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Blood gases and Blood gases and acid-base balanceacid-base balance

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What is pH?What is pH?

• a measure of the acidity or alkalinity of a a measure of the acidity or alkalinity of a solution solution

• the negative logarithm of [Hthe negative logarithm of [H++] ]

• the more acidic a solution, the smaller the the more acidic a solution, the smaller the pH value pH value

• neutral pH = 7neutral pH = 7

• change in pH of 1 = 10 fold change in [Hchange in pH of 1 = 10 fold change in [H++]]

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pH is determined by:pH is determined by:

• dissolved COdissolved CO2 2 = PCO= PCO22

• metabolic acidsmetabolic acids• buffersbuffers

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• PCOPCO22 indicates how well the lung is removing indicates how well the lung is removing COCO22 from the blood (ventilation) from the blood (ventilation)

• COCO22 (respiratory acid) accumulates when (respiratory acid) accumulates when breathing is insufficient: respiratory acidosisbreathing is insufficient: respiratory acidosis

COCO22 + H + H22O O ↔ H↔ H22COCO33 ↔ H ↔ H+ + + HCO+ HCO33--

LUNGLUNG KIDNEY KIDNEY

• Excessive elimination of COExcessive elimination of CO22 leads to leads to respiratory alkalosisrespiratory alkalosis

PCOPCO22

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• Lactic acid accumulates when respiration at a Lactic acid accumulates when respiration at a tissue level is insufficient, due to inadequate tissue level is insufficient, due to inadequate oxygen delivery: metabolic acidosisoxygen delivery: metabolic acidosis

• The bicarbonate (base) buffer is the main The bicarbonate (base) buffer is the main system that neutralizes metabolic acids to system that neutralizes metabolic acids to maintain a physiologic pH maintain a physiologic pH

basebaseacidacid

pHpH

Metabolic componentMetabolic component

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• The The base deficit (BD)base deficit (BD) is calculated to estimate is calculated to estimate the amount of excess metabolic acid.the amount of excess metabolic acid.

BDBD““How much alkali needs How much alkali needs to be added to return to be added to return the pH to 7.40 if the the pH to 7.40 if the PCOPCO22 is 40.” is 40.”

Metabolic componentMetabolic component

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Arterial, capillary or venous samplesArterial, capillary or venous samples are are nearly equally useful for the nearly equally useful for the determination of ventilation (PCOdetermination of ventilation (PCO22), pH ), pH

and base deficit and base deficit butbut quite different for quite different for oxygenation (POoxygenation (PO22, SaO, SaO22))

ApplicationApplication

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Dissolved oxygen

Ratio of 1 to 40Ratio of 1 to 40

BoundBoundDissolvedDissolved

OxygenOxygen

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pHpH 7.35 to 7.457.35 to 7.45

PCOPCO22 35 to 45 mmHg35 to 45 mmHg

BDBD -4 to +4 mmol/L-4 to +4 mmol/L

HCOHCO33 22 to 26 mmol/L22 to 26 mmol/L

PaOPaO22 50 to 80 mmHg50 to 80 mmHg

Normal blood gas valuesNormal blood gas values

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pHpH 7.25 to 7.407.25 to 7.40

PCOPCO22 45 to 55 mmHg45 to 55 mmHg

BDBD NANA

HCOHCO33 NANA

PaOPaO22 50 to 80 mmHg50 to 80 mmHg

Acceptable blood gas valuesAcceptable blood gas valuesfor acute respiratory illnessfor acute respiratory illness

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Blood gas interpretationBlood gas interpretation1.1. Is the pH Is the pH normalnormal

decreased (acidosis)decreased (acidosis)increased (alkalosis)increased (alkalosis)

2.2. Is the primary problemIs the primary problem respiratoryrespiratorymetabolicmetabolic

3.3. Is there compensationIs there compensation respiratoryrespiratoryrenalrenal

4.4. Is the arterial oxygenationIs the arterial oxygenation normalnormaldecreased (hypoxemia)decreased (hypoxemia)increased (hyperoxemia)increased (hyperoxemia)

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pHpH PCOPCO22 BDBD InterpretationInterpretation

↓↓ ↑↑ NN respiratory acidosisrespiratory acidosis

↓↓ NN ↑↑ metabolic acidosismetabolic acidosis

↑↑ ↓↓ NN respiratory alkalosisrespiratory alkalosis

↑↑ NN ↓↓ metabolic alkalosismetabolic alkalosis

1)1) Normal, acidosis or alkalosis?Normal, acidosis or alkalosis?2)2) Respiratory, metabolic, mixed, or compensated?Respiratory, metabolic, mixed, or compensated?

Blood gas interpretationBlood gas interpretation

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• most acid-base problems in babies result most acid-base problems in babies result from from inadequate ventilationinadequate ventilation ( (respiratory respiratory acidosis)acidosis)

• acidosis with pH ≤ 7.25 and PCOacidosis with pH ≤ 7.25 and PCO22 ≥ 55 ≥ 55

indicates poor ventilation requiring indicates poor ventilation requiring correction (ventilation)correction (ventilation)

• in metabolic acidosis correct the cause - in metabolic acidosis correct the cause - generally generally poorpoor tissue perfusiontissue perfusion

Blood gas interpretationBlood gas interpretation

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• HypoxiaHypoxia:: inadequate delivery of oxygen to inadequate delivery of oxygen to tissuestissues

• Generally due to arterial hypoxemiaGenerally due to arterial hypoxemia– PaOPaO22 less than 50 mmHg less than 50 mmHg

– SaOSaO22 (~ SpO (~ SpO22) less than 85%) less than 85%

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Case 1: BillyCase 1: Billy

• Billy was delivered by emergency Billy was delivered by emergency Caesarean delivery for fetal distressCaesarean delivery for fetal distress

• RR 26, HR 124, his respiratory effort is RR 26, HR 124, his respiratory effort is weakweak

• SpOSpO22 90% in 50% oxygen 90% in 50% oxygen

• He is hypotonicHe is hypotonic

• Blood gas: Blood gas:

– pH 7.16, PCOpH 7.16, PCO22 70, BD 7, HCO 70, BD 7, HCO33 21 21

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pH 7.16, PCOpH 7.16, PCO22 70, BD 7 70, BD 7

pHpH PCOPCO22 BDBD InterpretationInterpretation

↓↓ ↑↑ NN Respiratory acidosisRespiratory acidosis

↓↓ NN ↑↑ Metabolic acidosisMetabolic acidosis

↑↑ ↓↓ NN Respiratory alkalosisRespiratory alkalosis

↑↑ NN ↓↓ Metabolic alkalosisMetabolic alkalosis

1)1) Normal, acidosis or alkalosis?Normal, acidosis or alkalosis?2)2) Respiratory, metabolic, mixed or compensated?Respiratory, metabolic, mixed or compensated?

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Case 1: BillyCase 1: Billy

• Billy had a asphyxial insult. Billy had a asphyxial insult.

• Inadequate respirations Inadequate respirations high PCO high PCO2 2

(respiratory acidosis). (respiratory acidosis).

• Tissue hypoxia Tissue hypoxia metabolic acidosis. metabolic acidosis.

• Is this blood gas acceptable?Is this blood gas acceptable?

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Case 2: BelindaCase 2: Belinda

• 28 weeks’ gestation 28 weeks’ gestation • Developed RDSDeveloped RDS• Is ventilatedIs ventilated• Blood gas: Blood gas:

– pH 7.47, PCOpH 7.47, PCO22 28, HCO 28, HCO33 22, BD 4 22, BD 4

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pH 7.47, PCOpH 7.47, PCO22 28, BD 4 28, BD 4

pHpH PCOPCO22 BDBD InterpretationInterpretation

↓↓ ↑↑ NN Respiratory acidosisRespiratory acidosis

↓↓ NN ↑↑ Metabolic acidosisMetabolic acidosis

↑↑ ↓↓ NN Respiratory alkalosisRespiratory alkalosis

↑↑ NN ↓↓ Metabolic alkalosisMetabolic alkalosis

1)1) Normal, acidosis or alkalosis?Normal, acidosis or alkalosis?2)2) Respiratory, metabolic, mixed or compensated?Respiratory, metabolic, mixed or compensated?

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Case 2: BelindaCase 2: Belinda

• Belinda has respiratory alkalosis Belinda has respiratory alkalosis

• Excessive ventilationExcessive ventilation

• Correct by Correct by – decreasing ventilator ratedecreasing ventilator rate– decreasing tidal volume or decreasing tidal volume or – decreasing inspiratory pressuredecreasing inspiratory pressure

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Case 3: JacintaCase 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 days Poor 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, PCOBlood gas: pH 7.36, PCO2 2 25, HCO25, HCO3 3 12, 12,

BD 10BD 10..

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pH 7.36, PCOpH 7.36, PCO22 25, BD 8 25, BD 8

pHpH PCOPCO22 BDBD InterpretationInterpretation

↓↓ ↑↑ NN Respiratory acidosisRespiratory acidosis

↓↓ NN ↑↑ Metabolic acidosisMetabolic acidosis

↑↑ ↓↓ NN Respiratory alkalosisRespiratory alkalosis

↑↑ NN ↓↓ Metabolic alkalosisMetabolic alkalosis

1)1) Normal, acidosis or alkalosis?Normal, acidosis or alkalosis?2)2) Respiratory, metabolic, mixed or compensated?Respiratory, metabolic, mixed or compensated?

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Case 3: JacintaCase 3: Jacinta

• Compensated metabolic acidosisCompensated metabolic acidosis

• Fluid loss (diarrhea) plus poor intake Fluid loss (diarrhea) plus poor intake dehydration + metabolic acidosis dehydration + metabolic acidosis

• Mechanisms: Mechanisms:

– tissue hypoperfusion tissue hypoperfusion lactic acidosis lactic acidosis

– bicarbonate losses in stool bicarbonate losses in stool

– respiratory compensation by “blowing respiratory compensation by “blowing off” COoff” CO22

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