2. BASIC TERMINOLOGY pH : pH signifies free hydrogen ion concentration. pH is inversely related to H+ ion concentration. Increase in pH means H+ion is decreasing. Decrease in pH means H+ ion is increasing 9/15/2009
3. Acid and Base Acid : A substance that can “donate” H+ ion or when added to solution raises H+ ion (i.e., lowers pH). Base : A substance that can “accept” H+ ion or when added to solution lowers H+ ion (i.e., raises pH). 9/15/2009
4. Anion and Cation Anion : An ion with negative charge (i.e., Cl-, HCO3-) Cation : An ion with positive charge (i.e., Na+, K+, Mg+) 9/15/2009
5. Acidosis and Alkalosis Acidosis : Abnormal process or disease, which reduces pH due to increase in acid or decrease in alkali. Alkalosis : Abnormal process or disease, which increases pH due to decrease in acid or increase in alkali. 9/15/2009
7. Regulation of pH in a narrow range of 7.35 – 7.45 is essential for normal cellular enzymatic reaction and for normal ionic concentration. Extreme ranges of pH (less than 7.2 or more than 7.55) are potentially life threatening because of disruption of many vital cellular enzymatic reactions and physiological process. 9/15/2009
8. BASIC PHYSIOLOGY OF ACID BASE REGULATION The body maintains pH within a normal range in spite of variation in dietary intake of acid and alkali and endogenous acid production. The regulation of pH in a narrow range, is the function of buffers, lungs and kidneys. 9/15/2009
9. Buffers Buffers are chemical systems, which either release or accept H+ion. Buffers act fastest but have least buffering power. Most important buffers are - 1. Bicarbonate – carbonic acid systems 2. Plasma proteins 3. Haemoglobulin 4. Phosphates 9/15/2009
10. Respiratory regulation By excreting volatile acids, lung regulates PaCo2. Respiratory regulation acts rapidly (in seconds to minutes) and has double buffering power as compared to chemical buffer. Failure of regulation causes Respiratory acidosis Respiratory alkalosis 9/15/2009
11. Renal regulation The role of kidney is to maintain plasma Hco3 concentration. It has the most powerful buffering system, which starts within hours and takes 5-6 days for the peak effect. Failure of regulation Metabolic acidosis Metabolic alkalosis 9/15/2009
12. If initial disturbance affects Hco3 Metabolic acidosis (fall in bicarbonate) Metabolic alkalosis (raise in bicarbonate) If initial disturbance affects PaCo2 Respiratory acidosis (raise in PaCo2) Respiratory alkalosis (fall in PaCo2) When you see “Metabolic” – Think of Hco3 and when you see “Respiratory” – Think of PaCo2. 9/15/2009 4 primary acid base disorders
14. Evaluation and investigations History and examination : Careful history and examination can provide clue for underlying clinical disorders. Diarrhea or ketoacidosis metabolic acidosis Presence of Kussmaul’s breathing Metabolic acidosis. 9/15/2009
15. Primary investigations Basic investigations are essential as they may provide clue for underlying disorders. Most useful investigations are serum sodium, potassium, chloride, Hco3 and anion gap. Other relevant investigations are CBC, urine examination, urine electrolytes, blood sugar, renal function test etc. 9/15/2009
16. Arterial blood gases (ABG) : As blood gases require arterial puncture and is relatively expensive, it should not be performed as a routine investigation. 9/15/2009
17. Indications for ABG Critical and unstable patients where significant acid base disorder is suspected. If history, examination and serum electrolytes suggest severe progressive acid base disorders. Sick patient with significant respiratory distress, secondary to acute respiratory diseases or exacerbation of chronic respiratory diseases 9/15/2009
18. Interpretation of basic Investigations 1) pH Normal value : 7.4 (7.35 - 7.45) Normal pH : It suggests either absence of disorders or presence of mixed disorder. Low pH (<7.35) : Suggests acidosis High pH (> 7.45) : Suggests alkalosis. 9/15/2009
21. 4. Anion gap (AG) The charge difference between unmeasured anion and cation is termed as Anion gap (AG). Anion gap (AG) = Na – (Cl + Hco3) = 122 mEq/L (Normal value). 9/15/2009
22. Importance of Anion Gap : In case of mixed acid-base disorders (eg. Metabolic acidosis and metabolic alkalosis co exist) in same patient, the pH may be normal or near normal. When pH is normal an elevated AG denotes presence of metabolic acidosis. Anion gap is most useful to establish etiological diagnosis of metabolic acidosis. 9/15/2009
23. METABOLIC ACIDOSIS Metabolic acidosis is characterized by Fall in pH Fall in HCO3 Fall in PaCO2 (compensatory change – hyperventilation). 9/15/2009
24. Pathophysiology : Metabolic acidosis can result from – 1. Loss of base – HCO3 via GI tract or kidneys (diarrhea, proximal RTA). 2. Over production of metabolic acids in the body (ketoacidosis or lactic acidosis) 3. Ingestion or infusion of acid or potential acids (salicylates or NH4Cl). 4. Failure of H+ excretion by kidney (renal failure). 9/15/2009
25. Etiology : Calculation of anion gap is helpful in narrowing etiological diagnosis. On the basis of AG, causes of metabolic acidosis can be divided into 2 groups. Normal anion gap (hyperchloraemic) acidosis High anion gap (normochloraemic) acidosis 9/15/2009
26. Treatment : 1) Specific management of underlying disorder 2) Alkali therapy Indications : a. When blood pH drops below 7.15 b. When HCO3 fall below 10 mEq/L c. Treatment of hyperkalemia with metabolic acidosis. 9/15/2009
27. Amount of HCO3 required (mEq/L) = (desired HCO3 – actual HCO3) x 0.5 x body weight in kg. 9/15/2009
28. 50% of the calculated NaHCO3 is infused with 5% dextrose over 4-5 hours. Remaining 50% of NaHCO3 is infused gradually over 24 hours. 3) Correct volume and electrolyte deficits 9/15/2009
29. METABOLIC ALKALOSIS Metabolic alkalosis is characterized by Elevated pH Elevated HCO3 Elevated PaCO2 (compensatory change – hypoventilation) 9/15/2009
30. Pathophysiology Generation of metabolic alkalosis : The loss of hydrogen ion form upper GI tract (vomiting) or urine (diuretics). Addition of alkali : Administration of HCO3 or its precursors as citrate (multiple transfusions of citrated blood). 9/15/2009
31. Urine chloride concentration differentiates metabolic alkalosis in to saline responsive and saline resistant metabolic alkalosis. The differential diagnosis is based on urine Cl- <20 mEq/L (chloride/saline responsive) Cl- > 20 mEq/L (chloride/saline resistant). 9/15/2009
32. Treatment : A) Treatment of underlying cause Saline (chloride / volume) responsive alkalosis. Aim of therapy is adequate correction of volume, chloride and K+ deficit. IV isotonic saline with KCl or isolyte G are preferred infusion. Treatment with H2 inhibitors or proton pump inhibitors will reduce gastric acid secretion and will minimize further H+ loss due to vomiting or nasogastric suction. Avoid or discontinue exogenous source of alkali such as NaHCO3 infusions, ringer’s lactate, acetate or citrate. In severe metabolic alkalosis, diluted Hcl can be given IV to lower the plasma HCO3 concentration. Dialysis therapy may be useful in occasional patients with severe metabolic alkalosis, volume overload and renal failure. 9/15/2009
33. C) Saline (chloride/volume) resistant metabolic acidosis : Metabolic alkalosis due to certain causes like oedemaotus states, mineralocorticoid excess, may be saline resistant. It needs specific treatment of underlying causes (surgical treatment of pituitary tumor or adrenal adenoma in Cushings syndrome) or supportive treatment such as potassium sparing diuretics (amiloride, spiranolactone), correction of hypokalemia and sodium restriction. 9/15/2009
34. RESPIRATORY ACIDOSIS Respiratory acidosis is characterized by Fall in pH Elevated PaCO2 Elevated HCO3 (Compensatory change) It occurs when the effective alveolar ventilation (CO2 excretion by lung) fails to keep pace with the rate of CO2 production. 9/15/2009
35. Clinical features Mild to moderate hypercapnia – Sleep disturbances, day time somnolence, loss of memory, personality changes, tremor, and myoclonic jerks. Severe hypercapnia – Headache, dyspnoea, hallucination, psychosis, papilloedema and coma. 9/15/2009
36. Treatment A) General measures : The major goal of therapy is to identify and treat the underlying cause promptly. Establish patent airway and restore adequate oxygenation. If a patient with chronic hypercapnia develops sudden increase in PaCO2, search for the aggravating factor. Vigorous treatment of pulmonary infection, bronchodilator therapy and removal of secretions can offer considerable benefits in such patients. 9/15/2009
37. B) Oxygen therapy : Role of oxygen therapy in respiratory acidosis is like a “double edged sword”. In acute respiratory acidosis, major threat to life is hypoxia. So oxygen supplementation is needed. In chronic hypercapnia, hypoxemia may be the primary and only stimulus to respiratory, injudicious therapy can be lead to further reduction in alveolar ventilation and aggravate hypercapnia drastically. If PaO2 is 50mm of Hg or higher, O2 therapy is generally not needed. If PaO2 is less than 50 mm of Hg, O2 should be given. 9/15/2009
38. C) Mechanical ventilatory (MV) support : Indications : Unstable, symptomatic or progressively hypercapnic (PaCO2 > 80mm of Hg) patients. If the patient exhibits signs of muscle fatigue, start MV before respiratory failure occurs. Refractory severe hypoxia or apnoea. Depression of respiratory center (eg. Drug over dose) D) Alkali therapy : Avoid alkali therapy, except in patients with associated metabolic acidosis. 9/15/2009
39. RESPIRATORY ALKALOSIS Respiratory alkalosis is characterized by Elevated pH Fall in PaCO2 Fall in HCO3 (compensatory change) 9/15/2009
40. Clinical features Light headache, Tingling of extremities, Circumoral anaesthesia, Cardiac arrhythmias and Infrequent tetany or seizures. PaCO2 below 20-25 mm of Hg is a grave prognostic sign. 9/15/2009
41. Treatment Vigorous treatment of underlying cause. Mild alkalosis with few symptoms need no direct treatment. As hypoxemia is the common cause of hyperventilation, O2 supplementation is essential along with etiological diagnosis and treatment. In the absence of hypoxemia, hyperventilation needs reassurance and rebreathing in a paper bag. Pretreatment with acetazolamide minimizes symptoms due to hyperventilation at high altitude. Associated treatment of hypocalcemia. 9/15/2009
