ABG

Case Study No. 2
60 y/o male comes ER c/o SOB. Tachypneic, tachycardic, diaphoretic and Cyanotic. Dx acute resp. failure and ABG’s Show PaCO2 very high, low pH and PaO2 is moderately low. The blood gas document Resp. failure due to primarily ventilatory insufficiency.
Metabolic Acidosis
failure of kidney function blood HCO3 which results in availability of renal tubular HCO3 for H+ excretion pH < 7.35 HCO3 < 22
salicylate overdose
Metabolic Alkalosis
plasma bicarbonate pH > 7.45 HCO3 > 26
Causes of Metabolic Alkalosis
loss acid from stomach or kidney hypokalemia excessive alkali intake
Helps differentiate oxygen deficiencies from primary ventilatory deficiencies from primary metabolic acid-base abnormalities
What Is An ABG?
pH [H+] PCO2 Partial pressure CO2 PO2 Partial pressure O2
3. PCO2
4. HCO3
Four-step ABG Interpretation
Step 1: Examine PaO2 & SaO2 Determine oxygen status Low PaO2 (<80 mmHg) & SaO2 means hypoxia
NL/elevated oxygen means adequate oxygenation
pH < 7.35 PCO2 > 45
CO2 + H2CO3 pH
Causes of Respiratory Acidosis
emphysema
drug overdose narcosis respiratory arrest airway obstruction
metabolic irregularity if HCO3 abnl & PaCO2 NL
Four-step ABG Interpretation
Step 4: Determine if there is a compensatory mechanism working to try to cortudy No. 1
60 y/o male comes ER c/o SOB. Tachypneic, tachycardic, diaphoretic and Cyanotic. Dx acute resp. failure and ABG’s Show PaCO2 well below nl, pH above nl, PaO2 is very low. The blood gas document Resp. failure due to primary O2 problem.
Respiratory Component
function of the lungs
Carbonic acid H2CO3
Approximately 98% normal metabolites are in the form of CO2 CO2 + H2O H2CO3 excess CO2 exhaled by the lungs
ABG INTERPRETATION
Debbie Sander PAS-II
Objectives
What’s an ABG? Understanding Acid/Base Relationship General approach to ABG Interpretation Clinical causes Abnormal ABG’s Case studies
Metabolic Component
Function of the kidneys
base bicarbonate Na HCO3
Process of kidneys excreting H+ into the urine and reabsorbing HCO3- into the blood from the renal tubules 1) active exchange Na+ for H+ between the tubular cells and glomerular filtrate 2) carbonic anhydrase is an enzyme that accelerates hydration/dehydration CO2 in renal epithelial cells
PCO2, H2CO3 insufficiency = pH
pH > 7.45 PCO2 < 35
Causes of Respiratory Alkalosis
hyperventilation panic d/o pain pregnancy acute anemia
Respiratory Alkalosis
pH PaCO2 HCO3 7.50 30 22
Metabolic Acidosis
pH PaCO2 HCO3 7.30 40 15
Metabolic Alkalosis
pH PCO2 HCO3 7.50 40 30
What are the compensations?
HCO3 Bicarbonate BE SaO2 Base excess Oxygen Saturation
Acid/Base Relationship
This relationship is critical for homeostasis Significant deviations from normal pH ranges are poorly tolerated and may be life threatening Achieved by Respiratory and Renal systems
How to Analyze an ABG
1. PO2 2. pH NL = 80 – 100 mmHg NL = 7.35 – 7.45 Acidotic <7.35 Alkalotic >7.45 NL = 35 – 45 mmHg Acidotic >45 Alkalotic <35 NL = 22 – 26 mmol/L Acidotic < 22 Alkalotic > 26
CO2 Expected
CO2 Less Abnormal
Metabolic
Metabolic
Acidosis
Compensated Metabolic Acidosis
Compensated Respiratory Acidosis
Respiratory Acidosis
Mixed Respiratory Metabolic Acidosis
ABG Interpretation
Alkalosis CO2 Change c/w Abnormality CO2 Normal CO2 Change opposes Abnormality
CO2 More Abnormal
CO2 Expected
CO2 Less Abnormal
Metabolic Alkalosis
Buffers
There are two buffers that work in pairs
H2CO3 Carbonic acid
NaHCO3 base bicarbonate
These buffers are linked to the respiratory and renal compensatory system
2
80 60 40 30
7.20 7.30 7.40 7.50
20
7.60
ABG Interpretation
Acidosis CO2 Change c/w Abnormality CO2 Normal CO2 Change opposes Abnormality
CO2 More Abnormal
Compensated Metabolic Alkalosis
Compensated Respiratory Alkalosis
Respiratory Alkalosis
Mixed Respiratory Metabolic Alkalosis
Respiratory Acidosis
pH PaCO2 HCO3 7.30 60 26
Acid/Base Relationship
H2O + CO2

H2CO3
HCO3 + H+
Normal ABG values
pH PCO2 7.35 – 7.45 35 – 45 mmHg
PO2
HCO3
80 – 100 mmHg
22 – 26 mmol/L
BE
SaO2
-2 - +2
>95%
Four-step ABG Interpretation
Step 2: pH acidosis alkalosis <7.35 >7.45
Four-step ABG Interpretation
Step 3: study PaCO2 & HCO 3
respiratory irregularity if PaCO2 abnl & HCO3 NL
Acidosis
pH < 7.35
Alkalosis
pH > 7.45
PCO2 > 45 HCO3 < 22
PCO2 < 35
HCO3 > 26
Respiratory Acidosis
Think of CO2 as an acid
failure of the lungs to exhale adequate CO2
Take home
What is an ABG
Arterial Blood Gas Drawn from artery- radial, brachial, femoral
It is an invasive procedure.
Caution must be taken with patient on anticoagulants.
Causes of Metabolic Acidosis
renal failure
diabetic ketoacidosis
lactic acidosis
excessive diarrhea cardiac arrest
Respiratory Alkalosis
too much CO2 exhaled (hyperventilation)
Respiratory acidosis Respiratory alkalosis metabolic alkalosis metabolic acidosis
In respiratory conditions, therefore, the kidneys will attempt to compensate and visa versa. In chronic respiratory acidosis (COPD) the kidneys increase the elimination of H+ and absorb more HCO3. The ABG will Show NL pH, CO2 and HCO3. Buffers kick in within minutes. Respiratory compensation is rapid and starts within minutes and complete within 24 hours. Kidney compensation takes hours and up to 5 days.
ie: if have primary respiratory acidosis will have increased PaCO2 and decreased pH. Compensation occurs when the kidneys retain HCO3.
~ PaCO – pH Relationship