Acid-Base Disturbances - Physician Education Downloads for website/A… · Physiology zCarbonic...
Transcript of Acid-Base Disturbances - Physician Education Downloads for website/A… · Physiology zCarbonic...
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Acid-Base DisturbancesDr. Thomas VanderLaan
Dr. Melanie WalkerHuntington Memorial Hospital
Pasadena, California
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What is it?
Respiratory: breathing is inadequate and carbon dioxide accumulates
PCO2 contributes to an acid pH Metabolic:
normal metabolism is impaired - acid formsif severe, the patient may be in shock
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Physiology
Carbonic acid (H2CO3) is central to our understanding and evaluation of acid-base disturbances. The dissociation products and the ionization products are normally in equilibrium
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Physiology: The Cell Wall
Limits transfer of substancesDepends on pH
first, as the pH changes so will the degree of ionization and, hence, the concentration of ionizedsubstances; second, if the degree of ionization changes greatly, a substance may cease to be ionized and will, therefore, escape from the cell.
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Physiology: Extracellular Fluid
Treatable volumeExtracellular fluid is 20% of the body weightProvides:
NutritionOxygenationWaste removalTemperatureAlkalinity
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Acid-Base Disturbances
pH must be within small rangeNormal is 7.4
Large acid loads are produced by normalmetabolism
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Some definitions…
pH defines the blood [H+] concentrationLow (<7.35) = AcidemiaHigh (>7.45) = Alkalemia
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Some definitions…
[HCO3] defines the metabolic componentLow (<20 mmol/L) = Metabolic acidosis High (>33 mmol/L) = Metabolic alkalosis
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More definitions…
pCO2 defines the respiratory componentLow (<35 mmHg) = Respiratory alkalosisHigh (>45 mmHg) = Respiratory acidosis
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Basic Evaluation
High pH (>7.45) suggests: Respiratory alkalosis - pCO2 < 35mmHgMetabolic alkalosis - [HCO3] > 33 mmol/L.
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Basic Evaluation
Low pH (<7.35) suggests:Respiratory acidosis - pCO2 > 45 mmHgMetabolic acidosis - [HCO3] < 23 mmol/L
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Normal pH?
Normal pH (7.35-7.45) suggests:No acid-base disturbanceChronic respiratory alkalosis Chronic respiratory acidosis (mild)Mixed disturbance
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Buffer Systems
Bicarbonate – carbonic acid systemLungs excrete
Proteins and phosphatesKidneys excrete
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Respiratory Acidosis
• respiratory exchange with retention of CO2 results in a pCO2 which then causes renal retention of bicarbonate
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Respiratory Acidosis: Causes
respiratory exchange CNS Depressiontrauma/infections/tumorcerebrovascular accidentsdrug overdoseNeuromuscular disorders Myopathies
Thoracic disordershydrothoraxpneumothoraxLung disorder bronchial obstruction emphysema (chronic obstructive airway disease) severe pulmonary edema
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Respiratory Acidosis: Compensation
Problem: pCO2 and this results in a blood pH (high H+)[H+] stimulates kidney to generate and retain bicarbonate
respiratory acidosis.is compensated for by the development of a metabolic alkalosis
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Respiratory Acidosis: Compensation
Compensation is complete ([HCO3] levels out) in 2-4 daysFinal HCO3 can be calculated from the following equation:
HCO3 mmol/L = 0.44 X pCO2 mmHg + 7.6 (+/-2).
Limit of compensation is a HCO3 of 45 mmol/L
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Respiratory Acidosis: Treatment
Acute: correct underlying source of alveolar hypoventilation
BronchodilatorsOxygen Antibiotics/Drug therapy Dialysis
If it is chronic: try to avoid excessive supplemental oxygen
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Respiratory Alkalosis
respiratory exchange with loss of CO2 results in a pCO2 which then stimulates renal excretion of bicarbonate
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Respiratory Alkalosis: Causes
respiratory exchangeCNS disturbancesPsychogenic (anxiety)PregnancyHypoxiaDrug toxicity / overdose
Pulmonary disorders
EmbolismEdemaAsthmaPneumonia
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Respiratory Alkalosis: Compensation
Problem: pCO2 causing blood pH (low H+)
pH stimulates the kidney to excrete bicarbonate
respiratory alkalosis is compensated for by the development of a metabolic acidosis
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Respiratory Alkalosis: Compensation
If the condition has been present for 7 days or more full compensation may occur.Compensation is complete ([HCO3] levels out) in 7-10 days.The limit of compensation is a HCO3 of 12 mmol/L.
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Respiratory Alkalosis: Treatment
Treatment aims to eradicate the underlying condition
removal of ingested toxins treatment of fever or sepsis (toxin) treatment of CNS disease
In severe respiratory alkalosis:breathing into a paper bag, which helps relieve
acute anxiety and increases carbon dioxide levels
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Metabolic Acidosis
production or renal retention of H+ results in a low pH which stimulates respiration to the pCO2
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Metabolic Acidosis: Causes
High Anion GapRenal failuretoxins ketoacidosis
Normal anion gap (hyperchloremic)
Hyperkalemiaobstructive uropathy diarrhea renal tubular acidosisSome medications
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Metabolic Acidosis: Compensation
Problem: [HCO3] causing blood pH (high H+).[H+] stimulates respiration which lowers the blood pCO2
metabolic acidosis is compensated for by the development of a respiratory alkalosis
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Metabolic Acidosis: Compensation
Compensation is complete (pCO2 levels out) in 12-24 hours.The final pCO2 can be calculated from the following equation:
pCO2 mmHg = 1.5 x [HCO3] (mmol/L) + 8 (+/-2).
The limit of compensation is a pCO2 of 10 mmHg
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Metabolic Acidosis: Treatment
Try to restore perfusion and correction of underlying disturbanceIt is rarely necessary to administer sodium bicarbonate to patients with acute metabolic acidosis
Not recommended for stable patients with pH 7.2 or higher
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Metabolic Alkalosis
production or renal retention of HCO3 results in a high pH which inhibits respiration to increase the pCO2
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Metabolic Alkalosis: Causes
Urinary chlorideGut H+ loss
Vomiting, suctionRenal H+ loss
Diuretic therapyContraction alkalosis
Urinary chlorideMineralocorticoid excessDiuretic therapy
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Metabolic Alkalosis: Compensation
Problem: [HCO3] causing blood pH (low H+)Low [H+] suppresses respiration which blood pCO2
metabolic alkalosis is compensated for by the development of a respiratory acidosis
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Metabolic Alkalosis: Compensation
Compensation is complete (pCO2 levels out) in 12-24 hours.The final pCO2 can be calculated from the following equation:
pCO2 mmHg = 0.9 X [HCO3] (mmol/L) + 9 (+/-2)
The limit of compensation is a pCO2 of 60 mmHg
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Metabolic Alkalosis: Treatment
When metabolic alkalosis is potentially life-threatening (pH > 7.6 or [HCO3-] > 40 mEq/L):
the carbonic anhydrase inhibitor acetazolamideshould be considered; however, this agent is associated with renal loss of potassium
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Metabolic Alkalosis: Treatment
If acetazolamide is not effective or the metabolic alkalosis worsens:
exogenous acid, in the form of a 0.1N solution of hydrochloric acid (100 mEq/L), should be administered through a central venous catheter