to prevent aspiration of gastric contents in otherwise unstable patients during gastric lavage for suspected drug overdose or during gastrointestinal endoscopy. In critically ill patients, intubation and mechanical ventilation may be indicated before the performance of essential diagnostic or therapeutic studies if it appears that respiratory failure may occur during those maneuvers.
It is implemented with devices that can support ventilatory function and improve oxygenation through the application of high-oxygen-content gas and positive pressure.
There are two general classes of mechanical ventilation: noninvasive ventilation (NIV) and conventional mechanical ventilation. NIV, administered through a tightly fitting nasal or facial mask, is widely used in acute-on-chronic respiratory failure related to COPD exacerbations. NIV typically involves a preset positive pressure applied during inspiration and a lower pressure applied during expiration; it is associated with fewer complications such as nosocomial pneumonia than conventional mechanical ventilation through an endotracheal tube. However, NIV is contraindicated in cardiopulmonary arrest, severe encephalopathy, severe GI hemorrhage, hemodynamic instability, unstable coronary artery disease, facial surgery or trauma, upper airway obstruction, inability to protect the airway, and inability to clear secretions.
Most pts with acute respiratory failure require conventional mechanical ventilation via a cuffed endotracheal tube. The goal of mechanical ventilation is to optimize oxygenation while avoiding ventilator-induced lung injury. Various modes of conventional mechanical ventilation are commonly used; different modes are characterized by a trigger (what the ventilator senses to initiate a machine-delivered breath), a cycle (what determines the end of inspiration), and limiting factors (operator-specified values for key parameters that are monitored by the ventilator and not allowed to be exceeded). Three of the common modes of mechanical ventilation are described below; additional information is provided in Table 15-1.
TABLE 15-1CLINICAL CHARACTERISTICS OF COMMONLY USED MODES OF MECHANICAL VENTILATION
| Ventilator Mode | Independent Variables (Set by User) | Dependent Variables (Monitored by User) | Trigger/Cycle Limit | Advantages | Disadvantages |
|---|---|---|---|---|---|
| ACMV | FIO2 Tidal volume Ventilator rate Level of PEEP Inspiratory flow pattern Peak inspiratory flow Pressure limit |
Peak airway pressure Minute ventilation Plateau pressure Mean airway pressure I/E ratio |
Pt effort Timer Pressure limit |
Timer backup Pt-vent synchrony Pt controls minute ventilation |
Not useful for weaning Potential for dangerous respiratory alkalosis due to hyperventilation Barotrauma risk |
| SIMV | Same as for ACMV | Same as for ACMV | Same as for ACMV | Timer backup is useful for weaning Comfort from spontaneous breaths |
Potential dyssynchrony |
| PSV | FIO2 Inspiratory pressure level PEEP pressure limit |
Tidal volume Respiratory rate Minute ventilation ABG |
Inspiratory flow Pressure limit | Assures synchrony Good for weaning |
No timer backup; may result in hypoventilation |
| NIV | Inspiratory and expiratory pressure levels FIO2 |
Tidal volume Respiratory rate I/E ratio Minute ventilation ABG |
Pressure limit Inspiratory flow | Pt control | Discomfort and bruising from mask Leaks are common Hypoventilation risk |
Assist-control ventilation: The trigger for a machine-delivered breath is the pt’s inspiratory effort, which causes a synchronized breath to be delivered. If no effort is detected over a prespecified time interval, a timer-triggered machine breath is delivered. Assist-control is volume-cycled with an operator-determined tidal volume. Limiting factors include the minimum respiratory rate, which is specified by the operator; pt efforts can lead to higher respiratory rates. Other limiting factors include the airway pressure limit, which is also set by the operator. Because the pt will receive a full tidal breath with each inspiratory effort, tachypnea due to nonrespiratory factors, such as pain, can lead to respiratory alkalosis. In pts with airflow obstruction (e.g., asthma or COPD), auto-PEEP (positive end-expiratory pressure) can develop.
Synchronized intermittent mandatory ventilation (SIMV): As with assist-control, SIMV is volume-cycled, with similar limiting factors. As with assist-control, the trigger for a machine-delivered breath can be either pt effort or a specified time interval. However, if the pt’s next inspiratory effort occurs before the time interval for another mandatory breath has elapsed, only their spontaneous respiratory effort (without machine support) is delivered. Thus, the number of machine-delivered breaths is limited in SIMV, allowing pts to exercise their inspiratory muscles between assisted breaths.
Pressure-support ventilation (PSV): PSV is triggered by the pt’s inspiratory effort. The cycle of PSV is determined by the inspiratory flow rate. Because a specific respiratory rate is not provided, this mode of ventilation may be combined with SIMV to ensure that an adequate respiratory rate is achieved in pts with respiratory depression.
Other modes of ventilation may be appropriate in specific clinical situations; for example, pressure-control ventilation is helpful to regulate airway pressures in pts with barotrauma or in the postoperative period from thoracic surgery.
General care of mechanically ventilated pts is reviewed in Chap. 4, along with weaning from mechanical ventilation. A cuffed endotracheal tube is often used to provide positive pressure ventilation with conditioned gas. A protective ventilation approach is generally recommended, including the following elements: (1) target tidal volume of ~6 mL/kg of ideal body weight; (2) avoid plateau pressures >30 cm H2O; (3) use the lowest fraction of inspired oxygen (FIO2) to maintain arterial oxygen saturation ≥90%; and (4) apply PEEP to maintain alveolar patency while avoiding overdistention. This approach may result in a permissible degree of hypercapnia. After an endotracheal tube has been in place for an extended period of time, tracheostomy should be considered, primarily to improve pt comfort, reduce needs for sedative medications, and provide a more secure airway. No absolute time frame for tracheostomy placement exists, but pts who are likely to require mechanical ventilatory support for >2 weeks should be considered for a tracheostomy.
A variety of complications can result from mechanical ventilation. Barotrauma—overdistention and damage of lung tissue—can cause pneumomediastinum, subcutaneous emphysema, and pneumothorax. Ventilator-related pneumothorax typically requires treatment with tube thoracostomy. Ventilator-associated pneumonia is a major complication in intubated pts; common pathogens include Pseudomonas aeruginosa and other gram-negative bacilli, as well as Staphylococcus aureus.
Match the mode of ventilation with its description.
Pressure support ventilation