cp 50 10-18 2 blood gas and acid base balance

ABG Interpretation

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Case 1FiO2 0.21pH 7.21PaCO2 64BE 2PaO2 48SpO2 80

• Acute respiratory acidosis with moderate hypoxemia

Normal composition of clean, dry air near sea level

Nitrogen 78.084Oxygen 20.9476Argon 0.934Carbon dioxide 0.0314Neon 0.001818Helium 0.000524Krypton 0.000114Xenon 0.000087Hydrogen 0.00005Methane 0.0002Nitrous oxide 0.00005

Dalton’s Law of Partial Pressure

GAS PARTIAL PRESSURE = % GAS CONC x 7.13 mmHg

ABG DATApH 7.40 ( 7.35 - 7.45 )H+ 40 nM/LPaO2 97 ( 90- 100 ) torrPaCO2 40 ( 35 - 45 ) torrActual HCO324 ( 22 - 26 ) mEq/LBase excess +/- 2 mEq/LHb 15 gm%O2 sat 95-100%O2 content 15-23%

IMPLICATIONS OF ABG

• LUNG FUNCTION• OXYGENATION, VENTILATION, ACID BASE

STATUS• EARLY DIAGNOSIS OF ARF• DIAGNOSIS OF SECONDARY POLYCYTHEMIA

DUE TO PULMONARY DISEASE• HEART AND CIRCULATORY FUNCTION• KIDNEY FUNCTION• METABOLISM• THE USE OF SOME MEDICATIONS• QUALIFY PATIENTS FOR HOME OXYGEN USE• DETECT EXPOSURE TO CARBON MONOXIDE

AND OTHER CHEMICALS

TECHNIQUES

SITES: radial, brachial, femoral arteries.

Allen’s testVOLUME: 1 mL

ERRORS• PLASTIC SYRINGE• AIR MIXTURE • OVER HEPARINIZATION (acid)• VENOUS SAMPLE • PAIN• DELAY OR UNCOOL SPECIMEN• ERROR OF GAS ANALYSER

PaCO2

• 80% plasma NaHCO3

• 10% carboxy Hb + 2% carbamino comp

• 8% dissolved in plasma

EXCRETORY RATE OF CO2 = VA x Pa CO2

Pa CO2 = 1 / VA

VA = ( VT - VD ) x fINTERPRETATION OF PaCO2

PaCO2 > 45 mmHg = alveolar hypoventilation = resp acidosis

PaCO2 < 35 mmHg = alveolar hyperventilation = resp alkalosis

PaO2

Interpretation> 100 mmHg hyperoxemia90-100 normal60-80 mild

hypoxemia40-60 moderate

hypoxemia< 40 severe hypoxemia

Oxygenation and external respiration

Causes of hypoxemia

Low FIO2

HypoventilationDiffusion defectVentilation perfusion mismatchDead space and shuntingVenous admixture

Oxygen transport and internal respiration

Arterial oxygen contentVolume of dissolved oxygen

+Volume of combined oxygen with

hemoglobin-------------------------------------

Total oxygen content--------------------------------------

volume of dissolved O2 = PaO2 x CsO2 = 0.3 vol%O2

volume of combined O2 = Hb x SaO2 x 1.34 = 19.7vol%

CaO2 = dissolved O2 + combined O2 = 20 vol%

CaO2 - CvO2 = 20 - 15.2 = 4.8 vol%

O2 consumption = Q x C(a - v) O2 = 250 mL O2/min

total O2 transport = cardiac output x CaO2 = 1000mLO2/min

Oxygenation ratio (PaO2/%FiO2)

Pulmonary status O2 ratio

normal 4.0 - 5.0moderate pulmonary dysfunction

2.0 - 3.9substantial pulmonary dysfunction

< 2.0

pHHenderson’s equation

Kc = [H] [HCO3] / [H2CO3]

Hasselbalch’s equation pH = pKc + log [HCO3] /

[H2CO3]

pH = pKc + log [HCO3 / dissolve CO2]

pH = 6.1 + log 24 / 1.2 pH = 7.4

Determination of primary problem

pH> 7.4 Alkalosis is primary;

acidosis is compensatory< 7.4 Acidosis is primary;

alkalosis is compensatory

Severity of generalized acid-base disturbances

pH Degree of impairment

< 7.20 severe acidemia 7.20-7.29 moderate acidemia 7.30-7.34 mild acidemia 7.35-7.45 normal pH 7.46-7.50 mild alkalemia 7.51-7.55 moderate alkalemia > 7.55 severe alkalemia

Calculated bicarbonate• Actual bicarbonate• It is a calculated value based on the

Henderson-Hasselbalch equation.• Henderson’s equation

Kc = [H] [HCO3] / [H2CO3]

Hasselbalch’s equation pH = pKc + log [HCO3] / [H2CO3]

pH = pKc + log [HCO3 / dissolve CO2]

pH = 6.1 + log 24 / 1.2 pH = 7.4

Base excess of blood

BE = Observed BB - normal BB

Classification of laboratory metabolic acid-base compensationClassification BE

HCO3

normal metabolic component 0 +/-2 24+/-2

metabolic acidosis < - 2< 22

metabolic alkalosis > + 2 > 26

Stepwise approach to diagnosing acid-base disorders

• Step1: Acidemic or alkalemic?• Step2: Is the primary disturbance

respiratory or metabolic?• Step3: For a respiratory disturbance,

determine whether it is acute or chronic.

• Step4: For a metabolic acidosis, determine whether an anion gap is present.

• Step5: Determine whether other metabolic disturbances coexist with an anion gap acidosis.

• Step6: Assess the normal compensation by the respiratory system for a metabolic disturbance.

Step1: Acidemic or Alkalemic?

Normal arterial blood pH = 7.40 +/- 0.05

Acidemic: pH < 7.35Alkalemic: pH > 7.45

Step2: Is the primary disturbance respiratory or metabolic?

A respiratory disturbance alters the arterial PaCO2 (normal value 40, range 38-42). Go to step 3.

A metabolic disturbance alters the serum HCO3 (normal value 24, range 22-26)

• If HCO3 < 22, metabolic acidosis is present. Go to step 4.

• If HCO3 > 26, metabolic alkalosis is present, is respiratory compensation adequate? Go to step 6.

Step3: For a respiratory disturbance, determine whether it is acute or chronic.

• Ac resp acid: pH decrease = 0.08*(PaCO2-40)/10

• Ch resp acid: pH decrease = 0.03*(PaCO2-40)/10

• Ac resp alka: pH increase = 0.08*(40 - PaCO2)/10

• Ch resp alka: pH increase = 0.017*(40 - PaCO2)/10

Step4: For a metabolic acidosis, determine whether an anion gap is

present.

• Anion gap = Na - (Cl + HCO3)

• Anion gap metabolic acidosis, anion gap > 12

• Normal or non anion gap acidosis, anion gap </= 12

Anion gap reflects the unmeasured anion and cation.

Unmeasured Anions

Proteins, mostly albumin 15 mEq/L

Organic acids 5 mEq/LPhosphates 2 mEq/LSulfates 1 mEq/LTotal: 23 mEq/L

Measured AnionsChloride 104 mEq/LBicarbonate 24 mEq/LTotal: 128 mEq/L

Unmeasured Cations

Calcium 5 mEq/LPotassium 4.5 mEq/LMagnesium 1.5 mEq/LTotal: 11 mEq/L

Measured CationsSodium 140 mEq/LTotal: 140 mEq/L

Step5: Determine whether other metabolic disturbances coexist with an anion gap

acidosis.

Corrected HCO3 = measured HCO3

+ (anion gap - 12)

If the corrected HCO3 varies significantly above or below 24, then a mixed or more complex metabolic disturbance exists.

To be more specific, if the corrected HCO3 is greater than 24, a metabolic alkalosis coexists. If the corrected HCO3 is less than 24 then a non anion gap acidosis coexists.

Step6: Assess the normal compensation by the respiratory system for a metabolic disturbance.

Winter’s FormulaExpected PaCO2 = (1.5*HCO3) +(8+/-2)

Winter’s Formula does not predict the resp response to a metabolic alkalosis.

Two general rules• a pt will increase PaCO2 above 40 but not greater

than 50-55 to compensate for a metabolic alkalosis.• a pt will be alkalemic if the PaCO2 is elevated to

compensate for a met alk ( If the patient is acidemic,PH < 7.38, then an additional resp acid is present).

Steps in evaluation and classification of acid-base

compensation• Evaluate for the presence of

compensation.• Determine the probable primary

problem.• Classify the degree of

compensation.

Alerts to mixed acid-base disturbances

• If respiratory and metabolic parameters change proportionately, pH remains unchanged.

• Both parameters are altered in fashion that changes the pH in the same direction.

• Fails to compensate in the expected manner for a primary disorder after sufficient time has elapse.

• A metabolic alkalosis is accompanied by an increase in the anion gap.

• Absent of compensation.• Long standing pulmonary or renal disease.• Excessive compensation.• Respiratory assistance.• Settings conducive to mixed disturbances.• Triple disorders may also be encountered.

Case 2FiO2 0.21pH 7.22PaCO2 25HCO3 10PaO2 96SaO2 95creatinine 11 mg/dL• Simple metabolic acidosis with normoxemia

Case 4FiO2 0.21pH 7.35PaCO2 22HCO3 12PaO2 41SaO2 75

• Mixed respiratory alkalosis and metabolic acidosis.

Case 5FiO2 0.21pH 7.10PaCO2 95BE - 5HCO3 29PaO2 60SpO2 78%

• Partially compensated respiratory acidosis.

Case 6FiO2 0.21pH 7.53PaCO2 49HCO3 39PaO2 92SaO2 98

• Partially compensated metabolic alkalosis with normoxemia.

Case 7FiO2 0.21pH 7.58PaCO2 31HCO3 28PaO2 65SaO2 96

• Combined respiratory alkalosis and metabolic alkalosis with mild hypoxemia.

Case 9FiO2 0.21pH 7.04PaCO2 15BE -22PaO2 125SaO2 95

• Partially compensated metabolic acidosis with hyperoxemia.

Case 10FiO2 0.21pH 7.25PaCO2 80HCO3 34PaO2 39SaO2 52

• Partially compensated respiratory acidosis with severe hypoxemia.