Barotrauma
Pneumothorax, pneumomediastinum, subcutaneous emphysema, and other forms extra-alveolar air detected during mechanical ventilation are collectively termed ‘barotrauma.’ Although this term implies that excessive pressure is the cause, it is more likely that alveolar disruption results from overdistention (i.e., excessive peak inflating volume) rather than high pressure per se. Extra-alveolar air that first becomes evident during ventilatory support may also be unrelated to the ventilator.
Depending upon how much air enters the pulmonary interstitium and where the path of least resistance takes it, a variety of clinical manifestations can ensue. Pneumomediastinum, pneumoperitoneum, and subcutaneous emphysema are rarely of physiologic significance and do not require specific treatment. However, pneumothorax is different in that the air collects in a space (the pleural cavity) from which it cannot naturally escape as pressure increases. Pneumothorax in a patient who receives positive-pressure ventilation can quickly progress to tension pneumothorax, which may rapidly cause cardiovascular collapse and death; for this reason, it must always be relieved promptly if ventilatory support cannot immediately be discontinued.
Table 14 [16k PDF*] outlines steps to minimize the risk of alveolar rupture and barotrauma during mechanical ventilation. These steps are especially important in high-risk patients, such as those who have obstructive or unevenly-distributed lung disease, pulmonary infection, and late-phase ARDS (e.g., beyond 1–2 weeks).
Ventilator-induced parenchymal lung injury, apart from clinically apparent barotrauma as discussed here, has been shown to affect both lung dysfunction and clinical outcomes in acute lung injury. Repetitive opening and closing of collapsed alveoli subjects them to high local pressures and can lead to activation of inflammatory mediators (so-called "biotrauma"). Similarly, overdistention of alveoli or lung regions can injure them without producing extra-alveolar air. Current evidence indicates that the prevention or reduction of ventilator-induced lung injury is an important reason for improved survival and other outcomes when patients with acute lung injury or ARDS are managed by the lung-protective ventilatory strategy discussed earlier.
Other complications
Several important complications associated with invasive mechanical ventilation are not discussed in this chapter. However, three important topics are briefly discussed here that are not generally included in reviews of ventilator complications.
‘Fighting the ventilator’ Agitation and respiratory distress that develop in a patient on a mechanical ventilator who has previously appeared comfortable represents an important clinical circumstance for which the clinician needs an organized approach. The patient should not automatically be sedated, but must instead be evaluated for several potentially life-threatening developments that can present in this fashion.
The ventilator is first disconnected from the patient, and the patient ventilated manually using 100% oxygen and a self-inflating bag. If this procedure relieves the patient’s distress, the problem lies proximal to the endotracheal tube, and the ventilator and its circuitry must be inspected or replaced. If agitation and respiratory distress persist, the airway are suctioned and a brief physical examination performed, noting recent trends in vital signs and any other new developments in the hours that preceded the onset of distress. A more detailed examination is required if the likely cause is not apparent, which must include a new set of measurements of arterial blood gases and a bedside chest radiograph. On rare occasions, if the patient appears in extremis and tension pneumothorax cannot be excluded at the bedside, empiric measures to drain the chest may be warranted.
Deteriorating oxygenation in the ventilated patient Deteriorating arterial oxygenation during mechanical ventilation, a common reason for ‘fighting the ventilator’, should initiate a systematic search for specific mechanisms and therapy rather than simply an increase in inspired oxygen fraction or level of PEEP. Possible causes for worsening oxygenation fall into several categories.
The problem could be with the ventilator and its circuitry. The patient’s primary disease process (e.g., pneumonia, ARDS) could be worsening, or a new medical problem may have appeared. Examples of the last include pneumothorax, acute lobar atelectasis, pulmonary edema from fluid overload, nosocomial pneumonia or sepsis, aspiration of gastric contents, retained secretions, and bronchospasm. A fall in cardiac output can also cause worsening oxygenation in a patient who has significant pulmonary venous admixture.
Interventions and procedures can also lead to a decline in oxygenation. Examples include the effects of airway suctioning, chest
physical therapy, or even changes in body position, especially in patients affected by heterogeneously distributed pulmonary involvement. Bronchoscopy, thoracentesis, and hemodialysis can also lead to a decline in oxygenation. Finally, a number of drugs administered to patients undergoing mechanical ventilation can interfere with arterial oxygenation. Among these are vasodilators (which can decrease hypoxic vasoconstriction), b-blockers (which can depress cardiac output and induce bronchospasm), and bronchodilators (which can alter ventilation/perfusion ratios).
Effects of suboptimal ventilator management Some adverse effects of mechanical ventilation are iatrogenic. With increasing complexity of the ventilators and their modes, it becomes more likely that the physician, nurse, or respiratory therapist who adjusts the ventilator may not fully understand the consequences of a given adjustment. Table 15 [26k PDF*] lists several such iatrogenic problems, the clinical circumstances in which they typically occur, and steps to prevent or correct them.
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References
- Pierson DJ. Invasive mechanical ventilation. In Albert RK, Spiro SG, Jett JR, eds. Clinical respiratory medicine. London/Philadelphia, Saunders, 2nd edition, 2004:189-209
- MacIntyre NR, Cook DJ, Guyatt GH, eds. Evidence-based guidelines for weaning and discontinuing ventilatory support. American College of Chest Physicians, American Association for Respiratory Care, and American College of Critical Care Medicine. Chest. 2001 Dec;120(6 Suppl):375S-484S.
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