Mechanical Ventilation
Case 7 Answers
You are called to the bedside of a patient because the nurse is concerned that the ventilator’s pressure alarm is now going off. He was admitted for a COPD exacerbation and was intubated earlier in the day when he failed a trial of non-invasive ventilation. Earlier in the evening, the peak pressure was 45 cm H2O while the static pressure was 25 cm H2O. At the time she calls you, the peak pressure has risen to 60 cm H20 and the static pressure is now 40 mm Hg. His heart rate has increased from 90 beats/minute to 110 beats/minute while his blood pressure has fallen from 110/85 to 90/70. The physical exam is noteworthy for diminished breath sounds on the left side of the chest.
What do static pressures represent on the ventilator?
The static or “plateau” pressure is representative of the compliance of the respiratory system (lung, chest wall and abdomen). In essence, it is telling you how much pressure is necessary to inflate the alveoli with each breath. Any problem which causes a fall in the compliance of the respiratory system will cause static pressures to rise. Examples of such problems include the onset of ARDS or pulmonary edema, large pleural effusions, pneumothorax, abdominal distention, or circumferential chest wall burns. The ventilator does not display this pressure with every breath. Instead, you must use an inspiratory pause maneuver to see this value.
What do peak pressures represent on the ventilator?
The peak pressure is representative of the resistance in the system from the ventilator tubing all the way down to the segmental bronchi. Anything that affects the resistance of these tubes (mucous plugging, bronchospasm, blood clots, and kinked endotracheal tube) will cause the peak pressure to rise. The machine displays the peak pressure with every breath. It is important to know that, while some of the same factors contribute to both peak and static airway pressures, a number of things that affect peak pressure are external to the patient and do not necessarily reflect a change in the compliance of the patient’s lungs.
Where do you think the problem lies with this particular patient?
There are two basic patterns of abnormalities that arise when there are pressure problems in mechanical ventilation:
- the peak pressure rises but the static pressure remains unchanged. This situation suggests there is a resistance problem in the system;
- the peak pressure rises but the static pressure rises as well. This situation suggests that the problem involves a change in the compliance of the respiratory system.
In the case described above, both the peak and the static pressures have increased, suggesting this patient has a new “compliance” problem. Something has happened to the lungs, chest wall or abdomen to lower the compliance of the system. In a patient with COPD who develops unilateral diminished breath sounds, tachycardia and hypotension while on mechanical ventilation, you should be highly suspicious that the patient has developed a tension pneumothorax as the cause of the altered compliance.
What management steps should you institute at this point?
Whenever the static pressures change acutely and there is a change in lung compliance, you should obtain a chest x-ray to look for evidence of a pneumothorax, new pleural effusions, worsening edema or ARDS. You should also examine the patient’s abdomen for evidence of over-distention, which might impair downward movement of the diaphragm and impair respiratory system compliance. If the patient becomes hemodynamically unstable and you have a high suspicion for a tension pneumothorax, you should place a large bore needle into the second intercostal space along the mid-clavicular line to decompress the pneumothorax before the chest x-ray is performed. Patient’s who develop pneumothorax while on the ventilator will ultimately require tube thoracostomy (chest tube placement).
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