A 73 year old male brought in as a priority 1 for respiratory distress. He has a 1 week history of shortness of breath and had a collapse today. The patient has a past history of IHD and a valve replacement and CCF. The patient is on FiO2 0.7
ABG as below
pH 7.191 Na 134 mmol/l
pCO2 85mmHg K 4.9 mmol/l
pO2 70mmHg Cl 90 mmol/l
HCO3 29 mmol/l Glu 9.6mmol/l
Lactate 2.2 Cr 93umol/l
- Describe and interpret the ABG
- What is the A-a gradient and what are causes of a raised A-a gradient?
Answer
pH 7.19 severe acidaemia
pCO2 raised – respiratory acidosis
Compensation – no given history of COPD – expected HCO3 – for every 10 increase in CO2, there is a increase of 1 HCO3 = 28.5 mmol/l
A-a gradient:
Expected for age = age/4 + 4 =22
PAO2 = {FiO2 x (Patm-Ph2o)} -PCO2 x 1.25 = 393
PAO2 – PaO2 = 323
The patient has a compensated acute respiratory acidosis with a raised A-a gradient. A raised CO2 is indicative of decreased ventilation. In this clinical context with no further information likely differentials would be hypoventilation secondary to head trauma, CVA, drugs leading to reduced RR or airway obstruction. Another possible cause could be underlying COPD with an acute exacerbation, or any other cause of obstruction to the airways – asthma, FB, oedema. Below DDx for the raised A-a gradient in this case.
The A-a gradient, measures the difference between the oxygen concentration in the alveoli and arterial system. The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia. Causes of raised A-a gradient include diffusion disorders (eg interstitial lung disease), V/Q mismatch (eg PE, mucous plugging, pneumonia, atelectasis) and shunting (eg anatomical – TOF. Physiological – atelectasis, consolidation)
NOTE: Hypoventalation alone will decrease PAO2 and cause hypoxia, and A-a gradient will be normal unless there is another underlying process causing shunting, v/q mismatch or diffusion disorder.