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Approach to the Respiratory Patient : Arterial Blood Gases

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Approach to Acid-Base Status

 

1. What is the pH acidemic (pH <7.35), alkalemic (pH >7.45), or normal (pH  7.35-7.45)

 

2. What is the primary disturbance? 

  • metabolic: change in HCO3 and pH in same direction
  • respiratory: change in HCO3 and pH in opposite direction

3. Has there been appropriate compensation? (Table below)

  • metabolic compensation occurs over 2-3 days reflecting altered renal HCO3 production/excretion
  • respiratory compensation through ventilation control of PaCO2 occurs immediately
  • inadequate compensation may indicate a second acid-base disorder

4. If there is metabolic acidosis, what is the anion gap and osmolar gap?

  • anion gap = [Na]-([Cl]+[ HCO3]); normal = 10-15 mmol/L
  • osmolar gap = measured osmolarity - calculated osmolarity  = measured - (2[Na] + glucose + urea); normal = 10

5. If anion gap is increased, is the change in bicarbonate the same as the change in anion gap?

  • if not, consider a mixed picture

Expected Compensation for Specific Acid-Base Disorders

Disturbance

PaCO2 (mmHg)

HCO3 (mmHg)

Respiratory Acidosis

Acute

Chronic

increase 10

increase 10


increase 1

increase 3

Respiratory Alkalosis

Acute
Chronic


decrease 10

decrease 10


decrease 2

decrease 5

Metabolic Acidosis

decrease 1

decrease 1

Metabolic Alkalosis

increase 5-7

increase 10

DIFFERENTIAL DIAGNOSIS OF RESPIRATORY ACIDOSIS


characterized by increased PaCO2 secondary to hypoventilation

  • respiratory centre depression (decreased RR)
    • drugs (anesthesia, sedatives, narcotics)
    • trauma
    • increased ICP
    • encephalitis
    • stroke
    • central apnea
    • supplemental O2 in chronic CO2 retainers (i.e. COPD)
  • neuromuscular disorders (decreased TV)
    • myasthenia gravis
    • Guillain-Barre syndrome
    • poliomyelitis
    • muscular dystrophies
    • ALS
    • myopathies
    • chest wall disease (obesity, kyphoscoliosis)
  • airway obstruction (asthma, foreign body) (decreased FEV)
  • parenchymal disease
    • COPD
    • pulmonary edema
    • pneumothorax
    • pneumonia
    • pneumoconiosis
    • acute respiratory distress syndrome (ARDS)
  • mechanical hypoventilation (inadequate mechanical ventilation)

    DIFFERENTIAL DIAGNOSIS OF RESPIRATORY ALKALOSIS


    characterized by decreased PaCO2 secondary to hyperventilation

    • hypoxemia
      • pulmonary disease (pneumonia, edema, PE, interstitial fibrosis)
      • severe anemia
      • heart failure
      • high altitude
    • respiratory centre stimulation
      • CNS disorders
      • hepatic failure
      • Gram-negative sepsis
      • drugs (ASA, progesterone, theophylline, catecholamines, psychotropics) 
      • pregnancy
      • anxiety
      • pain
    • mechanical hyperventilation (excessive mechanical ventilation)
    • see Nephrology for differential diagnosis of metabolic acidosis and alkalosis


    Calculation of A-aDO2 Gradient (Approach to Oxygenation Status)

    • calculate the oxygen gradient between the alveolus and the pulmonary capillaries
    • approach includes asking 3 questions:
      • 1.  What is the PaO2? (normal = 95-100 mm Hg)
      • 2.  What is the A-aDO2 (the gradient)? (normal <15 mm Hg)
        • A-aDO2 = PAO2 (alveolar)  - PaO2(arterial) = [FiO2 (Patm - PH2O) - PaCO2/RQ] - PaO2
          • On room air: FiO2 = 0.21, Patm = 760* mmHg, PH2O = 47 mmHg, RQ = 0.8
          • A-aDO2 = [150* – 1.25(PaCO2)] – PaO2 (*apply at sea level)
        • the normal A-aDO2 increases with age
      • 3. What is the cause of the hypoxemia? (see Figure  below)  

    Oxygen-Hb dissociation curve

     

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    Approach to Hypoxemia

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    Pathophysiology of Shunt

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    Approach to the Respiratory Patient : Arterial Blood Gases Approach to the Respiratory Patient : Arterial Blood Gases Reviewed by Radiology Madeeasy on November 07, 2010 Rating: 5
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