A 58-year-old female ventilated in intensive care for a week following a motor vehicle accident was noted to drop her oxygen saturation suddenly, requiring an increase in FiO2 from 0.4 to 0.6.
The nursing staff has performed an arterial blood gas.
|
Parameter |
Patient Value |
Normal Adult Range |
|
|
FiO2 |
0.6 |
||
|
pH |
7.48* |
7.36 – 7.44 |
|
|
PCO2 |
41 mmHg (5.4 KPa) |
35 – 45 (4.6 – 6.0) |
|
|
PO2 |
86 mmHg (11.3 kPa) |
||
Ventilator data:
Tidal Volume 700 mL
Respiratory rate 14 breaths/min
Peak pressures 28 cm H2O
Plateau pressures 18 cm H2O
PEEP 7.5 cm H2O
SpO2 94%
EtCO2 28 mmHg
What is the most likely diagnosis? Give the reasons for your diagnosis.
College Answer
The most likely diagnosis is a pulmonary embolus.
The reasons are as follows:
Sudden onset of hypoxemia raises a number of possibilities – mucus plugging, pneumothorax, LVF, aspiration etc. However, the ventilation data indicate preserved compliance, normal peak pressures (argue against a pneumothorax or plugging or LVF) and there is increased dead space, (raised A-et CO2 gradient)
Discussion
There are numerous possible causes for "sudden onset hypoxemia" in a trauma patient recovering from surgery. The college practically give this one away by offering the candidate an end-tidal CO2 measurement, which is substantially lower than the arterial CO2 measurement, suggesting that there is a large area of lung which is not participating in gas exchange, i.e. it is dead space. Capnometry and the arterial-expired carbon dioxide gradient is discussed elsewhere.
Of course, one should still go though the motions of calculating the A-a gradient.
PAO2 = 0.6 × (760 - 47) - (PaCO2 × 1.25) = 376.55;
thus, A-a = 290.55