Recapitulation of acid-base concepts, metabolic acidosis, metabolic alkalosis, respiratory control, medulla respiratory center, pons respiratory center

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Biomedical Science
BMS 420
Charles Miller

3 December Recapitulation of Acid-Base Concepts Blood gas analysis (PaO ; Pa2O ) and b2ood pH are the best overall tests of lung function. Arterial blood gas values cannot be determined precisely by clinical assessment – even by experience clinicians (examples: cyanosis doesn’t appear until PaO around 50 2 60 mm Hg and % saturation of 80%; auscultation for air exchange is poor indicator of lung function). If the PaCO is2normal, is the patient necessarily exhibiting normal lung function? Not necessarily since the individual could be hyperventilating to decrease the PaCO to 2 normal! Oximetry measures % saturation between hemoglobin and oxygen, NOT PaO 2 Henderson-Hasselbach equation and its importance: Analysis of acid-base status of blood for the bicarbonate buffer system. If the ratio of bicarbonate to CO is 20, 2he pH will be 7.4 (normal) Compensations brought about by the lungs or kidneys help maintain this ratio. The 4 possible alterations in A-B balance are respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis. Respiratory defects are brought about by alterations in ventilation whereas metabolic defects are due to a gain or loss of either hydrogen or bicarbonate ions. Metabolic Acidosis Primary change is a decrease in HCO - 3 Cause can be acids in blood due to diabetes mellitus or tissue hypoxia (increase in lactic acid) Shift initially is downward + Respiratory compensation is via increased ventilation (due to H stimulation of chemoreceptors) which then decreases pCO and then i2creases the ratio and the pH. Shift is to the right along the lower buffer curve. Metabolic Alkalosis Increased ratio due to increase in HCO 3- Cause can be ingestion of bases or antacids or loss of acids due to vomiting Shift initially is upward Respiratory compensation is via decreased ventilation which increases pCO and 2 movement is along the upper buffer line to the left. The compensation is minor. Metabolic Causes of Acidosis Drug ingestion (methanol, ethanol, ethylene glycol, ammonium chloride) Diarrhea Renal dysfunction Lactic acidosis (shock, acute respiratory distress syndrome (ARDS), carbon monoxide) Ketoacidosis (diabetes, starvation, alcoholism) Metabolic Causes of Alkalosis Vomiting (nasogastric suctioning) Diuretics Antacid ingestion Steps in analyzing arterial blood gas samples Step 1: Is there acidosis of alkalosis? Step 2: Is the primary disorder of respiratory or metabolic origin? In other words, does the pH move in appropriate direction with pCO ? If 2o, this is respiratory disorder. If pH does not move in the appropriate direction, then it is a metabolic disorder with respiratory compensation. Step 3: Is there evidence of compensation – respiratory or metabolic? If so, is it acute or chronic? Acute: For each increase in 10 mm Hg of PaCO , the pH2will drop .08 units if no time for renal compensation. Chronic: For each i
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