21: Mechanical Ventilation in Critical Illness

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CHAPTER 21 Mechanical Ventilation in Critical Illness

11 What are trigger variables?

All modern ICU ventilators constantly measure one or more of the phase variables (i.e., pressure, volume, flow, or time) (Table 21-1). Inspiration occurs when one of these variables reaches a preset value. Clinically this is referred to as triggering the ventilator. The following conditions are necessary to initiate a breath under each individual variable:

Two potentially hazardous forms of triggering have also been identified:

16 What is intrinsic or auto-positive end-expiratory pressure?

Intrinsic PEEP (PEEPi) is unrecognized positive alveolar pressure at end exhalation during MV. Patients with high minute ventilation requirements or patients with chronic obstructive pulmonary disease (COPD) or asthma are at risk for PEEPi. In healthy lungs during MV, if the respiratory rate is too rapid or the expiratory time too short, there is insufficient time for full exhalation, resulting in stacking of breaths and generation of positive airway pressure at end exhalation. Small-diameter endotracheal tubes may also limit exhalation and contribute to PEEPi. Patients with increased airway resistance and decreased pulmonary compliance are at high risk for PEEPi. Such patients have difficulty exhaling gas because of small airway obstruction/collapse and are prone to development of PEEPi during spontaneous ventilation and MV. PEEPi has the same side effects as PEEPe, but detecting it requires more vigilance.

Failure to recognize the presence of auto-PEEP can lead to inappropriate ventilator changes (Figure 21-1). The only way to detect and measure PEEPi is to occlude the expiratory port at end expiration while monitoring airway pressure. Decreasing rate or increasing inspiratory flow (to increase I:E ratio) may allow time for full exhalation. Administering a bronchodilator therapy in the setting of bronchospasm is usually beneficial.

26 How is the patient who is fighting the ventilator approached?

Initially the potential causes are separated into ventilator (machine, circuit, and airway) problems and patient-related problems. Patient-related causes include hypoxemia, secretions or mucous plugging, pneumothorax, bronchospasm, infection (pneumonia or sepsis), pulmonary embolus, myocardial ischemia, gastrointestinal bleeding, worsening PEEPi, and anxiety. The ventilator-related issues include system leak or disconnection; inadequate ventilator support or delivered FiO2; airway-related problems such as extubation, obstructed endotracheal tube, cuff herniation, or rupture; and improper triggering sensitivity or flows. Until the problem is sorted out, the patient should be ventilated manually with 100% oxygen. Breath sounds and vital signs should be checked immediately. Arterial blood gas analysis and a portable chest radiograph are valuable, but, if a tension pneumothorax is suspected, immediate decompression precedes the chest radiograph.

27 Should neuromuscular blockade be used to facilitate mechanical ventilation?

Neuromuscular blocking agents (NMBAs) are commonly used to facilitate MV during ARDS; but, despite wide acceptance, there are few data and as yet no consensus available for when these agents should be used. Gainnier and associates (2004) were the first to report the effects of a 48-hour NMBA infusion on gas exchange in patients with early ARDS. All patients were ventilated according to the ARDSNet protocol. Significant improvements in oxygenation and ability to lower PEEP occurred in the NMBA group and were sustained beyond the 48-hour infusion period. Although it remains to be elucidated why muscle paralysis improves oxygenation, NMBAs are thought to decrease oxygen consumption, promote patient-ventilator interface, and increase chest wall compliance.

Muscle paralysis may also be of benefit in specific situations such as intracranial hypertension or unconventional modes of ventilation (e.g., inverse ratio ventilation or extracorporeal techniques). Drawbacks to the use of these drugs include loss of neurologic examination, abolished cough, potential for an awake paralyzed patient, numerous medication and electrolyte interactions, potential for prolonged paralysis, and death associated with inadvertent ventilator disconnects. Use of NMBAs must not be taken lightly. Adequate sedation should be attempted first; if deemed absolutely necessary, use of NMBAs should be limited to 24 to 48 hours to prevent potential complications.