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  • The term used to define the acidity or alkalinity of solutions or blood, pH, is the negative logarithm of the hydrogen ion concentration ([H+]).
  • Normal pH = -log [H+] = 7.40 (7.35-7.45)
  • Acid = proton donor (H+)
  • Base = proton acceptor
  • Solution acidity reflects [H+]
  • Pa02: Partial pressure of oxygen in arterial blood (mmHg). Normal is 40
  • PaCo2: Partial pressure of carbon dioxide in arterial blood (mmHg). Normal 75-100 mm Hg


  • A buffer is solution containing a weak acid and its conjugate base or a weak base and its conjugate acid
  • It minimizes changes in pH by accepting or giving up H+
  • Examples: Bicarbonate, hemoglobin, proteins, phosphates, amnonia

Physiologic Responses to pH Changes

  • Immediate chemical buffering
  • Respiratory compensation (if possible)
  • Renal compensation (slower)

Pulmonary Compensation

  • Mediated by chemoreceptors in brain stem
  • VO2 1-4 L/min for every 1mm Hg PaCO2
  • Response to metabolic acidosis is hyperventilation, response to metabolic alkalosis is hypoventilation (but less predictable)
  • PaCO2 usually does not rise above 55 mmHg in response to metabolic alkalosis because hypoxemia produced by hypoventilation drives ventilation and limits compensation  
  • PaCO2 decreases 1-1.5 mmHg for every 1 mEq/L decrease in HCO3-
  • PaCO2 increases 0.25-1 mmHg for every 1 mEq/L increase in HCO3-  up to 55mmHg 

Renal Compensation

  • Enormous ability of kidneys to eliminate H+ and reabsorb or form new HCO3-
  • Acidosis:
  • Increased absorption of HCO3-
  • Increased excretion of acids
  • Increased NH3 production
  • Alkalosis
  • Increased secretion of HCO3-
  • Generally occurs only with concomitant Na+ deficit or with mineralocorticoid excess

Metabolic Acidosis


  • Metabolic acidosis, usually characterized by an acidemic pH (<7.35) and hypobicarbonatemia


  • Lactic Acidosis
  • Diabetic Ketoacidosis
  • Alcoholic Ketoacidosis
  • Toxins
  • Uremic Acidosis
  • Gastrointestinal HCO3- Loss
  • Renal Tubular Acidosis


  • Treatment is aimed at the underlying disorder, such as insulin and fluid therapy for diabetes and appropriate volume resuscitation to restore tissue perfusion.

Metabolic Alkalosis


  • Characterized by an alkalemic pH (>7.45)


  • Factors that generate metabolic alkalosis include nasogastric suction and diuretic administration 
  • Abnormalities that generate HCO3- within the body are called “initiation factors” of metabolic alkalosis, whereas abnormalities that promote renal conservation of HCO3- are called “maintenance factors.”


  • Mild alkalosis is generally well tolerated. 
  • Severe or symptomatic alkalosis (pH > 7.60) requires urgent treatment.
  • Depending on the degree of hypovolemia, adequate amounts of 0.9% NaCl and KCl should be administered. 
  • Discontinuation of diuretics and administration of H2-blockers in patients whose alkalosis is due to nasogastric suction can be useful.

Respiratory Acidosis


  • Increase in PaCO2 > 45 mmHg
  • Respiratory acidosis, usually characterized by a low pH (<7.35) and always characterized by hypercarbia, occurs when minute ventilation is insufficient to eliminate CO2 production without an increased capillary–alveolar CO2 gradient.
  • Occurs when an acute or chronic rise in arterial CO2


  • Respiratory acidosis results from decreased alveolar ventilation and subsequent hypercapnia
  • Inadequate Elimination of CO2
  • Increased Production of CO2

Symptoms and Signs

  • With acute onset, there is somnolence and confusion, and myoclonus with asterixis may be seen. Coma from CO2 narcosis ensues. 
  • Severe hypercapnia increases cerebral blood flow and cerebrospinal fluid pressure. 
  • Signs of increased intracranial pressure (papilledema, pseudotumor cerebri) may be seen.


  • Since drug overdose is an important reversible cause of acute respiratory acidosis, administration of naloxone (opiate antagonist), 0.04-2 mg intravenously is given to all such patients if no obvious cause for respiratory depression is present 
  • In all forms of respiratory acidosis, treatment is directed at the underlying disorder to improve ventilation.

Respiratory Alkalosis

Definition & etiology:

  • Decrease in PaCO2 < 35 mmHg

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