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This column is designed to provide the latest research findings in patient care in a format that is easy to understand and integrate into clinical practice. The information is drawn from individual protocols in the various Protocols for Practice series available from AACN, which cover research-based practice protocols in detail.
The various techniques used in mechanical ventilation to help patients breathe are called modes. Generally, modes are controlled or assisted. In controlled ventilation, the ventilator initiates the breath and does all the work of breathing. In assisted ventilation, the patient initiates and terminates some or all the breaths, with the ventilator giving various amounts of support throughout the respiratory cycle. Hence, the modes of ventilation vary in the degree of the patient’s effort versus ventilator support.
The mode chosen for a particular patient is determined by how much of the work of breathing the patient ought to perform. In full ventilatory support, the ventilator performs all the work of breathing. In partial ventilatory support, both the patient and the ventilator contribute. With some modes, adjustments can be made in the ventilator settings to provide gradations from partial to nearly full ventilatory support over the course of ventilation.
Modes of positive pressure ventilation can be divided into 2 groups: volume-targeted and pressure-targeted. This classification stems from the limit variable—the target value—set for inspiration. The limit variable is the variable that the ventilator maintains at a preset value during inspiration, but reaching the limit variable does not cause inspiration to end. In volume-targeted ventilation, the limit variable during inspiration is the preset tidal volume. Volume-targeted modes, such as continuous mandatory ventilation (CMV), assist/control (A/C), and synchronized intermittent mandatory ventilation (SIMV) have been the favored ventilatory support modes in adults for the past 25 years. In pressure-targeted ventilation, pressure, the target, is held constant at a preset level throughout inspiration. Modes that operate in this fashion are pressure-support (PS), pressure-control, pressure A/C, and airway pressure-release ventilation (APRV). In the past decade, use of pressure-targeted modes has become more widespread. Volume-and pressure-targeted modes can be integrated, as in SIMV + PS, in which the mandatory (SIMV) breaths are delivered a target volume during inspiration and the patient’s spontaneous breaths are supported with a target pressure. Further, some of the newest modes integrate volume-and pressure-targeted concepts in the same mode (eg, pressure-regulated volume control [PRVC], volume support [VS]). The key difference between volume- and pressure-targeted modes is either the assurance of a set tidal volume or the assurance of a set peak inspiratory pressure (see Table
).
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Use the A/C mode when the patient has a normal respiratory drive and it is desirable for the patient to assist in breathing yet perform minimal work. Use of this mode allows the ventilator to perform most of the work of breathing. During A/C ventilation, when the patient initiates a breath, the ventilator delivers a breath of the preset tidal volume. Research indicates that the work of breathing required of the patient is minimal (when the flow rate and sensitivity are properly set) but can be considerable if the patient’s flow demands are not met. This type of ventilation is desirable when the patient is too weak to perform the work of breathing (eg, when patients are emerging from anesthesia, pulmonary compliance is decreased, or a low oxygen cost of breathing is desired).
Use SIMV for a wide range of ventilatory support needs when it is desirable to allow the patient to assist in breathing and thus contribute more to the work of breathing. SIMV can also be used as a method for discontinuing mechanical ventilation. During SIMV, the patient receives a preset number of breaths of a preset tidal volume but is able to breathe spontaneously between mandatory breaths. Level of ventilatory support varies from full to partial and depends on the number of SIMV breaths prescribed. Weaning with SIMV is achieved by gradually reducing the number of SIMV breaths, thus requiring the patient to take on a greater percentage of the work of breathing.
Q: Under what conditions are pressure-targeted ventilatory modes indicated?
Pressure-targeted ventilation may be used at low levels (5–10 cm H2O) to overcome the patient’s work of breathing created by airflow resistance from an endotracheal tube or demand valve, which can produce an undesirable workload, cause discomfort, and compromise ventilatory function.1 Pressure ventilation can be titrated upward to achieve a desired tidal volume and minute ventilation or to relieve the ventilatory muscles of excessive work of breathing, as evidenced by reduction in tachypnea and abnormal breathing patterns.2,3
Pressure modes are indicated to improve synchrony between the patient and the ventilator, allowing volume and flow to synchronize with the patient’s ventilatory effort. Rapid flow is provided at the initiation of the breath and variable flow thereafter, meeting the patient’s inspiratory flow demand throughout inspiration. Patients’ effort is therefore reduced, and their comfort is increased. Further, during PS ventilation, patients can vary frequency and inspiratory time to maintain acid-base status. PS ventilation is also indicated as a weaning mode. Finally, pressure-control ventilation is indicated for patients in whom it is desirable to control peak inspiratory pressure (eg, patients with noncompliant lungs who have high airway pressure when conventional volume-targeted ventilation is used). Early initiation of pressure-targeted ventilation results in lower peak inspiratory pressure and improved pulmonary compliance, thereby potentially limiting injury to the lung from high distending pressures.4
Q: Why are lung-protecting ventilatory strategies recommended for patients with acute respiratory distress syndrome and acute respiratory failure associated with asthma and chronic obstructive airway disease?
In acute respiratory distress syndrome, lung function is not homogeneous. Less involved areas of the lung receive a disproportionate share of the tidal volume. Conventional ventilatory strategies that use high tidal volume, fraction of inspired oxygen, and levels of positive end-expiratory pressure (PEEP) may result in high peak inspiratory and alveolar pressures that increase the risk of barotrauma. Volutrauma (stretching of the lung) may also result from regional alveolar overdistention. Both barotrauma and volutrauma may worsen lung injury. Acute respiratory failure in asthma and chronic airway disease are associated with marked expiratory obstruction and hyperinflation of the lung. The increased expiratory lung volume causes an increased pressure in the lung that can cause volutrauma and barotrauma.
Q: What are the major components of a lung-protecting mechanical ventilatory strategy?
Maintain stable alveolar units by applying an adequate level of PEEP to recruit alveoli, increase the functional residual capacity, and keep recruited alveoli open throughout the respiratory cycle.5 Excessive PEEP stretches alveoli and leads to injury. Too little PEEP allows recruited alveoli to collapse during exhalation and requires the next positive-pressure breath to "snap open" the alveolus before the alveolus can fill (see Figure). Repetitive alveolar collapsing and opening contributes to parenchymal injury (shear stress) during positive-pressure breathing. Providing enough PEEP to prevent repeated small airway and alveolar opening and closing reduces alveolar injury.
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In order to accomplish the goal of limiting plateau pressure, a tidal volume of 8 to 10 mL per kilogram of body weight should be used. If plateau pressure remains greater than 35 cm H2O, the tidal volume should be reduced to 5 to 8 mL/kg and the PaCO2 should be permitted to increase (permissive hypercapnia) unless the presence, or risk, of increased intracranial pressure or other contraindication to an elevation in carbon dioxide level exists.5,7 The elevation in the PaCO2 should be controlled to allow renal compensation of the respiratory acidosis and to prevent complications related to rapid induction of respiratory acidosis.
Lung-protecting strategies also include limiting the fraction of inspired oxygen to 0.6 or less while maintaining an arterial oxygen saturation of 90% or greater. Inhalation of air with a high fraction of inspired oxygen increases the partial pressure of oxygen in the alveoli. High partial pressure of oxygen in the alveoli increases microvascular permeability to protein, can lead to absorption atelectasis and surfactant inactivation, and may impede tracheal mucus flow. Hyperoxia can also lead to excess oxidant production, lipid peroxidation, protein oxidant damage, and cell death.
Mechanical ventilation techniques are growing in sophistication and complexity in an effort to improve the efficiency of respiratory support. New techniques such as noninvasive positive pressure ventilation, ventilation delivered with a nasal or face mask, are also gaining widespread use (refer to Nontraditional Ventilatory Techniques : Noninvasive Positive Pressure Ventilation8). No one mode, technique, or strategy is best for managing patients with respiratory failure, and each one has its advantages, disadvantages, and indications. Critical care nurses should not only understand each mode and the monitoring required during its application but also contribute to optimal outcomes for patients by appreciating the research basis for application of these techniques in patients.
Note
This article was first published in Critical Care Nurse February 2000.
This article is based on the protocol Traditional and Nontraditional Modes of Mechanical Ventilation by Lynelle Pierce. It was taken from the Care of the Mechanically Ventilated Patient series of AACN’s Protocols for Practice. Protocols can be obtained from AACN, 101 Columbia, Aliso Viejo, CA 92656-1491, (800) 899-AACN, (949) 362-2000. Product #170724.
(Refer to Traditional and Nontraditional Modes of Mechanical Ventilation for an annotated bibliography and a more extensive reference list.)
References
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