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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


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


Hypoventilation

CNS dz, neuropathy, NMJ-opathy, myopathy

Airway obstruction




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




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