problem ( 1 ) : - Alkalosis . - Respiratory. - Compensated. - one possible cause : Prolonged Hyperventilation. problem ( 2 ) : - Acidosis. - Metabolic. - Compensated. - one possible cause : Renal Failure or Diabetic ketoacidosis.
Work Step by Step
Normal Blood gases & acid-base Data PH : 7.36 - 7.44 PaCO2 : 36 - 44 mmHg. PaO2 : 85 - 100 mmHg. SBC ( Standard Bicarbonate HCO3 ) : 22 - 28 mEq/L BE ( Base excess or deficit ) : +2.5 or -2.5 mEq/L SaO2 : 95 - 100 % How to interpret the Acid-Base data 1- look at the PH : the change in PH is in the direction of the primary change : Acidosis ( decrease PH ). Alkalosis ( increase PH ). 2- look at the PaCO2 : - this reflect the respiratory component . - if the PaCO2 is High- then there is Respiratory Acidosis. - if it is low - Respiratory Alkalosis. 3- look at the standard bicarbonate ( SBC ) & the base excess ( BE ). - these reflect the Metabolic component. - if the SBC is low or the BE is negative ( i.e. there is base deficit ) - then there is a Metablic Acidosis. - if the SBC is High & BE is positive - then there is Metabolic Alkalosis. 4- Note that the primary abnormality goes with the direction of the PH - so, * if PH was Acidic & the PaCO2 is High - then it is a Respiratory Acidosis. & * if the PH was Acidic & the SBC or BE are Low - it is a Metabolic Acidosis. & so on. 5- now, evaluate also the state of oxygenation by looking at the PaO2 & SaO2 in relation to FIO2 : - if SaO2 is greater than 90% - Oxygenation is adequate. ( Hypoxaemia may be the reason of Acid-Base disturbances ). Problem ( 1 ): PH : 7.63 PCO2 : 19 mmHg HCO3 ( SBC ) : 19.5 mEq / L - the PH is alkaline ( above 7.44 ). & PaCO2 goes with the PH i.e. is low too so, this is a Respiratory Alkalosis but with compensatory decrease amount of HCO3 in blood by alkalinazation of the urine. - examples of Respiratory Alkalosis: conditions of prolonged hyperventilation as occurs in : 1- high attitude . 2- some psychosomatic cases . 3- meningitis & certain trauma of the brain. 4- prolonged voluntary hyperventilation. Respiratory Alkalosis - this disorder develops due to decreased CO2 blood level ( PaCO2 ). Compensation ( correction ) in Respiratory alkalosis - there is renal correction only ( since the cause is a resiratory disorder ). - the CO2 decrease in the tubular cells, so H secretion decreases allover the renal tubules leading to : 1- incomplete HCO3 reabsorption in the PCTs & the excess is excreted in the urine. 2- decreased addition of new HCO3 at the DCTs & CDs. 3- Excretion of alkaline urine containing excess HCO3. 4- Minimal excretion of acid phophates & ammonium salts. Accordingly, the blood HCO3 level decreases, & in compensated cases, such decrease is proportional to the decrease in CO2. Problem ( 2 ) PH : 7.22 PCO2 : 30 mmHg. HCO3 ( SBC ) : 12 mEq / L. - the PH is Acidic ( below 7.36 ). & the metabolic component HCO3 ( SBC ) is Acidic ( low ) so, this is Metabolic Acidosis but with a compensatory hyperventilation that lowered the PaCO2 to 30 mmHg. - Examples of Metabolic Acidosis : 1- Renal Failure. 2- Shock, diarrhea & severe vomiting. 3- severe dehydration & starvation. 4- salicylate poisoning. 5- hypoxia. 6- diabetic ketoacidosis. 7- deficient secretion of aldosterone e.g. in Addison s disease. Metabolic Acidosis - the disorder develops due to decreased HCO3 blood level ( decrease SBC ). Compensation ( correction ) of Metabolic Acidosis : - there is both renal & respiratory correction: 1- Respiratory correction : - the respiratory rate increase due to stimulation of the Respiratory centre by the increase blood H . - this washes CO2 in the expired air, thus its blood level ( PaCO2 ) is decreased. 2- Renal correction : this is more efficient than the Respiratory correction but it is delayed & requires alonger time.