bg analysis
DESCRIPTION
Blood gas analysis for veterinariansTRANSCRIPT
-
Blood Gas Analysis Acid-base Melissa Claus, Lecturer in Emergency and Critical Care
-
Objectives
Know info obtained with a blood gas and when to perform one
Know how the sample is collected
Interpret acid-base abnormalities
Calculate the anion gap
Provide ddxs for acid-base abnormalities
-
Whats measured or calculated? Acid-base parameters
pH
PCO2 Bicarbonate
Base Excess
Ventilation parameters PCO2
Pulmonary function parameters Arterial blood gas ONLY
PaO2 SaO2
Electrolytes, glucose, lactate
-
When is it indicated?
Hospitalized patients
Electrolytes
Acid-base status
Pulmonary function
Lactate
Anesthetized patients
Ventilation
Pulmonary function
-
Arterial
Dorsal metatarsal artery
Femoral artery
Lingual artery
Auricular artery
Catheter placement
Venous
Central venous catheter
Jugular vein
Any peripheral vein
Arterial or venous sampling
-
Interpret the Results
Youve collected your sample.
The machine has provided numbers.
Now you have to
-
Acids and Bases: Definitions
Acid = proton donor (HA)
Base = proton acceptor (A-)
pKa = pH at which acid is 50% dissociated in an aqueous solution
Depicts the strength of the acid
Low pKa (
-
Acids in the body
Volatile acid
CO2 (can form H2CO3)
Balance maintained by ventilation
Non-volatile acid = noncarbonic acids
All acids other than H2CO3
Phosphoric acid, sulfuric acid
Lactic acid, ketoacids
Toxins (e.g. ethylene glycol metabolites)
Balance maintained by excretion/retention (kidney) or metabolism to CO2 and H2O
-
Buffers
Resist change when HA or A- are added
Weak acids, pKa within 1 unit of blood pH
Scavenge H+ or OH-
Essential to life
-
Buffers in the body
Carbonic acid/bicarbonate system = open
Others:
Hemoglobin
Albumin
Phosphate
Bone
HCO3- + H
+ H2CO3 CO2 + H2O
Carbonic Anhydrase
-
How to assess the acid-base status
Reference Ranges
pH: 7.34-7.39 PvO2: 49-67 PvCO2: 38-46
AG: 8-21 HCO3: 22-24 BE: -2.3 to -0.1
Step 1: Assess pH
Step 2: Assess respiratory contribution
Step 3: Assess metabolic contribution
-
Respiratory contribution
CO2 is a volatile acid controlled by ventilation
Hypoventilation = hypercapnia = acidosis
Hyperventilation = hypocapnia = alkalosis
-
HCO3- + H
+ H2CO3 CO2 + H2O
Metabolic contribution
HCO3-
Primary buffer, regulated by the kidneys
Hypobicarbonemia = acidosis
Hyperbicarbonemia = alkalosis
Also affected by PCO2 (ventilation)
Law of mass action
-
Metabolic contribution
Base Excess
The mmol/L of strong acid or base required to return the plasma to a normal pH (7.4)
PCO2 held constant at 40 mmHg
Temperature held constant at 37 C
Best parameter to use to assess metabolic aspect
Negative BE metabolic acidosis
Positive BE metabolic alkalosis
-
pH Primary Disorder Primary derangement
Compensatory change
pH Metabolic Acidosis HCO3, -BE PCO2
pH Metabolic Alkalosis HCO3, +BE PCO2
pH Respiratory Acidosis PCO2 HCO3, +BE
pH Respiratory Alkalosis PCO2 HCO3, -BE
Compensation for pH changes pH is tightly maintained around normal
Respiratory = minutes
Metabolic = hours to days
NEVER OVERCOMPENSATES
NEVER brings pH to NORMAL
-
Causes of metabolic acidosis Bicarbonate buffers an acid High anion gap
Unmeasured anions: L.U.K.E.
Bicarbonate is lost from the body Normal anion gap, elevated chloride
Diarrhea
Renal tubular acidosis, CAI
Bicarbonate is diluted by Cl-containing solution
Compensation for respiratory alkalosis
Lactic acid
Uremic acids Ketoacids
Ethylene glycol metabolites
-
Anion Gap
Na+
K+
Cl-
HCO3-
Anion Gap
AG = (Na+ + K+) (Cl- + HCO3-)
Unmeasured Anions: L.U.K.E.
Loss of bicarb from GI or kidney = Chloride retention Excess chloride administration
-
Causes of metabolic alkalosis
Gastric acid loss
Pyloric obstruction
Gastric suctioning
Loop diuretics
Bicarb administration
Compensation
for respiratory acidosis
-
Causes of respiratory acidosis
Hypoventilation
Neuromuscular disease
Airway obstruction
Severe abdominal distension
Severe pleural space disease
End-stage pulmonary disease
Rebreathing
Compensation for metabolic alkalosis
Malignant hyperthermia
-
Causes of respiratory alkalosis
Excitement
Exercise
Pain
Pulmonary parenchymal disease
Fever, SIRS/Sepsis
Hypotension
Compensation for metabolic acidosis
-
Putting it all together
1. Assess pH
2. Assess respiratory contribution
3. Assess metabolic contribution
4. Decide which is the primary process
5. Determine if there is compensation
6. OR is this a mixed acid-base disorder?
7. If metabolic acidosis, calculate the AG
8. Differentials?
-
Stimpy
5 year old MC DSH
Straining, vomiting, anorexic, PD for 2 days
Indoors only, no toxins, previously healthy
Physical examination:
Markedly obtunded
HR 100
Firm 10 cm abd structure, painful when palpated
-
Stimpy
1. pH:
2. Resp:
3. Metab:
4. Primary:
5. Compensation:
6. Mixed?
7. AG:
8. Differentials?
Acidemia
Acidosis
Metabolic
Yup
Nope
29.3
Alkalosis
146-157
3.5-4.8
116-126
1.1-1.4
3.7-9.3
0.5-2.0
7.33-7.41
35-45
34-38
12-16
15-21
-9 to -3
Ref Ranges
(Na+ + K
+) (Cl
- + HCO3-)
-
Causes of metabolic acidosis with high AG
Unmeasured anions: L.U.K.E.
Lactic Acid
Uremic Acids
Ketoacids
Ethylene glycol metabolites
3.7-9.3
0.5-2.0
Ref Ranges
Uremia secondary to urethral obstruction
-
Carl
8 year old MC Cocker Spaniel
Found collapsed outside, unresponsive
Spends most of his time at owners car shop
Previously healthy
PE: Comatose, T 36.0, HR 120, RR 15.
-
Carl
1. pH:
2. Resp:
3. Metab:
4. Primary:
5. Compensation:
6. Mixed?
7. AG:
8. Differentials?
140-150
3.9-4.9
109-120
1.2-1.5
3.6-6.2
0.5-2.0
7.34-7.38
49-67
38-42
8-21
22-24
-2.3 0
Ref Ranges
Acidemia
Acidosis
Acidosis
Neither
Nope
YES
22.4
1.4 5.9
0.8
-
Met acidosis with high AG Unmeasured anions: L.U.K.E.
Lactic Acid
Uremic Acids
Ketoacids
Ethylene glycol metabolites
Hypoventilation
CNS dz, neuropathy, NMJ-opathy, myopathy
Airway obstruction
Severe abdominal distension
Severe pleural space disease
End-stage pulmonary disease
Respiratory acidosis
-
Ivan
5 year old M Rottweiler
Acute onset of diarrhea yesterday, persisting through today. Also anorexic and lethargic
PE: T 39.7, HR 120, RR 50. Markedly painful on abdominal palpation. BP 120/80 (100).
-
Ivan
1. pH:
2. Resp:
3. Metab:
4. Primary:
5. Compensation:
6. Mixed?
7. AG:
8. Differentials?
140-150
3.9-4.9
109-120
1.2-1.5
3.6-6.2
0.5-2.0
7.34-7.38
49-67
38-42
8-21
22-24
-2.3 0
Ref Ranges
Normal
Alkalosis
Acidosis
None
Nope
YES
19.4 7.375
7.401
-
Met acidosis with normal AG Bicarbonate has been lost from the body
Diarrhea
Renal tubular acidosis
CAI
Bicarbonate is diluted by Cl-containing solution
Excitement
Exercise
Pain
Pulmonary parenchymal disease
Fever/SIRS/Sepsis
Hypotension
Respiratory alkalosis
-
Millhouse
2 year old M greyhound
2 day history of vomiting, lethargy
No bowel movement in 3 days
Dietary indiscretion is his middle name
6 months ago, surgery for an intestinal F.B.
PE: ~7% dehydrated, mildly painful and very nauseous on palpation of cranial abdomen
-
Millhouse
1. pH:
2. Resp:
3. Metab:
4. Primary:
5. Compensation:
6. Mixed?
7. AG:
8. Differentials?
140-150
3.9-4.9
109-120
1.2-1.5
3.6-6.2
0.5-2.0
7.34-7.38
49-67
38-42
8-21
22-24
-2.3 0
Ref Ranges
Alkalemia
Alkalosis
Metabolic
Yup
Nope
N/A
Acidosis
-
Causes of metabolic alkalosis
Gastric acid loss
Pyloric obstruction
Gastric suctioning
Loop diuretics
Bicarb administration
-
Pearl
2 year old FS Nova Scotia Duck Tolling Retriever
Found sitting in the backyard next to a dead snake. Difficulty rising, wobbly when walking
PE: QAR, RR 45, RE seems shallow. Unable to ambulate weak in all 4 limbs.
-
Pearl
1. pH:
2. Resp:
3. Metab:
4. Primary:
5. Compensation:
6. Mixed?
7. AG:
8. Differentials?
140-150
3.9-4.9
109-120
1.2-1.5
3.6-6.2
0.5-2.0
7.34-7.38
49-67
38-42
8-21
22-24
-2.3 0
Ref Ranges
Acidemia
Acidosis
Normal
Respiratory
Nope
Nope
N/A
-
Causes of respiratory acidosis
Hypoventilation
CNS dz, neuropathy, NMJ-opathy, or myopathy
Airway obstruction
Severe abdominal distension
Severe pleural space disease
End-stage pulmonary disease