aka Pulmonary Puzzle 014
Having overcome his worrying episode of post-intubation hypotension in the emergency department, the man who was intubated for severe asthma in Pulmonary Puzzle 013 is now in the intensive care department. He remains mechanically ventilated.
You are asked to review him because the high airway pressure alarm is going off.
Q1. Why is the airway pressure alarm important?
For at least three reasons:
- Excessive airway pressure may have adverse effects on the patient.
- High airway pressures may indicate a deterioration of the patient’s condition.
- It may indicate an equipment problem that needs to be addressed.
Q2. What are the potential adverse effects of excessive airway pressure?
High airway pressures do not correlate with lung barotrauma.
Airway pressure itself is not particularly deleterious unless it reflects excessive alveloar pressure. Excessive alveolar pressure can have a number of adverse effects:
- barotrauma may result in acute lung injury (leading to ARDS) or air leaks (e.g. pneumothorax, pneumomediastinum).
- Excessive intrathoracic pressure may also result, with potential hemodynamic consequences (particularly decreased venous return, leading to hypotension and potentially cardiac arrest).
- High airway pressures may result in inadequate ventilation.
Inadequate ventilation can occur because many ventilators are set to terminate the inspiratory flow if the upper pressure limit setting is reached. When this occurs inspiratory volumes are markedly reduced, resulting in low tidal volumes and minute ventilation. Other ventilators do not do this but will simply hold the airway pressure at the pressure limit, resulting in a smaller reduction in tidal volume.
Q3. How is airway pressure related to alveolar pressure?
airway pressure = flow x resistance + alveolar pressure
Thus if flow or resistance is markedly altered, a change in airway pressure will not be indicative of a change in the alveolar pressure.
Airway pressure is more conveniently measured than alveolar pressure. Peak inspiratory pressure (PIP) is displayed on most ventilators.
A maximum acceptable PIP of <35 cmH20 is widely used.
Q4. How can alveolar pressure be estimated?
Alveolar pressure is estimated by determining the inspiratory pause pressure, which corresponds to the plateau pressure.
The inspiratory pause pressure is determined by observing the plateau pressure in an apneic ventilated patient when when the ‘inspiratory pause hold‘ control is activated. Because flow is reduced to zero, airway pressure and alveolar pressures will equalise and the airway pressure will correspond to the alveolar pressure at full inspiration.
airway pressure = 0 x resistance + alveolar pressure = alveolar pressure
Q5. What is an excessive alveolar pressure?
To prevent lung injury, alveolar pressure (aka the plateau pressure) should be kept <30 cmH2O.
High alveolar pressures can be due to excessive tidal volume, gas trapping, PEEP or low compliance as shown by this relationship:
alveolar pressure = (volume/ compliance) + PEEP
Q6. What are the two main categories of causes of high airway pressure?
As always, when it comes to ventilated patients you need to think:
Man versus Machine
Q7. What is the most useful first step in isolating the cause of high airway pressures?
Disconnect the patient from the ventilator and manually bag the patient using high-flow oxygen.
If the patient is difficult to ventilate the problem is not with the ventilator or the circuit, but the patient or endotracheal tube.
That’s right — deja vu all over again!
Q8. What are the possible causes of high airway pressures in the intubated and ventilated patient?
First, let’s consider the machine:
- inappropriate settings
- ventilator malfunction
- pooling of condensed water vapour
- wet filters causing increased resistance
- Endotracheal tube
- displacement, e.g. endobronchial intubation
- obstruction with foreign material
And now the man:
- bronchospasm (e.g. asthma)
- decreased compliance
- lung (e.g. collapse, consolidation, pulmonary edema)
- pleural (e.g. pneumothorax, pleural effusion)
- chest wall (e.g. abdominal distention, kyhposcoliosis, obesity)
- patient-ventilator dysynchrony, coughing
Q9. How will you go about identifying the cause of the high airway pressure?
The key steps are:
- While the patient is disconnected from the ventilator assess the “feel” of the lungs while bagging:
- If the patient is not difficult to ventilate then the problem is with the ventilator or the circuit.
- If the patient is difficult to ventilate it is a problem with the endotracheal tube or the respiratory system.
- For ventilator and circuit problems:
- check the ventilator settings that it is functioning correctly (e.g. on a test lung)
- check the circuit for obstruction or kinking.
- For patient or ETT problems:
- examine the patient looking for:
- asymmetrical chest expansion
- evidence of collapse
- evidence of dyssynchrony
- Pass a suction catheter through the ETT to check its patency.
- perform a CXR to check ETT position and look for potential causes of decreased compliance.
- examine the patient looking for:
- If the cause is still not clear measure inspiratory pause pressure to estimate alveolar pressure, in addition to the peak inspiratory pressure that reflects airway pressure.
- If both airway and alveolar pressure are high the problem is due to poor compliance (e.g. pulmonary edema).
- If only the airway pressure is high the problem is one of high resistance (e.g. bronchospasm).
Upon assessing the patient you decide that the high airway pressure alarms are due to patient-ventilator dyssynchrony, in addition to ongoing bronchoconstriction.
Look out for an upcoming post on patient-ventilator dyssynchrony….
- Bersten AD, Soni N. Oh’s Intensive Care Manual (6th edition). Butterworth-Heinemann, 2008.
- Gomersall C. ICU Web — Trouble-shooting mechanical ventilation; Asthma.
- Life in the Fast Lane — ICU Mind Maps: Abnormal ventilator waveforms; Acute Severe Asthma
- Weingart S. EMCrit Podcast 16 – Coding Asthmatic, DOPES, & Finger Thoracostomy.