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Acid-Base Disturbances Physiological Approach

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Acid-Base Disturbances Physiological Approach Acid-Base disturbances Physiological approach Pieter Roel Tuinman, M.D., PhD, intensivist Department of Intensive Care, VU Medical Center, Amsterdam, The Netherlands [email protected] Content • Introduction • Regulation of acid-base balance • Diagnosis of acid-base disturbances • Examples Acid-Base balance • definition: Bronsted-Lowry (1923) • Homeostasis • Physiologic effects of pH on protein function • normal A:B ratio 1:20 – strength is defined in terms of the tendency to donate (or accept) the hydrogen ion to (from) the solvent (i.e. water in biological systems) Relation pH and H+ pH H+ nanomol/L 7,80 16 7,60 26 7,40 40 7,20 63 7,00 100 Henderson-Hasselbalch equation + - 0.03 PCO2 H + HCO3 + • H = 800 x --------------- K d = ————— - CO HCO3 2 disolved - HCO3 Metabolic by kidneys • pH = 6.1 + log --------- CO2 disolved Respiratory by lungs pH pH is and indirect measure of [H+] CAVE! Hydrogen ions (i.e. protons) do not exist free in solution but are linked to adjacent water molecules by + hydrogen bonds (H3O ) [H+] by a factor of 2 causes a pH of 0.3 normal plasma pH pH 7.4 (7.36-7.44) normal Acidosis <7.35 pH 7.45> alkalosis Range compatible with life 6.80-7.80 (H+ 160-16 nM) Acid-base disturbances Why of interest? • Frequently encountered • Are (first) sign of illness • Can be used to diagnose the disease • Require early treatment Examples of diseases resulting in acid-base disturbance • CZS: CVA, bleeding, trauma • Circulation: hypotension, myocardial infarction • Respiratory: COPD, asthma. • Renal: acute / chronic kidney injury • Tr dig: vomiting, chronic diarrhea. • Liver: acute / chronic liverfailure H+-regulation • Chemical buffering • Control of PCO₂ - • Control of plasma HCO3 Regulation of acid-base balance How ? • Dilution (distribution) • Chemical buffering (intra – and extracellular) • Regulation of CO2 concentration (respiratory) + - • Regulation of H and HCO3 concentration (metabolic) Acid excretion Lungs • 20 – 50 mol volatile acid/day – (2 – 4 liter concentrated HCL) Kidneys • 80 mmol/l non-volatile, sulfate, phosphate, urineacid, citrate, ammoniumsalts. x 1000 times more acid trough lungs than kidneys Respiratory system - CO2 + differences in the stimulation of respiration by pCO2, H and pO2 alveolar ventilation disturbances acidemia respiratory centre of the brain alveolar ventilation CO2 alkalemia respiratory centre of the brain alveolar ventilation CO2 Respiratory regulation Influence of pH on breathing Daily CO2 production : 15.000 mmol/day H+ chemoreceptors medulla oblongata • pH ↑ alveolar ventilation /Minute volume • pH alveolar ventilation /Minute volume↑ Renal regulation - • Serum pH ↑ Urine pH↑ = netto HCO3 excretion • Serum pH Urine pH = netto H+ excretion Regulation kidney in summary serum pH↑ - HCO3 resorption H+ secretion = Netto base loss Urine buffers serum pH - HCO3 resorption ↑ H+ secretion↑ = Netto acid loss Urine buffers ↑ Quantative rules (see Berend et al. NEJM) • Compensation of acid-base disturbance is bound by quantative rules • Are this rules disregarded, than there is a mixed-acid-base disturbance • Over - or undercompensation does not exist - HCO3 ( nier ) Rules in general pH =pK + log ---------------- CO2 ( long ) - • Definition acid-base distubance pH, PCO2 en HCO3 • Compensation is usually not complete pH < 7.35 acidosis pH > 7.45 alkalosis • Inadequate compensation Mixed disturbance - • Direction compensation the same HCO3 / CO2 ratio. Assessment of A-B balance Arterial blood Mixed venous blood range range pH 7.40 7.35-7.45 pH 7.33-7.43 pCO 40 mmHg 35 – 45 pCO2 41 – 51 pO2 95 mmHg 80 – 95 pO2 35 – 49 Saturation 95 % 80 – 95 Saturation 70 – 75 BE 2 BE - - HCO3 24 mEq/l 22 - 26 HCO3 24 - 28 Disorders of A-B balance Acidosis: abnormal condition lowering arterial pH Alkalosis: abnormal condition raising arterial pH Homeostatic response predictable Simple A-B disorders: there is a single primary aetiological acid-base disorder Mixed A-B disorders: more primary aetiological disorders are present simultaneously Causes Respiratory abnormal processes which tend to alter pH because of a primary change in pCO2 levels acidosis alkalosis Metabolic abnormal processes which tend to alter pH because of a - primary change in [HCO3 ] acidosis alkalosis Stepwise approach 1. History 2. Look at the pH 3. Look at PCO2 and HCO3- 4. In metabolic acidosis: what is anion gap? 5. With high AG: delta-ratio? 6. Normal AG: calculate urine AG 7. Is compensation adequate? 8. In respiratory process: acute or chronic? Metabolic acidosis (MA) - • primary disorder is a pH due to HCO3 : – fixed [H+] = high anion gap - – loss or reabsorption of HCO3 = normal anion gap • Anion Gap Use and limitations of Anion Gap • [Na+]- [Cl⁻]-[HCO³⁻] • Calculate the excess of unmeasured anions • Range 8-12 mM/L Correction for albumin (alb ↓1 g/L->↑ 2.5 AG) High Anion Gap Acidosis Berend K, et al. NEJM 2014 (oct): 371:15 Normal Anion Gap Acidosis Berend K, et al. NEJM 2014 (oct): 371:15 Primary acid-base disturbances Disturbance Compensation Classification - pH HCO3 PCO2 metabolic acidosis with respiratory compensation - PCO2 ↑ HCO3 ↑ respiratory acidosis with metabolic compensation - pH ↑ HCO3 ↑ PCO2 ↑ metabolic alkalosis with respiratory compensation - PCO2 HCO3 respiratory alkalosis with metabolic compensation Diagnosis mixed acid-base disturbances There is a discrepancy between real and expected compensation • compensation CO2 ↑↑ : extra acidosis • CO2 : extra alkalosis - • compensation HCO3 ↑↑ : extra alkalosis - HCO3 : extra acidosis Berend K, et al. NEJM 2014 (oct): 371:15 Case 1 44 yr man dehydrated with sever diarrhea. pH 7.31/33/bic 16/93 Na 134/K 2.9/Cl 108/Cr 150/Ur 25 What is the acid base disorder? Answer 1.Based on history: normal AG because of diarrhea or elevated AG because of lactic acidosis due to hypovolemia 2.Look at pH 3.Look at the process (HCO3 and PCO2) 4.Calculate the AG: =10 5.Compensation adequate PCO2= 1.5 x bic +8 (+/-2)= 30-34 C/ Normal AG acidosis with adequate compensation, most likely due to diarrhea Case 2 22 yr female with DM1, N/V+, polyuria, abnormal breathing pH 7.27/23/bic 10 Na 132/K 6/Cl 93/gluc 36/Cr 200 What is acid-base disorder? Answer 1.History: elevated AG because of DKA or lactic acidosis secondary to hypovolemia due to vomiting and polyuria; metabolic alkalosis due to vomiting 2.Look at the pH 3.What is the primary process (bic/pCO2)? 4.Calculate AG=28 5.Is compensation adequate? = 1.5 ×11 + 8 ± 2 = 22.5 - 26.5. C/ High AG metabolic acidosis due to diabetic ketoacidosis Case 3 70 yr man, with history of CHF, increased dyspnoe and leg swelling pH 7.24/60/bic 27/ 52 What is acid-base disorder? Answer 1.History acute respiratory acidosis due to acute pulmonary edema 2.Look at pH 3.What is process? (PCO2/bic)? 4.Acute/chronic? 60-40=20/10=2 +24=26 (is almost bic 27) so acute respiratory acidosis C/ Acute respiratory acidosis secondary to pulmonary edema Questions? http://fitsweb.uchc.edu/student/selectives/TimurGraha m/Welcome.html .
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