1. BLOOD GAS INTERPRETATION
Josée Gaudreault, APN
Montreal Children’s Hospital
McGill University Health Center

2. Why do we do blood gases?

3. Why do we do blood gases? Assessment of oxygenation
Assessment of ventilation
Assessment of acid-base balance

4. What’s NOT a good reason to do a blood gas?

5. When not to get a blood gas…
When results won’t change the treatment
Clinical status prevails
Just because the policy says to (ie. Blood gas Qam)
When pulse oxymetry would give the same information
When there is a risk of complications
Bleeding

6. Few definitions Equilibrium is achieved at a normal pH Acid Base
substance able to give up hydrogen ions during chemical exchanges
Base
substance able of accepting hydrogen ions during chemical exchanges

7. Types of BG Venous Capillary Arterial No oxygen value
Affected by local circulatory and metabolic environment
Capillary
Accurate pCO2 and pH
Avoid excessive squeezing – why?
Free flowing blood.
Arterial
Gold standard

8. Information from BG pH
pCO2
 pO2
Bicarbonate HCO3
 Lytes, glucose

9. pH - log of [H+] ; logarithmic relationship (not linear)
Small change in [H+ ] = fatal
Low pH = acidic solution, high pH = basic solution
pH, pCO2 and HCO3 give information about effectiveness of regulatory mechanisms of lungs and kidneys

11. determine the primary problem!!!pH
pH, pCO2 & HCO3
determine the primary problem!!!

12. pO2 O2 is carried by Hgb and plasma
paO2 = Measure of how much oxygen is delivered to the tissues
Represents oxygenation; regulated by lungs
Can be correlated with oxygen saturation as measured by pulse oximeter

13. pCO2 Represents ventilation; regulated by lungs
By-products of cells metabolism
CO2 is a gas that diffuses easily in the blood
CO2 accumulates in blood when ventilation is impaired
Comes from acid (carbonic acid) in blood

14. Bicarbonate (HCO3-) Base
Most important buffer in blood (can accept 2 H+ for every molecule of HCO3)
Controlled at the level of the kidneys
Keeps pH within a tight range
GI losses below the pylorus = loss of bicarbonate

15. Normal blood gas values
ABG
Normal value pH
pO2
80-100
pCO2
35-45
HCO3-
22-26
Base excess
-2 to +2

16. Definitions Acidosis Alkalosis
relative increase in H+ or a deficit of bicarbonate
Alkalosis
relative excess of bicarbonate or a deficit of H+

17. Back to the buffer systems
Lungs regulate pCO2 by  resp rate. It is a quick response
2. Kidneys excrete H+, bicarbonate, and ammonium (NH4+). It is a slow compensatory mechanism

18. Compensation
Restoration of pH to near normal levels with resp or renal buffer system
Never results in overcorrection
Example: respiratory acidosis pH 7.30, CO2 60: when compensation occurs from the kidney, pH stays on the acidotic side  7.40

19. Acid-Base Imbalances: Four conditions
Respiratory
acidosis
alkalosis
Metabolic

20. Respiratory acidosis
Failure of alveolar exchange of CO2 (CO2 in blood, low pH)
Causes:
Respiratory conditions
Central nervous system depression
Obstructive or restrictive
Pain
Extreme obesity
Impaired respiratory muscles
Respiratory arrest/prolonged hypoxia

21. Respiratory alkalosis
Hyperventilation causes CO2 in blood, high pH
Causes:
CNS disorders
drug toxicity
anxiety
fever
pain
mechanical ventilation

22. Metabolic acidosis Too many acids Not enough bases exogenous sources
3 causes
Too many acids
exogenous sources
metabolic by-products
Not enough bases

23. Common causes: metabolic acidosis
Diarrhea (loss of HCO3- from bile salts in the stool)
 H+ in dehydration (hemoconcentration)
Administration of solution with high content of chloride (eg NaCl) (Cl- exchanges with HCO3- in proximal tubules >> more Cl out of cell = more HCO3 in)
Renal failure – kidneys cannot resorb bicarb
DKA
Infection - sepsis - septic shock – why?

24. Metabolic alkalosis Causes
Too much base from meds, accumulation of organic acid
Loss of H+ activates H+/K+ exchange pump (H+ out of cell, K+ into cell) – can lead to hypokalemia
Same thing goes other way – loss of K+ (thiazide diuretics, furosemide, losses above pylorus) activates H+/K+ pump (K+ out of cell for H+ into cell) >> alkalosis
Compensation results in hypoventilation in body’s attempt to retain CO2

26. Blood gas interpretation
First, is there a problem? Any number that is out of range?
Second, does the patient have an acidosis or an alkalosis? Is the pH less than 7.4 or more than 7.4?
Third, what is the primary problem – metabolic or respiratory? – Look at source. Is it your PCO2 that is out of range, or your HCO3?
Third, is there any compensation by the patient – respiratory compensation is immediate while renal compensation takes time. If pH is within range of 7.35 – 7.45, but other numbers are out of range, compensation has occurred.
Fourth, evaluate the effectiveness of oxygenation (only for arterial).

27. Few exercises…

28. Case 1
Jonathan (3yo) got into some of grand-ma’s barbiturates. He is found sleeping and unarousable. His respiration is very shallow. You see him in the ER 3 hours after ingestion with a respiratory rate of 4.
First intervention???
Blood gas
pH = 7.16
pCO2 = 70
HCO3 = 22

29. Case 1 What is the acid/base abnormality?
Uncompensated metabolic acidosis
Compensated respiratory acidosis
Uncompensated respiratory acidosis
Compensated metabolic alkalosis

30. Case 2
Julie has had vomiting and diarrhea for 3 days. Mom says: “She can’t keep anything down.” She has had 1 wet diaper in the last 24 hours. She appears lethargic and cool to touch with a prolonged capillary refill time. Priorities: ABC pH=7.34, pCO2=26, HCO3=12

31. Case 2 What is the acid/base abnormality?
Uncompensated metabolic acidosis
Compensated respiratory alkalosis
Uncompensated respiratory acidosis
Compensated metabolic acidosis

32. Case 3
A 10 year old female is brought to the ER by ambulance from school because of abdominal pain and vomiting. She has recently lost 10 lbs over the past 2 months and has polyuria for the past 2 weeks. She is alert and oriented, afebrile, HR 115, RR 26 and regular, BP 114/75, pulse ox 95% on RA.

33. Case 3
pH = 7.21 pCO2= 24 pO2 = 45 HCO3 = 10 BE = -10

34. Case 3 What is the blood gas interpretation?
Uncompensated respiratory acidosis with severe hypoxia
Uncompensated metabolic alkalosis
Combined metabolic acidosis and respiratory acidosis with severe hypoxia
Metabolic acidosis with some respiratory compensation

35. Case 4
6 week old girl presents to the ER
pO2 48
pH 6.9
pCO2 75
Bic 8

36. Case 4 Uncompensated respiratory acidosis
Compensated respiratory alkalosis
Uncompensated metabolic acidosis
Compensated metabolic acidosis

37. Case 5
Known asthmatic 4yo, 2 hospitalisations in the ICU in last year, sick for 3 days, URTI symptoms, had 4 ventolin at home, no improvement. To ER:
What are the vitals?
What findings are you expecting from the BG?

38. Case 5
Symptoms
RR 30 per minute
HR 150
Indrawing +++
Able to talk

39. Case 5 BG
pH 7.39
pCO2 32
Bic 23
Comments:

40. Evolution Case 5
He receives ventolin back to back with no improvement.
PICU is consulted 2 hrs post presentation
Symptoms
RR 35, HR 155
Not able to talk
Marked indrawing
BG?

41. Evolution Case 5 BG Decision is made to admit to PICU WHY??
pH 7.35, pCO2 44, Bic 22
Decision is made to admit to PICU
WHY??

42. Last one!!!
pH 7.38
pCO2 51
Bic 31
What is that ???
Causes ???

43. Merci!!!

44. Base excess
Calculated number (mEq/L) of acid needed to get to equilibrium (pH of 7.4) when respiratory factors are taken out of the equation
How much base is required to correct the arterial pH to 7.4 if pCO2 = 40 ?
Base deficit vs base excess