Hydrogen ion homeostasis and blood gases
Overview Hydrogen homeostasis Sources of hydrogen in the body Acid base control Assessment of acid base status Disorders of hydrogen ion homeostasis Acidosis Alkalosis
Hydrogen homeostasis Hydrogen ion homeostasis is essential for life. Examples: Mitochondrial functioning Charge and shape of proteins Ionization of Ca++ and Mg++
Normal [H+] Concentrations 35 – 45 nmol/L in the ECF 40 – 40,000 nmol/L in urine 1,000,000,000 nmol/L in gastric acid
Sources of hydrogen ion Carbon dioxide is the major source of acid in the body. CO2 + H2O ↔ H2CO3 H2CO3 ↔ HCO3- + H+
Other sources of Hydrogen Glycolysis lactic acid Lipolysis Free fatty acid Ureagenesis Urea synthesis Ketogenesis Ketoacids Renal excretions of buffered acids H+
The Body and pH: Homeostasis of pH is controlled through extracellular & intracellular buffering systems Respiratory: eliminate CO2 Renal: conserve HCO3- and eliminate H+ ions Electrolytes: composition of extracellular (ECF) & intracellular fluids (ICF) - ECF is maintained at 7.40
Respiratory Control Mechanisms Works within minutes to control pH; maximal in 12-24 hours Only about 50-75% effective in returning pH to normal Excess CO2 & H+ in the blood act directly on respiratory centers in the brain CO2 readily crosses blood-brain barrier reacting w/ H2O to form H2CO3 H2CO3 splits into H+ & HCO3- & the H+ stimulates an increase or decrease in respirations
Renal Control Mechanisms: Don’t work as fast as the respiratory system; function for days to restore pH to, or close to, normal Regulate pH through excreting acidic or alkaline urine; excreting excess H+ & regenerating or reabsorbing HCO3- Excreting acidic urine decreases acid in the EC fluid & excreting alkaline urine removes base
Assessment of acid base status Direct measurements: H+ PH PCO2 PO2 Derived measures: Bicarbonate (HCO3)
Use heparinized blood and measure within 10 minutes
Metabolic Disturbances: Alkalosis: elevated HCO3- (>26 mEq/L) Causes include: Cl- depletion (vomiting, prolonged nasogastric suctioning), Cushing’s syndrome, K+ deficiency, massive blood transfusions, ingestion of antacids, etc. Acidosis: decreased HCO3- (<22 mEq/L) Causes include: DKA, shock, sepsis, renal failure, diarrhea, salicylates (aspirin), etc.
Acid base disorders Metabolic acidosis (↓ HCO3, ↓ pCO2) Metabolic alkalosis (↑ HCO3, ↑ pCO2) Respiratory acidosis (↑ pCO2, ↑ HCO3) Respiratory alkalosis (↓ pCO2, ↓ HCO3)
Acid-Base w/o Compensation: Parameters: pH PaCO2 HCO3- Metabolic Alkalosis Normal Acidosis Respiratory
Interpretation Practice: pH: 7.31 PaCO2: 48 HCO3-: 24 pH: 7.47 PaCO2 : 45 HCO3- : 33 Normal values Arterial pH: 7.35 – 7.45 HCO3-: 22 – 26 mEq/L PaCO2: 35 – 45 mmHg
Acid-Base Fully Compensated: Parameters: pH PaCO2 HCO3- Metabolic Alkalosis Normal >7.40 Acidosis <7.40 Respiratory
Interpretation Practice: pH: 7.36 PaCO2: 56 HCO3-: 31.4 pH: 7.43 PaCO2 : 32 HCO3: 21 Normal values Arterial pH: 7.35 – 7.45 HCO3-: 22 – 26 mEq/L PaCO2: 35 – 45 mmHg
Acid-Base Partially Compensated: Parameters: pH PaCO2 HCO3- Metabolic Alkalosis Acidosis Respiratory
Interpretation Practice: pH: 7.47 PaCO2: 49 HCO3-: 33.1 pH: 7.33 PaCO2 : 31 HCO3- : 16 Normal values Arterial pH: 7.35 – 7.45 HCO3-: 22 – 26 mEq/L PaCO2: 35 – 45 mmHg
Case Study 4: Mrs. D is admitted to the ICU. She has missed her last 3 dialysis treatments. Her ABG reveals the following: pH: 7.32 ( 7.35-7.45) PaCO2: 32 ( 35-45mmHg) HCO3-: 18 (22-26mEq/L) Assess the pH, PaCO2 & HCO3-. Are the values high, low or WNL?
References http://www.slideshare.net/parcellus/ss-ulecture1- presentation http://www.slideserve.com/Anita/acid-base-balance- interactive-tutorial Marshall, W. and Bangert, S. (2008). Clinical chemistry (6th ed.). Edinburgh, London: Mosby Elsevier. ISBN 0723434557 (chapter 3) Gaw, A. et al. (2004). Clinical Biochemistry (3rd ed.) Beckett, G. et al. (2008). Clinical Biochemistry (8th ed.) Bishop., et al. (2000). Clinical Chemistry (4th ed.)