Intubation & Ventilation Scenario 9

Failure to Oxygenate: acute lung injury (ARDS)

Solution

The combination of acute confusion and hypoxemia following multiple long bone fractures is almost pathgnomic of fat embolus syndrome. The lung injury is similar to other causes of ARDS – damage to the pulmonary capillary-alveolar interface leading to flooding of the alveoli and interstitium with protein rich exudates. The pathology is a combination of a diffusion defect and ventilation perfusion mismatch.

In the diagram above you can see that ventilation-perfusion mismatches exist along a spectrum of either completely wasted ventilation or perfusion without ventilation. In most situations the V/Q mismatch exists somewhere between these two ends, usually in the low V/Q territory. It is important to understand that mismatches do not necessarily occur during both phases of the respiratory cycle - the main timing of mismatch is during expiration, when vulnerable alveoli collapse en masse, in acute lung injury. If you run the mouse over the picture below you can see how this occurs:

The initial treatment for this patient is humidified oxygen to return to PaO2 to 60mmHg:
How much oxygen do you think he will need? Click here for solution.

If this patients clinical condition worsens - progressive hypoxemia or fatigue, then he will require positive pressure ventilation.

The treatment of choice for acute lung injury is positive end expiratory pressure -PEEP, because the fundamental problem is loss of lung volume and reduction in functional residual capacity (FRC). PEEP restores FRC, and improves overall gas exchange. If the work of breathing remains high in spite of this, additional support is added in inspiration, such as pressure support (click here for more information).

The cartoon below demonstrates what happens to vulnerable alveoli when PEEP is applied: run your mouse over the picture to see the effect of PEEP.