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Acid base Acid base Presentation Transcript

  • Acid/base balance Medhat Hashem, MD, FCARCSI, FRCA Professor of Anaesthesia, Cairo University Director of Surgical ICU
  • Objectives     Basic physiology Definitions How to read ABG Examples
  • Normal concentration in ECF (m mol/L) Na 140     H+ 0.00004 Normal H+ concentration: 40 n mol/L Viable limits (20-160 n mol/L) Na ion conc is 1000 times higher. ECF Electroneutrality essential for cellular activity.
  • pH  [H+] concentration 40 n mol/L (20-160)  100 = (10)2 Log10 100 = 2 Log10 1000 = 3 pH; negative logarithm of reciprocal [H+] concentration. pH = - Log10 1/[H+]    
  • pH [H+] (nmol/L) 7.6 25 7.5 32 7.4 40 7.3 50 7.2 63 7.1 80 7.0 100 6.9 125 6.8 160
  • pH of body fluids Plasma 7.4 Gastric HCl 0.8 Urine 4.5 Pancreatic juice 8
  • Definitions       Acid: A substance that dissociates in water to H+. Acidosis: A process that causes acids to accumulate. Acidaemia; if pH<7.36 Alkali: a substance that accepts H+ Alkalosis; A process that causes bases to accumulate. Alkalaemia: if pH > 7.44
  • Buffers and compensation  Buffers: Hb, proteins, H2PO4 , H2CO3 H2O + CO2 → H2CO3 → HCO3- + H+  Respiratory compensation: Immediate  Hypo/hyperventilation to ↑or↓ CO 2   Renal compensation: After 24-48 hours  Excretion or retention of filtered HCO 3 
  • Henderson Hasselbach Equation pH ≈ HCO3/CO2  pH >7.44 → Alkalosis (alkalaemia)  <7.36 → Acidosis (acidaemia)   PaCO2 and HCO3 (and Base deficit)  PaCO2 → Respiratory  HCO3 → Metabolic
  • Blood gas analysis Blood gas machine measures  pH (7.36 - 7.44)  PaO (75 -100 mmHg), room air 2  PaCO2 (35 – 45 mmHg)  HCO3 calculated (22 – 26 mmol/L)
  • Metabolic acidosis ↓pH (↑H+)  ↓HCO (22-26 mmol/l) (↑Base deficit) 3  PaCO → normal 2  Compensation: Hyperventilation to wash CO 2 pH ≈ HCO3/CO2 pH HCO3PaCo2 Compensation  ↓ ↓↓ ↓ Hyperventilation ↓PaCO2
  • Metabolic acidosis  Overproduction of organic acids Lactic acid: shock, infection, hypoxia  Urate: renal failure  Ketones: DM, alcohol  Drugs/toxins: (salicylate, biguanide, methanol)   Excessive loss of bicarbonate
  • Metabolic acidosis   Overproduction of organic acids Excessive loss of bicarbonate: Diarrhoea  Pancreatic or small intestinal fistula  Renal tubular acidosis  Uretero-sigmodostomy  Addison’s disease 
  • Metabolic acidosis (CP)     of the cause Tachypnea (↑rate and depth) Severe acidosis → cardiovascular collapse → cardiac failure (↓BP) Death
  • Rx of metabolic acidosis Rx of the cause (e.g., insulin if DKA)  Mild to moderate: beneficial (shift O2 dissociation curve to Rt → ↑O2 delivery at tissue level).  Severe (↓pH < 7.2) → iv NaHCO 3   Amount of HCO3= Bwt x base deficit x 0.3 Start with half correction  Repeat assessment 
  • 35 yrs, M, RTA, bilateral # femur & pelvis, pH ≈ HCO3/CO2       SaO2 98% pH 7.25 PaCO2 37 mmHg HCO3 17 mmol/L BE -7 PaO2 130 mmHg (O2 by face mask) Hb 4.1 gm/dl Rx: Blood Tx, ( open book #pelvis)
  • 17 yrs, M, unconscious, syringe next to him pH ≈ HCO3/CO2      SaO2 81% pH 7.2 PaCO2 80 mmHg HCO3 22 mmol/L BE 0 PaO2 45 mmHg (O2 40%) Hb 14.5 gm/dl Respiratory acidosis (uncompensated)
  • Respiratory acidosis   ↓pH (↑H+) ↑PaCO2 (35-45 mmHg)  HCO3 (22-26 mmol/l) normal  Renal compensation (after 24 hours):  pH ↓ Reabsorption of filtered HCO3 HCO3- ↑ pH ≈ HCO3/CO2 PaCo2 Compensation ↑↑ Renal retention of HCO3-
  • Respiratory Acidosis   Always associated with hypoxia ↑PaCO2 → ↑respiratory depression (CO2 narcosis) → more hypoxia
  • Respiratory Acidosis (causes)  ↓Respiratory center Drugs (opioids-anaesthesia)  ICT brain tumours-head injury       Cervical spinal cord lesions: trans-section of SC AHC: e.g. poliomyelitis Chest wall: #ribs (flail), obesity Intercostal ms: Myasthenia Gravis Lung disease: severe COPD (blue bloater)
  • Respiratory Acidosis (CP)  Acute CO2 retention: restlessness  flapping tremors   Further CO2 rise: VD (warm limbs, papilloedema)  Drowsiness, confusion, coma   Chronic CO2 retention:   poor sleeping →day somnolence Cyanosis
  • Respiratory Acidosis (treatment)     Depends on the cause and timescale Patent airway Oxygen therapy Rx of the cause e.g., Naloxone (opioid overdose)  Drainage of hydrocephalus  fixation of flail ribs   Mechanical ventilation
  • 69 yrs, M, smoker, COPD, SOB pH ≈ HCO3/CO2      SaO2 91% pH 7.36 PaCO2 61 mmHg HCO3 32 mmol/L BE 7 PaO2 79 mmHg (O2 40%) Hb 21.5 gm/dl Compensated respiratory acidosis
  • Nurse story, post colecystectomy, 40 yrs, fit pH ≈ HCO3/CO2      SaO2 98% pH 7.55 PaCO2 36 mmHg HCO3 31 mmol/L BE 6 PaO2 111 mmHg (O2 40%) Hb 10.1 gm/dl Metabolic alkalosis (uncompensated)
  • Metabolic alkalosis   ↑pH (↓H+) PaCO2 (35-45 mmHg) normal  ↑ HCO3 (22-26 mmol/l)  Compensation: Respiratory inhibition but hypoxia limit rise of PaCO2 50 mmHg pH HCO3- ↑ ↑↑ pH ≈ HCO3/CO2 PaCo2 Compensation ↑ Hypoventilation ↑ PaCO2
  • Metabolic alkalosis  Loss of gastric HCl Repeated vomiting (pyloric stenosis)  Gastric suction   Hypokalaemia (intracellular K moves out exchange with K (maintain electro-neutrality)   diuretics Bicarbonate retention NaHCO3 administration  Milk alkali syndrome 
  • Metabolic alkalosis (CP)    The cuase Bradypnea (Chyne Stokes respiration) Tetany (↓ionized Ca)
  • Metabolic alkalosis (treatment)      Rx of the cause Cl replacement (Normal Saline 0.9%) Correcion of hypokalaemia (irritant, CVC) Iv ammonium chloride???? Rx of tetany: Ca (gluconate-chloride) 10ml SLOWLY
  • 22 yrs, M, soldier, spasticity pH ≈ HCO3/CO2      SaO2 98% pH 7.55 PaCO2 25 mmHg HCO3 25 mmol/L BE 1 PaO2 189 mmHg (O2 by face mask) Hb 13.6 gm/dl Respiratory alkalosis (uncompensated)
  • Respiratory alkalosis  ↑ pH (↓H+)  ↓PaCO2 (35-45 mmHg) 2  HCO3 (22-26 mmol/l) normal  Renal compensation (after 24 hours):  Increased renal excretion of HCO3 pH HCO3- ↑ ↓ pH ≈ HCO3/CO2 PaCo2 Compensation ↓↓ ↑Renal excretion of HCO3-
  • Respiratory alkalosis (causes)  Hyperventilation (wash out of CO2) Hysteria  Hyperpyrexia   Iatrogenic hyperventilation
  • Respiratory alkalosis (CP)     The cause Short lived, well tolerated ↓Ca+ → carpopedal spasm, parasthesia Severe respiratory alkalosis → respiratory arrest
  • Respiratory alkalosis (treatment)    Breath into paper bag (rebreathing) Readjust ventilator parameters Addition of CO2 ????
  • Base Excess  “The amount of acid or base (mmol/l) required to return the pH of 1L blood to normal at a PaCO2 40 mmHg.”  It is a measure of the magnitude of the metabolic component to the acid-base disorder
  • Standard Bicarbonate   Not the actual bicarbonate in the sample “Estimate of bicarbonate concentration after elimination of any abnormal respiratory contribution to HCO3 (i.e., an estimate of HCO3 at PaCO2 40 mmHg).
  • Thank You
  • QUESTIONS? medhashem@yahoo.com medhashem@yahoo.com