Acid-Base Disturbances

Lidia M. Guerrero, DMD

Acid-base homeostasis

! Regulation of the hydrogen ion concentration –  Chemical buffers of the body

! Bicarbonate-carbonic acid buffer pair – H(+) + HCO3 ! " H2CO3 " CO2 + H2O

–  Lungs ! Eliminates CO2 & H2O

–  Kidneys ! Reabsorbs filtered bicarbonate and regenerates new

bicarbonate via the titration of urinary buffers and the excretion of ammonium

Approach to the analysis of Acid-base disorders

! Careful history ! Arterial blood gases ! Serum electrolytes ! Urine electrolytes ! Rules of compensation

Arterial blood gasses (ABG s)

! Normal values –  Oxygen saturation = 93% - 98% –  Arterial pH = 7.35 – 7.45 –  P O2 = 80 – 95 mm Hg –  P CO2 = 36 – 43 mm Hg –  {HCO3!} = 20 – 30 meq/L

Acid-base disorders

! Type –  Acidosis –  Alkalosis

! Origin –  Metabolic –  Respiratory

! Compensation or mixed disorder

Primary Acid-base disorders

Type of disorder

pH P CO2 Plasma HCO3!

Respiratory acidosis

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Respiratory alkalosis

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Metabolic acidosis

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Metabolic alkalosis

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Respiratory acidosis

! Primary increase in P CO2 –  Interference in pulmonary excretion of CO2

! Compensatory response –  Increase in renal resorption & generation of

bicarbonate ! Leads to a rise in serum {HCO3!}

Respiratory acidosis

!  Common causes –  Depression of respiratory

center !  Drugs !  Infections !  CNS injury

–  Neuromuscular disorders !  Myopathy !  Guillian-Barré Syndrome

–  Pulmonary diseases !  COPD !  Asthma !  Kyphoskoliosis !  Pneumothorax

!  Manifestations –  Agitation –  Asterixis –  Papilledema –  Headache –  Somnolence –  Hypertension –  Tachycardia –  Heart failure –  Cardiac arrhythmias

Respiratory acidosis

! COPD patients –  Particularly serious condition

! Post operative period may accentuate preexisting chronic respiratory acidosis

–  Airway obstruction –  Atelectasis –  Pneumonia –  Pleural effusion –  Abdominal pain or distention

Respiratory acidosis

! Signs indicating inadequate ventilation with hypercapnia –  Similar to those associated with post

operative pain ! Restlessness ! Hypertension ! Tachycardia

–  Narcotics should be used cautiously or avoided

Respiratory acidosis

! Management –  Prompt correction of pulmonary defect –  Ensure adequate ventilation –  Encourage deep breathing and coughing –  Avoid over-sedation

Respiratory alkalosis

! Decrease in P CO2

–  Primary hyperventilation

! Compensatory response –  Increased renal bicarbonate excretion

! Fall in serum {HCO3!}

Respiratory alkalosis

! Very common in the surgical/dental patient ! Common causes

–  Hyperventilation ! Apprehension ! Pain ! Hypoxia ! CNS injury (encephalitis) ! Assisted ventilation

Respiratory alkalosis

!  Common causes –  CSN disorders

! Anxiety ! Brainstem lesions !  Infection

–  Pulmonary diseases ! Pneumonia ! Pulmonary emboli ! Pulmonary edema !  Interstitial lung disease

!  Common causes –  Drugs

! Salycilates ! Theophyline ! Catecholamines

–  Gram-negative sepsis –  Liver disease –  Pregnancy –  Excessive ventilation

from mechanical ventilators

Respiratory alkalosis

! Manifestations –  Lightheadedness –  Paresthesias –  Tetany –  Syncope –  Seizures –  Cardiac arrhythmias

Respiratory alkalosis

! Dangers –  Poor prognosis –  Related to potassium depletion

! Ventricular arrhythmias ! Fibrillation

–  Shift of the oxy-hemoglobin dissociation curve to the left ! Limits the ability of hemoglobin to unload oxygen at the

tissue level

Oxyhemoglobin dissociation curve

Respiratory alkalosis

! Management –  Paper bag ???

! Patient’s hands over mouth and nose –  Proper use of mechanical ventilation –  Correction of preexisting potassium deficits

Metabolic acidosis

! Reduction in {HCO3!} –  Accumulation of fixed acids –  Loss of alkali

! Compensatory response –  Increased ventilation (rate & depth)

! Fall in P CO2

Metabolic acidosis

!  Common causes –  Diabetes –  Alcohol withdrawal syndrome –  Renal damage

! Azotemia or uremia –  Lactic acid accumulation

! Reflection of tissue hypoxia –  Starvation –  Drug intoxication –  Diarrhea –  Small bowel fistulae

Metabolic acidosis

! Management –  Correction of the underlying disorder –  Restoration of adequate tissue perfusion by

proper volume replacement –  Avoid bicarbonate replacement

Metabolic alkalosis

!  Primary increase in {HCO3!} –  Loss of acids –  Gain of bicarbonate –  Aggravated by any preexisting potassium depletion

!  Compensatory response –  Primarily renal mechanisms

! Urinary excretion of bicarbonate –  Hypoventilation

! Rise in P CO2

Metabolic alkalosis

!  Common causes –  Vomiting –  Gastric suction with

pyloric obstruction –  Excessive intake of

bicarbonate –  Diuretics

!  Same dangers as in respiratory alkalosis

!  Manifestations –  Impaired mental status –  Hypotension –  Cardiac arrhythmias –  Hypoventilation –  Hypokalemia –  Decreased ionized

calcium

Metabolic alkalosis

! Management –  Replacement of extracellular fluid volume deficit –  Replacement of potassium

Thank you!!!