Hopefully you're here because you've just finished reading Blood Gases (the theory)- part one covering A and B, and part two covering C and D. If not, I'd really suggest reading it (they're long, but they're good. I know I'm biased).
In this (much shorter) post, I'm going to share some example blood gases so that you can have a go at putting the theory into practice. These are a combination of anonymised patient results, and my own personal results from when I've been in hospital and asked to see my results (don't ask...)
So, without further ado.....
QUESTION 1
Is this pH normal? Is it a metabolic or respiratory problem? Is there any compensation?
ANSWER- pH is low (acidotic). PCO2 is high (acidotic). HCO3 is high (alkalotic). Therefore, this is a partially compensated respiratory acidosis. HCO3 has not increased enough to return pH to normal so not fully compensated.
Is there any degree of respiratory failure? If yes, type 1 or type 2?
ANSWER- no signs of respiratory failure! PO2 is above 8kPa, so no hypoxia. There is hypercapnia (seen in T2RF), but not alongside hypoxia.
What could be a possible cause for this respiratory acidosis?
ANSWER- any kind of hypoventilation really. Could be a chronic problem, such as COPD with chronic CO2 retention and acidosis, or could be an acute problem with any kind of respiratory/CNS depression.
This patient was admitted following an overdose and was still very drowsy (GCS 6/15) so was not achieving a tidal volume sufficient to clear CO2, leading to acidosis. Hypercapnia was likely contributing to somnolence, which contributes to further hypoventilation, As GCS recovered, the CO2 dropped and the acidosis resolved.
QUESTION 2
Does this patient require a blood transfusion? What criteria could they meet to have a higher transfusion trigger than usual?
ANSWER- no need for a transfusion here. Usual trigger is 70, with a higher trigger of 80 for oncology patients, trauma patients, patient in severe septic shock, and cardiac patients with unstable ischaemic symptoms.
This patient had a significant cardiac background of coronary disease and Hx of MI, but had no current unstable ischaemic changes (high troponin or unstable angina) so Hb trigger remained at 70.
Does this haematocrit indicate dehydration? What actions would you take if so?
ANSWER- no action required, haematocrit is not high so unlikely to be dehydrated. If yes (so high haematocrit), give fluid bolus and recheck
What other parameter on a blood gas could indicate a need for a fluid bolus, and why?
ANSWER- lactate. High lactate is often seen in sepsis and can be due to anaerobic respiration of cells- therefore signifies oxygen not reaching cells due to hypovolaemia in shocked state (shock = state of inadequate tissue perfusion) so giving a fluid bolus increases intravascular volume to maintain perfusion.
QUESTION 3
This patient has an insulin infusion running- which readings could be affected by this?
ANSWER- blood glucose and potassium. Insulin alters blood sugar by enabling metabolism of glucose and movement into cells. Potassium is affected as insulin makes it move into the intravascular space, so serum potassium will drop.
This patient was a someone who had an SKPT (simultaneous kidney pancreas transplant) but the kidney had become infected and pancreatic graft started failing. So insulin was started to keep blood sugars within range, and lactate was slightly high due to the infection. She was on a filter to help with potassium clearance- important to monitor calcium as using citrate anticoagulation.
On your next gas, blood glucose has increased to 19.2. What actions would you take?
ANSWER- check ketones, run another gas to establish pH- rule out DKA. Increase the insulin infusion as per local policy to lower blood glucose to target range (4-8mmol/L). Monitor potassium if increasing insulin.
QUESTION 4
From this blood gas, identify a problem list and rank it from most to least important.
ANSWER- (open to debate)
pH acidotic (7.25)- high PCO2 (8.41), so respiratory acidosis
Lactate high (3.1) ?infection, ?generally unwell
Blood glucose high (21.1), ?is patient diabetic
Haematocrit high (48.8%), ?dehydration
HCO3- low (21.5), not compensating for acidosis
PO2 low (2.97), venous gas
electrolytes WNL (within normal limits)
What actions could you take in response to these issues?
ANSWER-
pH/PCO2- consider I+V to control minute volume. Increase tidal volume or respiratory rate to clear CO2. Give sodium bicarb 8,4% as HCO3 not compensating.
Lactate and haematocrit- fluid bolus. Improve intravascular volume, improve tissue perfusion.
Blood glucose- Check ketones, rule out DKA. Start insulin infusion
QUESTION 5
This is your repeat gas for the same patient as above. Identify your new problem list.
ANSWER-
Potassium low (3.0)
Blood sugar drop (21.1 -> 4.7)
Hct still high (44.3%)
What actions would you take in response to this problem list?
ANSWER-
Reduce insulin (if diabetic- should not stop infusion as can cause DKA) or stop insulin (if not diabetic) to avoid hypoglycaemia. Give long acting carbohydrate (if E+D) to prevent hypoglycaemia.
Consider continuous cardiac monitor for hypokalaemia. Start fluids containing KCl 0.3% via large vein or high concentration 20mmol/L in 50mL KCl if central access present.
Another fluid bolus for Hct
The above patient had taken an insulin overdose- hence the drop in blood glucose and the low potassium.
When you have a physical gas report in your hand, it can be useful to annotate it somehow. Either circle abnormal results, tick normal values or use up and down arrows to work out your pH- whatever you choose, physically marking it off can help those results to stick in your head and help you to develop a problem list, and then create a management plan.
And that concludes everything I have on blood gases! I hope this little series has been interesting- I've definitely enjoyed making it. If you want to look back over the theory, part one (PO2, pH, PCO2, HCO3, BE) is available here, and part two (Hb, Hct, Na+, K+, Cl-, Ca2+, glucose) is available here .
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