PH = 6.88, that is severe acidaemia
HCO3= 5.3 mmol/L. So, we have metabolic acidosis.
Next step is to calculate the anion gap to know which type of metabolic acidosis and we need to calculate the respiratory compensation.
Anion gap = Na – (Cl + HCO3) = 137 – (112 + 5.3) = 19.7, so we have HAGMA.
To calculate the compensation, we use Winter’s formula, that is Expected CO2=1.5xHCO3+8 (+/-2) = 15.95 (Range 14-18). This patient’s pCO2 is 30 mmHg. According to this the patient had additional respiratory acidosis.
Because we have HAGMA we need to calculate the delta ratio. That is calculated as Delta Ratio = (AG – 12)/24 – HCO3 = 0.41. Delta ration of 0.4 to 0.8 means we have combined HAGMA and NAGMA.
This patient has combined HAGMA, NAGMA and respiratory acidosis.
Other abnormal findings for this patient.
The most significant abnormality is Hb level of 66 g/L. This is severe anaemia.
Cl = 112 mmol/L that is hyperchloraemia.
Glucose = 11 mmol/L, that is hyperglycemia.
Lactate = 15 mmol/L. that is massively elevated.
Creatinine = 313 umol/L. If this is a chronic condition, then that will give this patient eGFR level of 15. This means stage 4 chronic kidney disease.
Next we need to look at the potential causes of this patient blood gases changes.
For the differential diagnosis of HAGMA we use the mnemonic CAT MUDPILES
- C = cyanide, carbon monoxide
- A = alcoholic ketoacidosis and starvation Ketoacidosis.
- T = toluene
- M = methanol, metformin
- U = uraemia
- D = diabetic ketoacidosis
- P = phenformin, pyroglutamic acid, paraldehyde, propylene glycol, paracetamol
- I = iron, isoniazid
- L = lactate
- E = ethanol, ethylene glycol
- S = salicylates
From the list above, the causes of this patient’s HAGMA are hyperlactataemia and potentially uraemia (Urea level is not provided, however Creatinine level is high).
For the differential diagnosis of NAGMA, we use the mnemonic USED CARP
- U = Ureteroenterostomy.
- S = Small bowel fistula.
- E = Extra chloride.
- D = Diarrhea.
- C = Carbonic anhydrase inhibitors.
- A = Adrenal insufficiency/ Addison’s disease.
- R = Renal tubular acidosis.
- P = Pancreatic fistula
From the list above, RTA is the most probable cause, it usually presents as hyperchloraemic NAGMA, and this is secondary to acute or chronic kidney disease.
For the causes of respiratory acidosis, this can be caused by:
Inadequate alveolar ventilation
- Central – drugs, trauma, haemorrhage, tumour, spinal trauma, tetanus
- Nerve / muscle – GBS, MGravis, muscle relaxants, toxins (organoPO4, snake venom), myopathies, diaphragm injury/ paralysis
- Lung/ Chest wall – COPD, trauma (flail, contusion, haemothorax), pneumothorax, pulmonary oedema, ARDS, restrictive lung disease, aspiration
- Airway – upper airway obstruction, laryngospasm, asthma (bronchospasm)
- External – inadequate mechanical ventilation
- Malignant hyperthermia
- Iatrogenic – rebreathing CO2, addition of CO2 to inspired gas, laparoscopic surgery
Most likely the cause of this patient respiratory acidosis is lung related.
These blood gases are arterial, then we can rely on pO2 level. Expected pO2 level is 4 to 5 times the inspired O2. This patient is on 3L O2, that is equivalent to FiO2 of 32%. Accordingly, this patient’s pO2 is within expected level. (32×4=128).
We can calculate the A-a gradient for this patient.
pAO2 is calculated as 713 x FiO2 – pCO2/0.8 = 190.
A-a gradient for this patient = 190 – 124 = 66. this is elevated. The expected A-a gradient is calculated as (Age in years/4 + 4) = 25 for this patient.
Because of extremely low HB level, this patient was started on the massive transfusion protocol. This patient had spontaneous massive psoas muscle haemorrhage secondary to acquired factor 8 deficiency.
Blood gases and lactate levels gradually normalised.
That patient recovered well after correcting the causes and he was discharged home after 1 week.