Acid and base balance physiology and disturbances Dr. Ayad Abbas Salman

PH If hydrogen ion of pure H2O at room temperature is equal to 1/10.000.000 mol/L → [H+] = 1/10.000.000 → [H+] = 10-7 → log [H+] = -7 → -log [H+] = 7 → pH = 7

solutions Neutral solution → [H+] = [OH-] = 10-7 → pH =7 Acidic solution → [H+] > [OH-] → pH < 7 Alkaline solution → [H+] < [OH-] → pH > 7 Range of blood pH which compatible with life is only (7-7.8) 6 times change in hydrogen ion concentration.

Acidosis : process that tend to lower pH Acidosis : process that tend to lower pH. ◊ Alkalosis: process that tend to rise pH ◊ Acidaemia: ↓ in blood pH < 7.35 ◊ Alkalaemia: ↑ in blood pH > 7.45

Compensatory Mechanism Physiological responses to changes in [H+] are characterized by 3 phases: 1.Immediate chemical buffering. 2.Respiatory compensation. 3.Slower but more effective renal response that may nearly normalize arterial pH.

Chemical buffering 1.Bicarbonate (H2CO3/HCO3-): is most important buffering in the extracellular fluid. H2O + CO2 (ac)↔(al) H2CO3 (ac) ↔(al) H+ +Hco3   ↑ PaCO2 Normally eliminated by lung

2.Haemoglobin (HbH/Hb-): Although restricted inside RBC, but it is an important buffer in the blood. H+ + KHb (al) ↔(ac) HHb +K+ 

3.Protein other than Hb: play a major role in buffering of intracellular fluid. Protein possess both acidic & alkaline groups. Acidic group called carboxyl group. Alkaline group called amino group. NH2 + H+ (al) ↔ (ac) NH3 COOH + OH- (ac) ↔ (al) COO + H2O

4.Phosphate (H2PO4-/ HPO4-2): it is important urinary buffer.  ↑ [H+] →HCl + Na2HPO4 →NaH2­PO4 (kidney) + NaCl di (Na) phosphate mono (Na) phosphate    ↑[OH-] →NaOH + NaH2PO4 → NaHPO4 + H2O excreted by kidney

5-Amonia (NH3/NH4+): it is also important urinary buffer. NH3 + H+ → NH4 (excreted by kidney)

Respiratory compensation Change in pH sensed by : chemoreceptors in brain stem ↑pH → hypoventilation → ↑PaCO2 ↓ pH → hyperventilation → ↓PaCO2

Renal compensation 1.Ability of kidney to control the amount of HCO3- reabsorption. 2.Ability of kidney to form new HCO3-. 3.Ability of kidney to eliminate H+ in the form of titrable acid and ammonium ion. EXAMPLES: ► H+ + HPO4-2 → H2PO4 (excreted by kidney)   ► NH3 + H+ NH4 (excreted by kidney)

Normal values and analysis PaCO2 = 40 +- 5 HCO3 = 22 +- 2 SBC* = 24+- 2 BE** = +- 3 -zero. * SBC: it is plasma [HCO3] from fully oxygenated (PaO2 = 100) blood which has been equilibrated to PCO2 40 mmHg at 37○C. ** BE: Base Excess: is an amount of strong acid or base required to titrate the pH to 7.4 at PCO2 40 mmHg

Clinical disorders PaCO2 Change: Respiratory acidosis Respiratory alkalosis Hc03 Change: Metabolic acidosis Metabolic alkalosis

examples Diag. BE Hco3 Paco2 PH UMac -20 10.8 41 7.05 URal 5 24 21 7.58 CMac -15 13.4 23 7.27 CMal 20 42.8 55 7.55

Physiological effect of acidosis 1.pH < 7.20 → direct myocardiac depression and ↓ peripheral vascular resistance → hypotension. 2.Tissue hypoxia (although O2 dissociation curve shift to the left) 3.Hyperkalaemia 4.CNS depression mainly in respiratory acidosis (CO2 narcosis) → ↑ cerebral blood flow → ↑ intracranial pressure.

Respiratory acidosis Causes of respiratory acidosis: 1.Alveolar hyperventilation ● CNS depression e.g: 1.cerebral ischemia 2.cerebral trauma 3.drug induced ● Neuromuscular disorder: myopathy, neuropathy ● Chest wall abnormality e.g kyphoscoliosis ● Pleural abnormality e.g pneumothorax, pleural effusion ● Airway obstruction: foreign body, tumor → upper airway asthma, COPD → lower airway ● Paranchymal lung disease:1.Pulmonary Oedema 2.Pulmonary Emboli 3.Pneumonia 4.Aspiration 5.Interstitial lung disease

2.↑ CO2 production   ● ↑ in enteral or paraentral nutrition of carbohydrate ● Intense shivering ● Prolong seizure activity ● Thyroid storm ● Excessive thermal injur

management ● Treatment of the cause   ● Treatment of the cause ● May → respiratory depression → mechanical ventilation ● I.V NaHCO3 is rarely necessary (i.e not used)

Metabolic acidosis causes anion Gap: defined as the difference between major measured cations and major measured anions. Anion gap = [Na+] -([Cl-] + [HCO3-] ) normally (9-15).

● 1.Incresed anion gap ↑ production of non volatile acids e.g renal failure, DKA (diabetic ketoacidosis), lactic acidosis. Ingestion of toxins e.g salicylatye, methanol, sulfer Rhabdomyositis

2.Normal anion gap ↑ GIT loss of HCO3: diarrhoea, ingestion of CaCl2/MgCl2, fistula (pancreatic, biliary, small bowel) ↑ renal loss of HCO3: renal tubular acidosis, hypoaldosteronism Dilutional: large amount of HCO3 free fluid Total paraenteral nutrition ↑ intake of chloride containing acid e.g ammonium chloride

management ph < 7.2 → Na HCO3 (8.4 %) should be given NaHCO3 = (BE X body weight) /3 giving 1/2 the dose and reassess the values ● Treatment of the cause

Alkalosis (physiological effect) 1.Hypokalemia 2.Difficult to give up O2 to tissue → hypoxia. 3.↓ ionized Ca+2 → circulatory depression and neuromuscular irritability. 4.Respiratory alkalosis → ↓ cerebral blood flow, ↑ systemic vascular resistance and may → coronary vasospasm.

Resp. Alkalosis 1○ ↓ in PaCO2 by inappropriate ↑ in alveolar ventilation relative to CO2 product. It is compensated by ↓ [HCO3] in plasma.

causes ● Central stimulation: 1.pain 2.anxiety (hystria) 3.stroke 4.ischemia 5.feve 6.infection 7.drug induced e.g salicylate ● Peripheral stimulation: 1.hypoxia 2.high altitude 3.pulmonary disease (CHF, pulmonary oedema, pulmonary emboli, asthma (severe) ● Iatrogenic: ventilator induced

management Treatment of the cause is enough For severe (pH > 7.6) i.v HCl or NH4Cl4 (ammonium chloride) my be indicated

II.Metabolic alkalosis 1○ ↑ in HCO3 Causes 1. Chloride sensitive: those causes associated with ↓ NaCl and ECF depletion ● GIT: vomiting, gastric drainage, chloride diarrhoea ● renal: diuretic ● sweat: cystic fibrosis

2.Chloride resistance: those associated with enhanced mineralocorticoid activity ● hyperaldosteronism, cushing syndrome 3.Miscellanous ● massive blood transfusion ● alkaline treatment with renal insufficiency ● hyper Ca+2 e.g metastasis

management Treatment of underlying cause For chloride sensitive causes i.v normal saline and K+ replacement When pH > 7.6 i.v : 1.HCl 2.ammonium chloride 3.Vit.C (5-10 g/day) 4.haemodialysis may be considered

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