Unfinished Acid-Base Disorders (Heeter)

_____ donates H+ and _____ accepts H+

Acid donates H+ and Base accepts H+

Normal pH of human blood

• What is Acidemia

• What is Alkalemia

• Normal pH → 7.36-7.44

  • Alcidemia = ≤ 7.35

  • Alkalemia = ≥ 7.45

What buffer system prevents acute acidosis

Carbonic acid/Bicarbonate Buffer system

• CO₂ + H₂O ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

CO₂ is the acid form of the buffer system

How is it most accurately measured

ABG (PaCO₂)

Is CO₂ value in chemistry panel a measure of acid

No

It is a measure of bicarbonate (base)

pH calculation (Henderson-Hasselbalch)

• pH = pKa + log (base/acid)

• pH = 6.1 log [(HCO₃^-) / (pCO₂ × 0.03)]

*pH = -log [H+]

*6.1 = pKa (-log of dissociation constant for the reaction)

*HCO3- as measured by ABG or chemistry panel

*pCO2 = partial pressure of carbon dioxide

*0.03 = solubility constant of CO2 in physiologic fluid

Respiratory regulation

• Principle role

• Rate & Tidal volume

• Regulate PaCO₂

  • Lungs attempt to compensate for metabolic acid-base imbalances

• Rate = breaths/min

• Tidal volume = amount of air exhaled in one breath

  • Increasing rate and tidal volume → increased CO2 excretion → decreased blood PCO2

  • Decreasing rate and tidal volume → decreased CO2 excretion → increased blood PCO2

Renal Regulation

• Principle role

• Kidneys compensate for respiratory acid-base disorders

  • Excretes non-volatile acids and H+ produced in body

  • Retains/excretes bicarbonate to control blood pH

ABGs are performed routinely in ICU to ventilated pts. Define each measurement

• PaCO₂

• PvCO₂

• PaO₂

• PaCO₂

• PaCO₂ = Arterial sample

• PvCO₂ = Venous/mixed sample

• PaO₂ = partial pressure of O₂

  • Measures oxygention of arterial blood

  • Hypoventilation and ↑ PCO₂ → ↓ PaO₂

• PaCO₂ = partial pressure of CO₂

  • Provides info about adequacy of lung function in excreting CO₂

  • ↑ PaCO₂ implies inadequate ventilation

Laboratory findings in Acid-Base Disorders

Disorder	pH	Primary change	Compensation
Metabolic acidosis	↓	↓↓↓HCO3-	↓↓PaCO2
Metabolic alkalosis	↑	↑↑↑HCO3-	↑↑PaCO2
Resp. acidosis (acute/chronic)	↓↓/↓	↑↑↑PaCO2	↑HCO3- (acute) ↑↑HCO3- (chronic)
Resp. alkalosis (acute/chronic)	↑↑/↑	↓↓↓PaCO2	↓HCO3- (acute) ↓↓HCO3- (chronic)

Determination of acid-base diagnosis

• Obtain ABGs and Chemistry panel simultaneously

• Compare bicarb on ABG and chem panel

• Calculate serum anion gap (SAG)

  • correct for albumin

• Determine if acidemia or alkalemia present

• Determine if primarily respiratory or metabolic

  • PaCO₂ or HCO₃^-

• Estimate compensatory response

• Evaluate PO₂ and SO₂

What acid-base disorder is present in a patient with the following ABG

• pH= 7.47, paCO2= 65, pO2= 78, serum HCO3= 38

Metabolic alkalosis and Respiratory acidosis

• pH > 7.44 → alkalosis

• HCO3 = 38 (norm 22-26) → metabolic

• PaCO2 = 65 (norm 35-45) → respiratory acidosis

What acid-base disorder is present in a patient with the following ABG?

• pH= 7.58, pCO2= 42, pO2= 72, serum HCO3= 38

Metabolic alkalosis

• pH > 7.44 → alkalosis

• HCO3 = 38 (norm 22-26) → metabolic

• PaCO2 = 42 (norm 35-45) → normal

What acid-base disorder is present in a patient with the following ABG?

• pH= 7.42, pCO2= 39, pO2= 94, serum HCO3= 25

Normal

• pH = 7.42 (norm 7.36-7.44) → normal

• HCO3 = 25 (norm 22-26) → normal

• PaCO2 = 39 (norm 35-45) → normal

What acid-base disorder is present in a patient with the following ABG?

• 7.2/30/84/16

Metabolic acidosis and Respiratory alkalosis

• pH = 7.2 (norm 7.36-7.44) → acidosis

• HCO3 = 16 (norm 22-26) → metabolic

• PaCO2 = 30 (norm 35-45) → respiratory alkalosis

What acid-base disorder is present in a patient with the following ABG?

• 7.12/58/60/32

Metabolic alkalosis and Respiratory acidosis

• pH = 7.12 (norm 7.36-7.44) → acidosis

• HCO3 = 32 (norm 22-26) → metabolic

• PaCO2 = 58 (norm 35-45) → respiratory alkalosis

What acid-base disorder is present in a patient with the following ABG?

• 7.56/25/70/38

Metabolic alkalosis and Respiratory alkalosis

• pH = 7.56 → alkalosis

• HCO3- = 38 (norm 25-26) → metabolic

• PaCO2 = 25 (norm 35-45) → respiratory alkalosis

Metabolic Acidosis

• Etiology

• Clinical presentation

• Treatment

• Decreased pH → result of primary decrease in HCO3-

  • from organic acid accumulation (lactic acid, ketoacidosis)

  • impaired renal excretion

  • HCO3- loss → diarrhea, biliary drainage, pancreatic fistula

• Chronic is often asymptomatic

• Severe acute (pH <7.2) = Kussmaul respirations, tachycardia, ↓ BP and CO

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