Monday, November 30, 2009

Respiratory Failure

Figure 1

This 57-year old man, who has history of chronic obstructive airway disease complicated with cor pulmonale, who is still an active smoker, was intubated this morning in view of a progressive deteriorating respiratory failure type 2. Figure 1 shows the chest X-ray AFTER intubation, and Figure 2 is the chest X-ray PRIOR to intubation.

Figure 2

This was his ABG results just prior to intubation:
pH 7.14
PCO2 73.3 mmHg
PO2 41.5 mHg
HCO3- 16.7 mmol/l

and this was his second ABG after intubation:
pH 7.10
PCO2 60.9 mmHg
PO2 108 mmHg
HCO3- 14.7 mmol/l

Note: This patient also had a huge lung mass on the left side, currently still be investigated.

The Chinese University of Hong Kong has a concise 2-page notes on Respiratory failure. Click here to download.

To learn more about ABG, I have re-posted a concise lists of formula and steps in interpreting ABG below:

Concept and Formula In Acid Base Equations

A. Metabolic Acidosis

Winter's equation:
Expected pCO2 = 1.5 * [HCO3-] + 8 (+/-2)
If the measured PaCO2 is less than the expected → concurrent respiratory alkalosis
If the measured PaCO2 is more than the expected → concurrent respiratory acidosis

Winter’s rule states that the last 2 digits of pH greater than 7 predicts the PaCO2 in a compensatory respiratory mechanisms
Pt with metabolic acidosis with pH 7.25, the predicted PaCO2 would be 25 mmHg

Delta Gap = Measured AG – normal AG (Normal AG taken as from 3 - 11, average 7)

Delta gap + measured HCO3- MUST BE equal back to the normal HCO3-

If Delta Gap + measured HCO3 more than normal HCO3-, this means there are too much HCO3-, therefore there is co-existing metabolic alkalosis

If Delta Gap + measured HCO3 less than normal HCO3, there is co-existing normal anion gap metabolic acidosis (besides the wide anion gap metabolic acidosis)

The presence of very high AG (more than 20) suggests wide anion gap metabolic acidosis EVEN IN THE PRESENCE OF NORMAL pH!!!

Bicarbonate therapy

Return blood pH to 7.2
↑ plasma bicarbonate 8 to 10 mmol/l
Sodium bicarb needed = (body weight * [target – measured HCO3-] * 0.5) mmol/l

B. Metabolic Alkalosis
Expected PCO2 would be
(0.6 * [HCO3- - 24] + 40) mmHg
If measured PCO2 is less than expected → concurrent respiratory alkalosis
If measured PCO2 is more than expected → concurrent respiratory acidosis

C. Respiratory Acidosis/Alkalosis
In acute setting,
For every 10 mmHg change of PCO2
[HCO3-] changes 1 – 2 mmol/l and
pH changes 0.08

In chronic setting,
For every 10 mmHg change of PCO2
[HCO3-] changes 4 – 5 mmol/l and
pH changes 0.03

D. Partial Pressure of Arterial Oxygen
Predicted PaO2 that should be achieved = FiO2 * 6 (FiO2 in percentage; e.g. FiO2 of 1.0 is 100%; therefore expected PaO2 would be 100*6 = 600)

Predicted PaO2 (in supine position) = [100 – 1/3 * (age in years)]

pH of blood
A rise or fall of pH by 0.10 results in PaO2 falls or rise (opposite direction) by 10%

E. PaO2/FiO2 Ratio
A quick way to estimate impairment of oxygenation
Normal PaO2/FiO2 ratio is 500 – 600 (when FiO2 is expressed in decimal)
Example: in PaO2 80mmHg, FiO2 40%, then PaO2/FiO2 ratio is 80/0.4= 200

PaO2/FiO2 is 300 – indicates ALI
PaO2/FiO2 is 200 – indicates ARDS

F. A-a Gradient
A-a gradient = PAO2 - PaO2
Therefore, A-a gradient = (713*FiO2) - (PaCO2*1.25) - PaO2
Normal A-a gradient = 10 – 20 mmHg
Normal A-a gradient for the particular age = (Age/4 + 4)
High A-a gradient in V/Q mismatch, Rt to Lt shunt, diffusion abnormalities

Adrogue, H. J. and Madias, N. E. (1998). Management of life-threatening acid-base disorders. First of two parts. N Engl J Med, 338(1), 26-34.

Leong, B. and Lee, K. W. (2004) In Guide To The Essentials In Emergency Medicine (Eds, Ooi, S. and Manning, P.) McGraw-Hill, Singapore, pp. 158-68.

Click here for an excellent tutorial on ABG.

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