Approach to the Respiratory Patient : Arterial Blood Gases

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. W...

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

 

 

Approach to Hypoxemia

 

Pathophysiology of Shunt