arterial blood gas interpretation
TRANSCRIPT
ParameterNormal Range
UnitDefinition
pHH+ concentration= log 1 . [H+]
7.35 – 7.45
Negative log of hydrogen ion activityIf [H+] = 10-7.4 then pH = 7.4
PaO2Partial pressure of O2
80 – 100mm Hg
Partial pressure of oxygen in arterial blood
PaCO2Partial pressure of CO2
35 - 45mm Hg
Partial pressure of CO2 in arterial blood
HCO3-Bicarbonate
Level21 – 27mEq/LBicarbonate plasma concentration
(calculated and not measured)SaO2Oxygen
saturation95 - 100%Percent of oxygen content to maximum
oxygen carrying capacity of bloodFiO2Fraction of
inspired oxygen21%Fraction of oxygen in relation to
inspired airPaO2 with age (= 100 – age in years above 40)
FiO2 at time of ABG sampling should be included in the report.
Arterial Blood Gas AnalysisArterial Blood Gas Analysis(ABG)(ABG)
StepLook atThink: Is there…
1PaO2, PaCO2Respiratory Failure
2PaO2/FiO2
Acute Respiratory Distress Syndrome
)ARDS(
3 - 5pH, PaCO2, Bicarb
Acid/Base Disturbance
6If severe metabolic acidosis ,Calculate IV Na Bicarb required
Respiratory failure is a syndrome of inadequate gas exchange due to dysfunction of one or more essential components of the respiratory system.(Airways, Lungs, Respiratory Muscles, Nerve and Blood Supply)
Step 1)Step 1) Look at PaO Look at PaO22 & PaCO & PaCO22
Is there Respiratory Failure?Is there Respiratory Failure?
TypeNameDefinitionMechanisms,Examples
1Hypoxic RFPaO2 < 60 mm HgShunt, DD, V/Q Mismatch: ARDS, IPF, Pneumonia, PE
2Hypercapnic RFPaCO2 > 46 mm HgHypoventilation:Central and sleep related hypoventilation
3Hypoxic/Hypercapnoeic RF
PaO2 < 60 mm Hg,PaCO2 > 46 mm Hg
Advanced or combined disorders
ARDS Severity PaO2/FiO2 MortalityMild< 30027%Moderate< 20032%Severe< 10045%
Step 2)Step 2) Calculate PaO Calculate PaO22/FiO/FiO22
Is there ARDS?Is there ARDS?Acute Respiratory Distress Syndrome is an acute condition characterized by bilateral pulmonary infiltrates and severe hypoxaemia in absence of evidence for cardiogenic pulmonary oedema (Non-Cardiogenic Pulmonary Oedema).PaO2/FiO2 (Carrico Index) is a quick and simple measure for integrity of lung tissue and its capacity to oxygenate the blood.Normal > 300 – 500 mmHg (at sea level)
PaO2/FiO2 can also indicate the degree of lung tissue injury in other pulmonary disorders, eg Pneumonia
Examples for Calculation of PaO2/FiO2
PaO2FiO2PaO2/FiO2Comment
950.21452Normal
800.5160Moderate ARDS
Two cases of ARDS with bilateral patchy opacities in middle and lower lung zones.
Common Causes: severe infection, aspiration, irritant gases, extensive trauma, multi-organ failure.
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pH pCO2Bicarb
Steps 3-5)Steps 3-5) Look at Look at pH, PaOpH, PaO22, PaCO, PaCO22 Is there Acid/Base Is there Acid/Base Disturbance?Disturbance?
7.337.347.357.367.377.387.397.407.417.427.437.447.457.467.477.487.497.5
7.517.527.537.547.55
7.287.297.307.317.32
7.247.257.267.27
33343536373839404142434445464748495
5152535455
2829303132
24252627
24252627
20212223
16171819
1415
3233
28293031
pH PCO2BicarbSevere Diarrhoea
1ry Defect Compensation
Bicarb(M Ac)
PCO2
(R Alk)
M Ac & Comp R Alk
M Ac & R Ac
M Ac & R Alk
7.337.347.357.367.377.387.397.407.417.427.437.447.457.467.477.487.497.5
7.517.527.537.547.55
7.287.297.307.317.32
7.247.257.267.27
33343536373839404142434445464748495
5152535455
2829303132
24252627
24252627
20212223
16171819
1415
3233
28293031
pH PCO2Bicarb
Persistent Vomiting
1ry Defect Compensation
Bicarb(M Alk)
PCO2
(R Ac)
M Alk & Comp R Ac
M Alk & R Ac
M Alk & R Alk
7.337.347.357.367.377.387.397.407.417.427.437.447.457.467.477.487.497.57.517.527.537.547.55
7.287.297.307.317.32
7.247.257.267.27
333435363738394041424344454647484955152535455
2829303132
24252627
24252627
20212223
16171819
1415
3233
28293031
pH PCO2Bicarb
Hypoventilation
1ry Defect Compensation
PCO2
(R Ac) Bicarb(M Alk)
R Ac & Comp M Alk
R Ac & M Alk
R Ac & M Ac
7.337.347.357.367.377.387.397.407.417.427.437.447.457.467.477.487.497.57.517.527.537.547.55
7.287.297.307.317.32
7.247.257.267.27
333435363738394041424344454647484955152535455
2829303132
24252627
24252627
20212223
16171819
1415
3233
28293031
pH PCO2BicarbHyperventilation
1ry Defect Compensation
PCO2
(R Alk) Bicarb(M Ac)
R Alk & Comp M Ac
R Alk & M Alk
R Alk & M Ac
Prediction of Compensatory Response
DisorderPredicted CompensationMetabolic AcidosispCO2 = (1.5 X Bicarb) + 8 + 2
Metabolic AlkalosispCO2 = (0.7 X Bicarb) + 20 + 5
Acute Respiratory Acidosis Bicarb = 0.1 pCO2
Chronic Respiratory Acidosis Bicarb = 0.4 pCO2
Acute Respiratory Alkalosis Bicarb = 0.2 pCO2
Chronic Respiratory Alkalosis Bicarb = 0.5 pCO2
or Bicarb in relation to 24 mEq/L or PCO2 in relation to 40 mm Hg
Another useful tool in estimating the PCO2 in metabolic acidosis is the recognition that pCO2 is approximately equal to the last 2 digits of the pH.
Na Bicarb Required =0.5 X Wt (Kg) X desired increase in serum bicarbonate (mEq/L)Apply only if severe metabolic acidosis (pH < 7.2)
Rather than the normal bicarb level of 24 mEq/L, target a lower level, eg. 12 mEq/L
Step 6)Step 6) If severe metabolic acidosis, If severe metabolic acidosis, calculate Na Bicarbonate Requiredcalculate Na Bicarbonate Required
Risks of Bicarbonate Therapy in Metabolic Acidosis:• Paradoxical transient intracellular acidosis.
• Shift of O2- Hb dissociation curve to the left.
• Hypokalaemia.
• Hypernatraemia and Hypervolaemia
Paradoxical Transient Intracellular Acidosis
Whereas the arterial pH tends to rapidly after administration of Na bicarb, intracellular pH more slowly:
• Slow passage of bicarb ion across the cell membrane.
• Rapid conversion of plasma bicarb to carbonic acid which dissociates producing CO2 and water. CO2 diffuses into cells more rapidly than bicarb intracellular bicarb/CO2 ratio intracellular pH.
The intracellular acidosis will persist as long as bicarb administration exceeds CO2 elimination.
Therefore, adequate tissue perfusion and ventilation should be secured in line with Na bicarb administration.
Shift of O2-Hb Dissociation Curve to the Left
( Hb Affinity for Oxygen)
Oxygen delivery to tissues tissue hypoxia tendency for lactic acidosis
Hypokalaemia
Acidosis stimulates movement of K+ from intracellular to extracellular compartment. Correction of acidosis by alkali has the reverse effect: movement of K+ from extracellular to intracellular compartment:
Roughly, for each 0.1 rise in pH, serum K+ by about 0.5 mEq/L
Hypernatraemia and Hypervolaemia
These are problematic in patients with cardiac and/or renal dysfunction
123456pH7.347.57.67.37.437.14
PaO2587580745594
PaCO2504130323928
HCO3-30322419258
FiO20.50.30.210.210.210.21DisorderPredicted CompensationMetabolic AcidosispCO2 = (1.5 X Bicarb) + 8 + 2
Metabolic AlkalosispCO2 = (0.7 X Bicarb) + 20 + 5
Acute Respiratory Acidosis Bicarb = 0.1 pCO2
Chronic Respiratory Acidosis Bicarb = 0.4 pCO2
Acute Respiratory Alkalosis Bicarb = 0.2 pCO2
Chronic Respiratory Alkalosis Bicarb = 0.5 pCO2