Blood%20Gas%20Interpretation

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Blood Gas Interpretation

• 2005/8/25

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Before beginning

• Allens test for radial and ulnar artery
• Common errors of arterial blood sampling
• Air in sample PCO2?, pH?, PO2?
• Venous mixture PCO2?, pH?, PO2?
• Excess anticoagulant (dilution) PCO2?, pH?, PO2?
• Metabolic effects PCO2?, pH?, PO2?
• Simultaneous electrolytes panel

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Acid Base Physiology

• The Law of Mass Action
• A B ? C D
• K1/K2 CD/AB
• Dissociation constant for an acid
• Ka HA-/HA

K1
K2
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Henderson-Hasselbalch Equation

• CO2 H2O ? H2CO3 ? H HCO3-
• H K x CO2/HCO3-
• 24 PCO2/HCO3-
• pH 6.1 log (HCO3-/0.0301xPCO2)

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Normal Range

• pH 7.35-7.45
• PCO2 35-45 mmHg (40 mmHg)
• HCO3- 22-26 mEq/L (24 mEq/L)

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Bicarbonate Buffering System
Metabolism Oral intake
Oral intake
Metabolism

• CO2 H2O ? H2CO3 ? H HCO3-

Kidney
Kidney Stomach
Lung
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Acid Production and Elimination

• Reaction Products
  Elimination
• Glucose H HCO3-
• Fat H
  HCO3-
• Glucose H lactate
• Cysteine H sulfate
• Phosphoproteins H phosphate

O2
Lungs 24,000 mEq/day Volatile acid
O2
Anaerobic
Kidneys 50-100 mEq/day Non-volatile acid
O2
O2
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Determinants of CO2 in the alveolus

• VA VE VD VT x f (1- VD/VT)
• PACO2 k x (VCO2/VA)
• Physiologic dead space anatomic dead space
  alveolar dead space

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PaCO2
PaCO2 gt 40 mmHg, MV 2x normal PaCO2 gt 80 mmHg ?
CO2 nacrosis
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Renal Regulation of Bicarbonate

• Reabsorption of filtered HCO3- (4000 mmol/day)
• Formation of titratable acid (4000 mmol/day H)
• Excretion of NH4 in the urine
• 80-90 of HCO3- reabsorbed in the proximal
  tubule
• Distal tubule reabsorption of remained
  bicarbonate and secretion of hydrogen ion

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Proximal Renal Tubule
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Distal Renal Tubule
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Distal Tubule NH4 excretion
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Acid Base Disturbance

• Metabolic acidosis HCO3-?
• Metabolic alkalosis HCO3- ?
• Respiratory acidosis PCO2?
• Respiratory alkalosis PCO2 ?
• Simple
• Primary
• Secondary
• mixed

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Metabolic Acidosis

• Indogenous acid production (lactic acidosis,
  ketoacidosis)
• Indogenous acid accumulation (renal failure)
• Loss of bicarbonate (diarrhea)
• High anion gap
• Normal (hyperchloremic )

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Pathophysiologic Effect of Metabolic Acidosis

• Kussmaul respiration
• Intrinsic cardiac contractility?, normal
  inotropic function
• Peripheral vasodilatation
• Central vasoconstriction ? pulmonary edema
• Depressed CNS function
• Glucose intolerance

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Anion Gap

• AG Na - (Cl- HCO3-)
• Unmeasured anions in plasma (normally 10 to 12
  mmol/L)
• Anionic proteins, phosphate, sulfate, and organic
  anions
• Correction if albumin lt 4
• Albumin ?1 ? AG ? 2.5

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Anion Gap

• Increase
• Increased unmeasured anions
• Decreased unmeasured cations (Ca, K, Mg)
• Increase in anionic albumin

• Decrease
• Increase in unmeasured cations
• Addition of abnormal cations
• Reduction in albumin concentration
• Decrease in the effective anionic charge on
  albumin by acidosis
• Hyperviscosity and severe hyperlipidemia (
  underestimation of sodium and chloride
  concentration)

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Causes of High-Anion-Gap Metabolic Acidosis Causes of High-Anion-Gap Metabolic Acidosis Causes of High-Anion-Gap Metabolic Acidosis
Lactic acidosis Toxins  
Ketoacidosis Ethylene glycol  
Diabetic Methanol  
Alcoholic Salicylates  
Starvation Renal failure (acute and chronic)  
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Metabolic Alkalosis

• Net gain of HCO3-
• Loss of nonvolatile acid (usually HCl by
  vomiting) from the extracellular fluid
• Kidneys fail to compensate by excreting HCO3-
  (volume contraction, a low GFR, or depletion of
  Cl- or K)

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Respiratory Acidosis

• Severe pulmonary disease
• Respiratory muscle fatigue
• Abnormal ventilatory control
• Acute vs. Chronic (gt 24 hrs)

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Respiratory Acidosis

• Acute anxiety, dyspnea, confusion, psychosis,
  and hallucinations and coma
• Chronic sleep disturbances, loss of memory,
  daytime somnolence, personality changes,
  impairment of coordination, and motor
  disturbances such as tremor, myoclonic jerks, and
  asterixis
• Headache vasocontriction

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Respiratory Alkalosis

• Strong ventilatory stimulus with alveolar
  hyperventilation
• Consuming HCO3-
• gt 2-6 hrs renal compensation (decrease NH4/acid
  excretion and bicarbonate re-absorption)

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Respiratory Alkalosis

• Reduced cerebral blood flow
• dizziness, mental confusion, and seizures
• Minimal cardiovascular effect in normal health
• Cardiac output and blood pressure may fall in
  mechanically ventilated patients
• Bohr effect left shift of hemoglobin-O2
  dissociation curve ? tissue hypoxia (arrhythmia)
• intracellular shifts of Na, K, and PO4- and
  reduces free Ca2

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Stepwise Approach

• Do comprehensive history taking and physical
  examination
• Order simultaneous arterial blood gas measurement
  and chemistry profiles
• Assess accuracy of data
• Direction of pH always indicates the primary
  disturbance
• Calculate the expected compensation
• Second or third disorders

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Determination of primary acid-base disorders
Respiratory alkalosis
Metabolic alkalsosis
7.6
pH
N
7.4
Metabolic acidosis
Respiratory acidosis
7.2
30
40
50
PCO2 (mmHg)
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Compensatory Mechanisms

• Respiratory compensation
• Complete within 24 hrs
• Metabolic compensation
• Complete within several days
• Both the respiratory or renal compensation almost
  never over-compensates

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Prediction of Compensatory Responses on Simple Acid-Base Disturbances Prediction of Compensatory Responses on Simple Acid-Base Disturbances Prediction of Compensatory Responses on Simple Acid-Base Disturbances
Disorder Prediction of Compensation  
Metabolic acidosis PaCO2 (1.5x HCO3-) 8 or  
PaCO2 will ? 1.25 mmHg per mmol/L ? in HCO3- or  
PaCO2 HCO3- 15  
Metabolic alkalosis PaCO2 will ? 0.75 mmHg per mmol/L ? in HCO3- or  
PaCO2 will ? 6 mmHg per 10-mmol/L ? in HCO3- or  
PaCO2 HCO3- 15  
Respiratory alkalosis  
Acute HCO3- will ? 2 mmol/L per 10-mmHg ? in PaCO2  
Chronic HCO3- will ? 4 mmol/L per 10-mmHg ? in PaCO2  
Respiratory acidosis  
Acute HCO3- will ? 1 mmol/L per 10-mmHg ? in PaCO2  
Chronic HCO3- will ? 4 mmol/L per 10-mmHg ? in PaCO2  
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Mixed Acid Base Disorders
Primary Secondary Secondary Secondary Secondary
Primary Respiratory acidosis Respiratory alkalosis Metabolic acidosis Metabolic alkalosis
Respiratory acidosis ? ?
Respiratory alkalosis ? ?
Metabolic acidosis ? ? ?
Metabolic alkalosis ? ? ?
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Oxygenation

• Poor diffusion across alveolar membrane
• Small pressure gradient between PAO2 and PaO2
• Large alveolar area is required for gas transfer
• Hemoglobin carries the majority of oxygen in the
  blood

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Oxygenation

• Ventilation and alveolar disease
• Ventilation??PAO2 ??PaO2 ?, combined PCO2?
• Alveolar disease
• Reduced alveolar area
• Thickened alveolar membrane
• V/Q mismatch
• Shunt

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Alveolar-arterial Oxygen Gradient

• PAO2 FiO2 (PB-PH2O) PCO2/R
• 0.21(760-47) 40/0.8
• 100
• R respiratory quotient
• P(A-a)O2 PAO2 PaO2
• ( Age x 0.4)

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Oxygen Content and Saturation

• O2 content 1.34 x Hb x Saturation 0.0031xPO2

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Pulse Oximeters

• Percentage of oxygenated hemoglobin in blood
• Absorption of light in the red and infra-red
  spectra
• Continuous monitor
• Accurate (?3) at high saturation, less below 80
• Insensitive around the normal PO2
• COHb and MetHb

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Clinical Example 1

• 72 y/o male, COPD with acute exacerbation
• Under O2 2L/min
• pH 7.44, PCO2 54, PO2 60, HCO3 36
• Metabolic alkalosis with respiratory compensation
• Mixed respiratory acidosis

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Clinical Example 2

• 30 y/o male, sudden onset dyspnea
• Room air
• 7.33/24/111/12
• Metabolic acidosis
• Respiratory compensation
• Normal A-a O2 gradient
• O2? hyperventilation

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Clinical Example 3

• 70 y/o male, acute hemoptysis and dyspnea
• Room air
• 7.50/31/88/24
• Respiratory alkalosis
• Not been renal compensated yet
• Normal PO2, but A-a O2 gradient?

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Clinical Example 4

• 18 y/o female, chest tightness and dyspnea for 4
  hrs
• RR 28/min, distressed, widespread wheezing
• O2 mask 6L/min
• 7.31/49/115/26
• Respiratory acidosis
• Normal bicarbonate ? acute
• May have problems with oxygenation

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Clinical Example 5

• 37 y/o female, mild asthma history
• Wheezes for 3 weeks, increasing chest tightness
  and dyspnea for 24 hrs, call for ambulance with
  Oxygen use
• RR 18/min, anxious and distressed
• Room air
• 7.37/43/97/27
• Normal?
• r/o CO2 retention
• Low A-a O2 Oxygen use in the ambulance

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Clinical Example 6

• 19 y/o male, Duchenne muscular dystrophy on
  wheelchair for 7 yrs
• No previous respiratory problems but frequent UTI
• Room air
• 7.21/81/44/36
• Respiratory acidosis
• Metabolic compensation
• Normal A-a O2 ? pure ventilatory failure

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Clinical Example 7

• 57 y/o male, smoker, one week URI then 36 hrs
  productive cough, fever and dyspnea
• RR 36/min, distressed, CXR RLL pneumonia
• 7.33/27/51/22, 2L/min
• 7.34/32/58/24, 10L/min mask
• Early metabolic acidosis
• Severe hypoxemic respiratory failure
• Intra-pulmonary shunting

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Thank you for your attention