ACoRN © Blood gases and acid-base balance

ACoRN © What is pH? a measure of the acidity or alkalinity of a solutiona measure of the acidity or alkalinity of a solution the negative logarithm of [H + ]the negative logarithm of [H + ] the more acidic a solution, the smaller the pH valuethe more acidic a solution, the smaller the pH value neutral pH = 7neutral pH = 7 change in pH of 1 = 10 fold change in [H + ]change in pH of 1 = 10 fold change in [H + ]

ACoRN © pH is determined by: dissolved CO 2 = PCO 2dissolved CO 2 = PCO 2 metabolic acidsmetabolic acids buffersbuffers

ACoRN © pH estimates the blood total acid load, which mostly reflects dissolved CO 2 but may also include metabolic acids such as lactic acid PCO 2 indicates how well the lung is removing CO 2 from the blood (ventilation) Application

ACoRN © PCO 2 indicates how well the lung is removing CO 2 from the blood (ventilation)PCO 2 indicates how well the lung is removing CO 2 from the blood (ventilation) CO 2 (respiratory acid) accumulates when breathing is insufficient: respiratory acidosisCO 2 (respiratory acid) accumulates when breathing is insufficient: respiratory acidosis CO 2 + H 2 O ↔ H 2 CO 3 ↔ H + + HCO 3 - LUNG KIDNEY Excessive elimination of CO 2 leads to respiratory alkalosisExcessive elimination of CO 2 leads to respiratory alkalosis PCO 2

ACoRN © Lactic acid accumulates when respiration at a tissue level is insufficient, due to inadequate oxygen delivery: metabolic acidosisLactic acid accumulates when respiration at a tissue level is insufficient, due to inadequate oxygen delivery: metabolic acidosis The bicarbonate (base) buffer is the main system that neutralizes metabolic acids to maintain a physiologic pHThe bicarbonate (base) buffer is the main system that neutralizes metabolic acids to maintain a physiologic pH base acid pH Metabolic component

ACoRN © The base deficit (BD) is calculated to estimate the amount of excess metabolic acid.The base deficit (BD) is calculated to estimate the amount of excess metabolic acid. BD “How much alkali needs to be added to return the pH to 7.40 if the PCO 2 is 40.” Metabolic component

ACoRN © Arterial, capillary or venous samples are nearly equally useful for the determination of ventilation (PCO 2 ), pH and base deficit but quite different for oxygenation (PO 2, SaO 2 ) Application

ACoRN © Dissolved oxygen Ratio of 1 to 40 Bound Dissolved Oxygen

ACoRN © pH 7.35 to 7.45 PCO 2 35 to 45 mmHg BD -4 to +4 mmol/L HCO 3 22 to 26 mmol/L PaO 2 50 to 80 mmHg Normal blood gas values

ACoRN © pH 7.25 to 7.40 PCO 2 45 to 55 mmHg BDNA HCO 3 NA PaO 2 50 to 80 mmHg Acceptable blood gas values for acute respiratory illness

ACoRN © Blood gas interpretation 1.Is the pH normal decreased (acidosis) increased (alkalosis) 2.Is the primary problemrespiratory metabolic 3.Is there compensationrespiratory renal 4.Is the arterial oxygenationnormal decreased (hypoxemia) increased (hyperoxemia)

ACoRN © pH PCO 2 BDInterpretation ↓↑N respiratory acidosis ↓N↑ metabolic acidosis ↑↓N respiratory alkalosis ↑N↓ metabolic alkalosis 1)Normal, acidosis or alkalosis? 2)Respiratory, metabolic, mixed, or compensated? Blood gas interpretation

ACoRN © Compensation: secondary changes that dampen the pH change pH indicates the primary problempH indicates the primary problem respiratory compensation is rapid; renal compensation slowerrespiratory compensation is rapid; renal compensation slower overcompensation occurs only if we cause itovercompensation occurs only if we cause it

ACoRN © most acid-base problems in babies result from inadequate ventilation (respiratory acidosis)most acid-base problems in babies result from inadequate ventilation (respiratory acidosis) acidosis with pH ≤ 7.25 and PCO 2 ≥ 55 indicates poor ventilation requiring correction (ventilation)acidosis with pH ≤ 7.25 and PCO 2 ≥ 55 indicates poor ventilation requiring correction (ventilation) in metabolic acidosis correct the cause - generally poor tissue perfusionin metabolic acidosis correct the cause - generally poor tissue perfusion Blood gas interpretation

ACoRN © Hypoxia: inadequate delivery of oxygen to tissuesHypoxia: inadequate delivery of oxygen to tissues Generally due to arterial hypoxemiaGenerally due to arterial hypoxemia –PaO 2 less than 50 mmHg –SaO 2 (~ SpO 2 ) less than 85%

ACoRN © Case 1: Billy Billy was delivered by emergency Caesarean delivery for fetal distressBilly was delivered by emergency Caesarean delivery for fetal distress RR 26, HR 124, his respiratory effort is weakRR 26, HR 124, his respiratory effort is weak SpO 2 90% in 50% oxygenSpO 2 90% in 50% oxygen He is hypotonicHe is hypotonic Blood gas:Blood gas: –pH 7.16, PCO 2 70, BD 7, HCO 3 21

ACoRN © pH 7.16, PCO 2 70, BD 7 pH PCO 2 BDInterpretation ↓↑N Respiratory acidosis ↓N↑ Metabolic acidosis ↑↓N Respiratory alkalosis ↑N↓ Metabolic alkalosis 1)Normal, acidosis or alkalosis? 2)Respiratory, metabolic, mixed or compensated?

ACoRN © Case 1: Billy Billy had a asphyxial insult.Billy had a asphyxial insult. Inadequate respirations  high PCO 2 (respiratory acidosis).Inadequate respirations  high PCO 2 (respiratory acidosis). Tissue hypoxia  metabolic acidosis.Tissue hypoxia  metabolic acidosis. Is this blood gas acceptable?Is this blood gas acceptable?

ACoRN © Case 2: Belinda 28 weeks’ gestation28 weeks’ gestation Developed RDSDeveloped RDS Is ventilatedIs ventilated Blood gas:Blood gas: –pH 7.47, PCO 2 28, HCO 3 22, BD 4

ACoRN © pH 7.47, PCO 2 28, BD 4 pH PCO 2 BDInterpretation ↓↑N Respiratory acidosis ↓N↑ Metabolic acidosis ↑↓N Respiratory alkalosis ↑N↓ Metabolic alkalosis 1)Normal, acidosis or alkalosis? 2)Respiratory, metabolic, mixed or compensated?

ACoRN © Case 2: Belinda Belinda has respiratory alkalosisBelinda has respiratory alkalosis Excessive ventilationExcessive ventilation Correct byCorrect by –decreasing ventilator rate –decreasing tidal volume or –decreasing inspiratory pressure

ACoRN © Case 3: Jacinta 14 days, 38 weeks’ gestation, SGA14 days, 38 weeks’ gestation, SGA Mother is a substance userMother is a substance user Poor feeding and watery stools x 3 daysPoor feeding and watery stools x 3 days One wet diaper in the last 24 hoursOne wet diaper in the last 24 hours Cool to touch and delayed capillary refillCool to touch and delayed capillary refill Respirations easy but rapidRespirations easy but rapid Blood gas: pH 7.36, PCO 2 25, HCO 3 12, BD 10.Blood gas: pH 7.36, PCO 2 25, HCO 3 12, BD 10.

ACoRN © pH 7.36, PCO 2 25, BD 8 pH PCO 2 BDInterpretation ↓↑N Respiratory acidosis ↓N↑ Metabolic acidosis ↑↓N Respiratory alkalosis ↑N↓ Metabolic alkalosis 1)Normal, acidosis or alkalosis? 2)Respiratory, metabolic, mixed or compensated?

ACoRN © Case 3: Jacinta Compensated metabolic acidosisCompensated metabolic acidosis Fluid loss (diarrhea) plus poor intake  dehydration + metabolic acidosisFluid loss (diarrhea) plus poor intake  dehydration + metabolic acidosis Mechanisms:Mechanisms: –tissue hypoperfusion  lactic acidosis –bicarbonate losses in stool –respiratory compensation by “blowing off” CO 2

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