Clinical Chemistry Lab - 09 Acid-Base Balance Methodologies

CLINICAL CHEMISTRY LABORATORY ACID-BASE BALANCE METHODOLOGIES OUTLINE • Introduction • Acid-Base Balance • Bicarbonate-Carbonic Acid Buffer System • Acid-Base Disorders • Compensation • Blood Gas Analysis • Case Studies ACID-BASE BALANCE INTRODUCTION • Acid • Base • Buffer ACID-BASE BALANCE • H+ concentration (36-44 nmol/L) • pH: • Acidosis vs. Alkalosis o Systemic: -osis o Blood: -emia • Buffers: prevent sudden changes in the pH of a Solution o Usually contain weak acid and conjugate base o Conjugate base - dissociation product of weak acid • Buffer systems that regulate the blood pH o Main buffer system: Bicarbonate-carbonic acid buffer system ▪ High concentrations of buffers in the blood ▪ Concentration can be regulated ▪ Acidic blood - body will regulate carbonic acid ▪ Carbonic acid - assoc. with pCO2 ▪ Bicarbonate - regulated by kidneys o Phosphate buffer system • Control Centers : o Main buffer system: Bicarbonate-carbonic acid buffer system o Phosphate buffer syste, o Organs: • Significant Normal Values o Blood pH : 7.35-7.45 (7.40) ▪ Any value <7.40 can be considered as acidosis ▪ Any value >7.40 can be considered as alkalosis ▪ Compensated acidosis/alkalosis : normal blood pH, abnormal pCO2 & HCO3 o pCO 2 : 35-45 mmHg ▪ Acidic component ▪ >45 mmHg - makes blood pH acidic ▪ <35 mmHg - blood pH is alkaline (slight/significant) o HCO 3 : 22-26 mmol/L ▪ Alkaline component ▪ >33 mmol/L - makes blood pH alkaline ▪ <26 mmol/L - makes blood pH acidic BICARBONATE-CARBONIC ACID BUFFER SYSTEM • H2CO3 → CO2 + H2O • CO2 → • HCO3 → • Ratio: • ACID-BASE DISORDERS • Respiratory o Ventilatory dysfunction o Unregulated parameter: pCO 2 o Respiratory Acidosis - excess pCO 2 o Respiratory Alkalosis - deficit pCO 2 • Metabolic o Renal and/or metabolic dysfunction o Unregulated parameter: HCO 3 ▪ Kidneys - regulate bicarbonate o Metabolic Acidosis - deficit HCO 3 o Metabolic Alkalosis - excess HCO 3 ROME Acidosis (↓ pH) Alkalosis ( ↑ pH) pCO 2 (respiratory) ↑ ↓ HCO 3 (metabolic) ↓ ↑ COMPENSATION • Respiratory Acidosis/Alkalosis o Issues in the lungs o Compensated by kidney through HCO3 release/retain o Acidosis (↑ pCO 2 ): ↑ HCO 3 by ↑ HCO 3 retention o Alkalosis ( ↓ pCO 2 ): ↓ HCO 3 by ↑ HCO 3 excretion o Compensation: Complete within 2-4 days (Kumpleto) • Metabolic Acidosis/Alkalosis o Issues with the kidney o Compensated by lungs through ventilation/respiration o Acidosis ( ↓ HCO 3 ): ↓ pCO 2 by hyperventilation o Alkalosis ( ↑ HCO 3 ): ↑ pCO 2 by hypoventilation (to conserve CO 2 ) o Compensation: incomplete but immediate (kuLUNG) • Uncompensated o Metabolic - pCO 2 is still normal, HCO 3 is abnormal o Respiratory - bicarbonate is still normal o Metabolic Acidosis ( ↓ HCO3) ▪ normal pCO2 = no action from lungs • Partially Compensated o Everything is abnormal (pH, bicarbonate, CO 2 ) o pH = <7.35 or >7.45 o pH is not yet within the range but the compensatory mechanism is working o Metabolic Alkalosis = ↑ pH, ↑ pCO2, ↑ HCO3

• Compensated/Fully Compensated o pH is normal, the rest is abnormal (CO 2 and HCO 3 ) o pH = 7.35 - 7.45 ▪ <7.40 - from acidosis ▪ >7.40 - from alkalosis o Not all normal value indicate compensation • Eg. ↑ pH, ↓ pCO2, ↑ HCO3 = mixed alkalosis (uncompensated) BLOOD GAS ANALYSIS • Specimen: Arterial blood (heparinized) • Lithium heparin on ice • Methods of analysis o Glass membrane electrode (pH meter) o Severinghaus electrode o Clark electrode (O2) CASE STUDIES • Case 1 o Test Results : ▪ Blood pH = 7.30 ▪ pCO2 = 70 mmHg ▪ HCO3 = 24 mmol/L o Finding : Respiratory Acidosis, Uncompensated o Rationalization : pH pCO2 HCO3 ↓ (acid) ↑ = (normal) ▪ Low pH = acidosis ▪ High pCO2 = acidosis ▪ Normal HCO3 = kidney is not compensating • Case 2 o Test Results : ▪ Blood pH = 7.48 ▪ pCO2 = 20 mmHg ▪ HCO3 = 40 mmol/L o Finding : Mixed Alkalosis, Uncompensated o Rationalization : pH pCO2 HCO3 ↑ ↓ ↑ ▪ High pH = alkalosis ▪ Low pCO2 = respiratory alkalosis ▪ High HCO3 = metabolic alkalosis ▪ Both kidney & lungs contribute • Case 3 o Test Results : ▪ Blood pH = 7.36 ▪ pCO2 = 25 mmHg ▪ HCO3 = 15 mmol/L o Finding : Metabolic Acidosis, Fully Compensated o Rationalization : pH pCO2 HCO3 ↓/= ↓ ↓ ▪ Slightly low pH = slightly acidic • pH is within normal range ▪ Low pCO2 = blood should be alkaline ▪ Low HCO3 = metabolic alkalosis, decreased pCO2 is compensatory mechanism • Lungs attempts to alkalinize blood pH •