内科学课件：33 酸碱平衡

Acid production in the bodyCarbonic acid:the metabolism of carbohydrates and fats (primarily derived from the diet) results in the production of approximately 15,000 mmol of CO2 per day. Non-carbonic acid:Organic: lactate, metabolized by the liver and kidneyInorganic : the metabolism of proteins and other substances results in the generation of noncarbonic acids (50 100 mEq, 1mEq/kg). Methionine glucose + urea + SO4(2-) + 2 H+ Arginine+ glucose (or CO2) + urea + H+ R-H2PO4 + H2O ROH + 0.8 HPO42- / 0.2 H2PO4- + 1.8 H+ 机体对酸负荷的反应The homeostatic response to acid load 1. Chemical buffering by the extracellular and intracellular buffers.2. Changes in alveolar ventilation to control the PCO2.3. Alterations in renal H+ excretion to regulate the plasma HCO3- concentration.Chemical buffering Extracellular buffers Intracelluar: boneHenderson-Hasselbalch equation(Eq. 1) H+ + HCO3- H2CO3 H2O + CO2 PCO2(Eq. 2) H+ = 24 x HCO3-or by the Henderson-Hasselbalch equation HCO3-(Eq. 3) pH = 6.10 + log 0.03 PCO2 Henderson-Hasselbalch equation(Eq. 1) H+ + HCO3- H2CO3 H2O + CO2 PCO2(Eq. 2) H+ = 24 x HCO3-or by the Henderson-Hasselbalch equation HCO3-(Eq. 3) pH = 6.10 + log 0.03 PCO2Acidosis: PCO2=1.5 X HCO3 + 8 Chemical buffering Extracellular buffers Intracelluar buffer: bone, Ca+ release, osteoclast activationThe homeostatic response to acid load 1. Chemical buffering by the extracellular and intracellular buffers.2. Changes in alveolar ventilation to control the PCO2.3. Alterations in renal H+ excretion to regulate the plasma HCO3- concentration.The homeostatic response to acid load 1. Chemical buffering by the extracellular and intracellular buffers.2. Changes in alveolar ventilation to control the PCO2.3. Alterations in renal H+ excretion to regulate the plasma HCO3- concentration.RENAL HYDROGEN EXCRETION(1) reabsorption of the filtered HCO3-(2) excretion of the 50 to 100 meq of H+ produced per day1.Formation of titratable acid2.Excretion of NH4+ in the urineCollecting tubuleTubular LumenPeritubular capillaryH+H2O2OH- + CO23HCO3-CAH+Cl-ATPaseATPaseH+K+Excretion of H+ in a intercalated cellsH+H+Collecting tubuleTubular LumenPeritubular capillaryH+H2O2OH- + CO23HCO3-CAH+HPO42-H2PO4Cl-ATPaseATPaseH+K+Excretion of H+ in a intercalated cellsCollecting tubuleTubular LumenPeritubular capillaryH+H2O2OH- + CO23HCO3-CAH+NH3NH4+Cl-H+-ATPaseNH3Excretion of H+ in a intercalated cellsCan be stimulated by low KAcid-base balance The kidneys must excrete the 50 to 100 meq of noncarbonic acid generated each day. The daily acid load is excreted as NH4+ and H2(PO4). The daily acid load also cannot be excreted unless virtually all of the filtered HCO3- has been reabsorbed, because HCO3- loss in the urine is equivalent to adding H+ ions to the body. Regulation: The extracellular pH the effective circulating volume, aldosterone, and the plasma K+ concentrationCan be independent of serum pH酸碱测定指标 pH PaCO2 标准碳酸氢盐 实际碳酸氢盐 缓冲碱 碱剩余、碱缺乏 CO2CP AGAnion Gap AG=Na+-Cl-HCO3- = 122 albumin: negative charged. Low serum albumin will reduce AG. Paraprotein (Ig or light chains, MM): positive charged. Presence of large amount of paraprotein reduces AG.Metabolic acidosis Influx of organic acid into plasma (high anion gap) Ketoacidosis Lactic acidosis Poisoning Accumulation of endogenous acids (high anion gap) Renal failure External losses of bicarbonate (normal anion gap; hyperchloremic). GI loss Renal lossAnion GapRenal failure With mild to moderate reductions in GFR, the acidosis reflects decreased ammoniagenesis and is therefore hyperchloremic. As kidney failure worsens, the kidney loses its ability to excrete various anions, and the accumulation of sulfate, phosphate, and other anions, produces an elevated AG. Renal failure Despite a daily net positive acid balance, it is unusual for HCO3to fall lower than 15 mmol/L. The buffering of protons by bone results in loss of calcium and a negative calcium balance. Chronic acidosis causes protein breakdown, muscle wasting, and a negative nitrogen balance. Maintenance of the acid-base balance close to normal can prevent these consequencesTreatment Alkali replacement NaHCO3 Sodium citrate Causes: Renal loss of alkali RTA GI loss of alkali Reciprocal changes in Cl and HCO3 result in normal AG In the absence of such a relationship suggests a mixed disturbanceDiarrhea Metabolic acidosis Metabolic acidosis and hypokalemia increase renal synthesis and excretion of NH4+, thus urinary pH is around 6 Urinary NH4 levels are high: urine anion gap is negativeProximal RTA (type 2) The threshold for HCO3- reabsorption in the proximal tubule is lower (normal: 26 -28 mmol/l). The distal nephron has a low capacity for HCO3 reabsorption. In the steady state, the serum HCO3 concentration usually is 16 18 mmol/l, when all the filtered HCO3 is reabsorbed. Despite systemic acidemia development, the urine pH is alkaline. However under steady state, the urine can be acidified to a pH of less than 5.5.HCO3HCO3HCO3Proximal RTA (type 2) The threshold for HCO3- reabsorption in the proximal tubule is lower (normal: 26 -28 mmol/l). In the steady state, the serum HCO3 concentration usually is 16 18 mmol/l, when all the filtered HCO3 is reabsorbed. Despite systemic acidemia, the urine pH is alkaline. However under steady state, the urine can be acidified to a pH of less than 5.5.Proximal RTA: hypokalemia Increased distal Na+ delivery (NaHCO3) Increased aldosterone levels (dehydration because of loss of Na in the urine). Treatment of acidosis with HCO3 improves the acidosis but worsens the degree of hypokalemia.Causes of Proximal RTA Inherited pRTA: NBCe1/SLC4A4) mutation, accompanied by ocular abnormalities such as cataracts, glaucoma. Carbonic anhydrase inhibitor: acetazolamide Fanconi syndrome: inherited and acquired Adult with Fanconi: dysproteinemic condition such as multiple myelomadRTA (type 1)HCO3HCO3HCO3dRTA Hyperchloremic acidosis Kidney stone Hypokalemia Sjogren syndromedRTA: kidney stone Urinary calcium excretion is high Acidosis induced bone mineral dissolution Low intraluminal concentration of HCO3- because of acidosis Urinary citrate levels are low citrate serve as the major Ca+ chelator in the urine High urine pH decrease the solubility of calcium phosphate complexes.dRTA Primary: idiopathic or inherited (SLC4A1 mutation) Systemic disease: Sjogren syndromedRTA-diagnosis NH4Cl Furosemide + mineralocorticoid (fludrocortisone)Type 4 RTA Renal function compromised Hyporeninemic hypoaldosteronism Hyperkalemia Urinary ammonium excretion depressedMetabolic Alkalosis An elevated arterial pH An increase in the serum HCO3- and a increase in PCO2 Often accompanied by hypochloremia and hypokalemiaPathogenesis Generative stage: loss of acid Maintenance stage: volume contraction, a low GFR or depletion of Cl or KDifferential diagnosis Mineralocorticoid excess Bartters or Gitelmans Diuretics Gastrointenstinal HCO3 retention + volume contraction Renal origin Diuretics Nonreaborbable anions and magnesium deficiency Potassium depletion After treatment of lactic acidosis or ketoacidosis posthypercapnia Mineralocorticoid administration or excess production Related electrolyte abnormalities: hypokalemia治疗原则 治疗原发病 纠正容量、低血钾 补酸Respiratory acidosis Severe pulmonary disease, respiratory muscle fatigue or abnormalities in ventilatory control Acute: immediate compensatory elevation in HCO3, which increases 1 mmol/L for every 10 mmHg increase in pCO2 Chronic (24h): renal adaptation increases the HCO3 by 4 mmol/L Clinical features The clinical feature varies according to Severity and duration Underlying disease Whether there is hypoxemia A rapid increase in pCO2: anxiety, dyspnea, confusion coma Chronic hypercapnea: sleep disturbances, loss of memory, .Treatment Acute respiratory acidosis can be life-threatening, measures to reverse the underlying cause should be undertaken simultaneously with restoration of adequate alveolar ventilation Chronic respiratory acidosis Improving lung functionRespiratory alkalosis Alveolar hyperventilation decreases PaCO2 and increases the HCO3/PCO2病因 中枢性 非低氧性：癔症、脑外伤、药物、温度过高、肝性脑病、酸中毒 低氧因素：高原、肺部疾病、供血不足 外源性：呼吸机管理不当、胸廓腹部手术、呼吸道阻塞突然解除 The effect of respiratory alkalosis vary according to duration and severity but are primarily those of the underlying disease Hyperventilation syndrome Paresthesia, circumoral numbness, chest wall tightness, dizziness Salicylates are the most common cause of drug induced respiratory alkalosis Progesterone increases ventilation Respiratory alkalosis is often an early finding of G- septicemiaSteps in acid-base diagnosisObtain arterial blood gas (ABGs) and electrolytes simultaneouslyCompare HCO3-on ABGs and electrolytes to verify accuracyCalculate anion gap (AG)Know 4 causes of high AG acidosis Ketoacidsis Lactic acid acidosis Renal failure ToxinsKnow 2 causes of hyperchloremic or nongap acidosis Bicarbonate loss from GI, RTAEstimate compensatory responseCompare AG and HCO3-Compare change in Cl with change in NaHenderson-Hasselbalch equation(Eq. 1) H+ + HCO3- H2CO3 H2O + CO2 PCO2(Eq. 2) H+ = 24 x HCO3-or by the Henderson-Hasselbalch equation HCO3-(Eq. 3) pH = 6.10 + log 0.03 PCO2Acidosis: PCO2=1.5 X HCO3 + 8