Arterial Blood Gases (ABG)

What is an ABG? Answer- a measurement of the partial pressure of O2, CO2, pH, and HCO3 in arterial blood Who normally draws this bloodwork? Where is it drawn from? Answer- Respiratory therapists draw for this profile most commonly from the radial, brachial, and femoral arteries. What is acidosis? Answer- It is characterized by an abundance of H+ ions in the arterial blood, with a pH of less than 7.35. What are the acids? Answer- H+, CO2, and H2CO3 (carbonic acid) How are acids produced? Answer- They are produced during metabolic processes. How are acids excreted from the body? Answer- Some exit via the lungs; others are buffered and excreted via kidneys. What is alkalosis? Answer- It is characterized by a decrease of H+ ions in the arterial blood, with a pH of more than 7.45. What are the bases? Answer- HCO3 (bicarbonate) What is a buffer? Answer- A chemical substance that reduces changes in systemic pH by either releasing or binding H+ ions. It is considered to be a moment by moment regulation. Why is CO2 considered a "potential acid"? Answer- Because it forms carbonic acid when combined with H2O. What are compensatory mechanisms? Answer- Mechanisms that try to prevent large changes in pH and/or attempt to correct alterations in acid-base balance; compensatory mechanisms ALWAYS alter the pH towards a normal level. Give 2 examples of compensatory mechanisms. Answer- Lungs and Kidneys What is partial pressure? Answer- Pressure exerted by a single gas. What is the goal in the regulation of pH? Answer- To maintain or restore body pH to a normal level - either through buffers or compensatory mechanisms. What are some examples of buffers? Answer- proteins, carbonic acid - bicarbonate, phosphate How do the lungs function as a compensatory mechanism? Answer- By expiration, in which CO2 is eliminated from the body, thereby reducing the amount of acids. (note that the lungs cannot eliminate H+ ions) How is additional expiration from the lungs initiated? What is the timeframe involved? Answer- Through chemoreceptors in the brain. It occurs within 1-3 minutes of detected imbalance, and lasts 12-24 hours. How do the kidneys function as a compensatory mechanism? Answer- In alkalosis : they excrete HCO3 In acidosis : they excrete H+ in addition to increasing production and retention of HCO3 What is the timeframe involved in kidney compensation? Answer- It begins to work within approximately 24 hours and continues until pH is WNL What is the normal range for pH? Answer- 7.35 - 7.45 What are the normal limits for PCO2? Answer- 35 - 45 mmHg What are the normal limits for HCO3? Answer- 22 - 26 mEq/L What is the normal range for PO2? Answer- 80-100 mmHg What does a low PO2 indicate and what is the potential (respiratory) result? Answer- A low PO2 indicated hypoxia and can result in hyperventilation. ABG Interpretation : What does the relationship between pH and PCO2 indicate? Answer- A respiratory problem ABG Interpretation : What does the relationship between pH and PHCO3 indicate? Answer- A metabolic problem If pH and PCO2 are going in opposite directions, what does this indicate? Answer- A respiratory problem If pH and PHCO3 are going in the same direction, what does this indicate? Answer- A metabolic problem What type of compensation is indicated if either pH and PCO2 or pH and PHCO3 are out of range (i.e. only 2 components)? Answer- Uncompensated compensation What type of compensation is idicated if pH, PCO2, and PHCO3 are all out of range? Answer- Partially compensated compensation What type of compensation is indicated if the pH is WNL and PCO2 and PHCO3 are out of range? Answer- Fully compensated compensation (note : need to determine whether pH is closer to acidosis or alkalosis to determine respiratory vs. metabolic) How does the brain respond to retained CO2 in the body? Answer- The blood vessels dilate, resulting in confusion. What are the characteristic values of respiratory acidosis? Answer- low pH; high PCO2 (lungs to not rid the body of CO2, the excess of which binds to H2O to form H2CO3) What is the cause of respiratory acidosis? What can lead to this condition? Answer- CAUSE = HYPOVENTILATION factors leading to this include : trauma, disease, meds, CNS depression [CNS depression (affects respiratory center in the brain); narcotics/sedatives/anesthesia (slow breathing rate); trauma that results in impaired respiratory function; pneumonia, atelectasis, pulmonary edema/pulmonary embolism] What are some manifestations of respiratory acidosis? Answer- confusion (due to dilation of cerebral vessels) weakness tremors/paralysis stupor - coma warm/flushed skin What is the respiratory compensation for respiratory acidosis? Answer- an increase in respiration rate (unless CNS depression) What are the renal compensations for respiratory acidosis? Answer- excretion of H+ ions, and production and retention of HCO3 What are the characteristic values for respiratory alkalosis? Answer- high pH; low PCO2 What is the cause of respiratory alkalosis and what is this secondary to? Answer- CAUSE = HYPERVENTILATION secondary to pain anxiety/fear fever/sepsis CNS lesions What are some manifestations of respiratory alkalosis? Answer- CONSTRICTION of cerebral vessels CARDIAC DYSRHYTHMIAS dizziness/light-headedness tetany numbness/tingling of fingers/toes panic seizures What is the renal compensation for respiratory alkalosis? Answer- a decrease in production of HCO3 (at around 24 hours) Is metabolic acidosis or metabolic alkalosis seen more often in a clinical setting? Answer- metabolic acidosis What are the characteristic values of metabolic acidosis? Answer- low pH; low PHCO3 (note : this is usually secondary to significant health problems such as DKA and CKD) What are some causes of metabolic acidosis? Answer- *renal failure (decreased production of HCO3 as well as decreased excretion of H+ ions) *severe diarrhea (increased loss of HCO3) *lactic acidosis (increased production of acids) *asprin toxicity (AKA : ASA Toxicity) *starvation *anaerobic metabolism secondary to hypoxia (remember cell injury leads to lactic acid production and accumulation) What are some manifestations of metabolic acidosis? Answer- EXPECT HYPERKALEMIA abdominal pain cardiac dysrhythmias bone disease What are the respiratory compensations for metabolic acidosis? Answer- and increase in respiration rate & depth (Kussmauls respirations = deep, rapid resp. pattern to blow off CO2 - this is very characteristic) What are the characteristic values for metabolic alkalosis? Answer- high pH; high PHCO3 What are some causes of metabolic alkalosis? Answer- *increased renal production of HCO3 *excess ingestion of alkali (antacids/baking soda) *GI suctioning/vomiting/bulimia/diuretic therapy (all involve a loss of acids What are some manifestations of metabolic alkalosis? Answer- EXPECT HYPOKALEMIA hyperactive reflexes/tetany hypotension cardiac dysrhythmias What is the respiratory compensation for metabolic alkalosis? Answer- decreased respiration rate