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ZCH ARM AIN E L. YU MAN G SEPTEMBER 2010  ARTERIAL BLOOD GAS

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    Z C HA R MA I NE L . Y UM A NGS E P T E M B E R 2 0 1 0

    ARTERIAL BLOOD GAS

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    ABG???

    yArterial blood gas analysis

    is an essential part of diagnosing andmanaging a patients oxygenation status and

    acid-base balancemeasures the acidity (pH) and the levels of

    oxygen and carbon dioxide in the blood froman artery

    This test is used to check how well your lungsare able to move oxygen into the blood andremove carbon dioxide from the blood.

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    Acid Base Balance

    The pH is a measurement of the acidity oralkalinity of the blood.It is inversely proportional to the number of

    hydrogen ions (H+) in the blood.The more H+ present, the lower the pH will be.The fewer H+ present, the higher the pH will be.

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    Acid Base Balance

    y Normal blood pH range is 7.35 to 7.45

    In order for normal metabolism to take place, the body must maintain thisnarrow range at all times.

    y pH 7.45 interferes withtissue oxygenationandnormal neurological and muscular functioning.

    y Extreme acid-base derangements will interfere with cellular functioning, and if uncorrected, will lead to death.

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    Maintenance of Acid Base Balance

    y So how is the body able to self-regulate acid-basebalance in order to maintain pH within the

    normal range?

    BUFFER SYSTEMS

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    Respiratory Buffer System

    y A normal by-product of cellular metabolism is carbondioxide (CO2). CO2

    y CO2 is carried in the blood to the lungs, where excessCO2 combines with water (H2O) to form carbonic acid

    (H2CO3). CO2 + H2O = H2CO3y The bloodpH willchangeaccording to the level of

    carbonicacidpresent.

    y This triggers the lungs to either increase or decrease the

    rate and depth of ventilation until the appropriateamount of CO2 has been re-established.

    y Activation of the lungs to compensate for an imbalancestarts to occur within 1 to 3 minutes.

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    Renal Buffer System

    y In an effort to maintain the pH of the blood withinits normal range, the kidneys excrete or retain

    bicarbonate HCO3

    y blood pH decreases (acidic) - the kidneys willcompensate by retaining HCO3

    y blood pH rises (alkalotic) - the kidneys excreteHCO3 through the urine

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    COMPONENTS

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    y pH

    Measurement of acidity or alkalinity, based on the hydrogen(H+) ions present.

    The normal range is 7.35 to 7.45

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    y PaO2

    The partial pressure of oxygen that is dissolved in arterialblood.

    It measures how well oxygen is able to move from the airspaceof the lungs into the blood.

    The normal range is 80 to 100 mm Hg

    Desired PaO2

    80 YEARS ABOVE 60

    Example 75 y/o Female

    PaO2 = 80 (75-60) = 65

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    y SaO2

    The arterial oxygen saturation.

    Oxygen saturation measures how much of the hemoglobin in

    the red blood cells is carrying oxygen (O2 The normal range is 95% to 100%.

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    y PaCO2

    The amount of carbon dioxide dissolved in arterial blood.

    how well carbon dioxide is able to move out of the body

    The normal range is 35 to 45 mm Hg

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    y HCO3

    The calculated value of the amount of bicarbonate in thebloodstream.

    The normal range is 22 to 26 mEq/liter

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    ACID BASE DISORDERS

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    Respiratory Acidosis

    y Low pH

    y High PaCO2

    y Primary increase in pCO2 resulting from alveolar

    hypoventilation

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    Respiratory Acidosis

    y Respiratory center depression Brainstem lesions narcotics, sedatives, or anesthesia

    y Neuromuscular failure Impaired respiratory muscle function related to spinal cord injury,

    neuromuscular diseases, or neuromuscular blocking drugsy Decreased compliance

    Parenchymal (e.g pulmonary fibrosis, ARDS) Extraparenchymal (e.g. Abdominal distention, severe kyphoscoliosis)

    y Increased airway resistance

    COPD OSA

    y Increased dead space Large pulmonary embolus

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    Respiratory Alkalosis

    y High pH

    y Low pCO2

    y Primary decreased in pCO2 resulting from alveolar

    hyperventilation

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    Respiratory Alkalosis

    y Central nervous system stimulation Pain

    Meningoencephalitis

    SAH

    Hepatic encephalopathy

    y Hypoxemia Moderate asthma exacerbation

    Acute pulmonary edema

    Pulmonary embolus High altitude

    pneumonia

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    Respiratory Alkalosis

    y Drugs

    Progesterone

    Salicylate poisoning

    Xanthinesy Miscellaneous

    Sepsis

    Mechanical hyperventilation

    pregnancy

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    Metabolic Acidosis

    y Low pH

    y low HCO3

    y Primary decreased in plasma HCO3 due to either

    HCO3 loss or accumulation of acid

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    Metabolic Acidosis

    y High anion gap Ketoacidosis diabetic, alcoholic, starvation

    Lactic Acidosis

    Intoxications e.g. Ethylene glycol, methanol, salicylate

    Advanced Renal Failure Severe rhabdomyolysis

    y Normal anion gap GI HCO3 loses (lower GI fistulas, diarrhea, ureterosigmoidostomy)

    Renal tubular acidosis

    Moderate renal insufficiency Acetazolamide use

    Large volume saline resuscitation

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    Metabolic Alkalosis

    y High pH

    y High HCO3

    y Primary increase in the plasma HCO3 due to either

    H+ loss or HCO3 gain

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    Metabolic Alkalosis

    y Chloride Responsive

    Upper GI losses

    Previous diuretic use

    Recovery from chronic hypercapniay Chloride unresponsive

    Effective mineralocorticoid excess

    Current diuretic use

    Bartters or Gitelmans syndrome Severe hypokalemia

    Excessive alkali administration

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    Steps on Dissecting Acid Base Disorder

    y Step 1 : Predict what underlying mechanisms mightbe present based on the clinical scenario.

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    Steps on Dissecting Acid Base Disorder

    y Step 2: Verify that the ABG values are internallyaccurate

    Simplified form of the Henderson-Hasselbach Equation:

    [H+] (nmol/L) = 24 x pCO2 (mmHg) / [HCO3-] (meq/L)

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    Steps on Dissecting Acid Base Disorder

    y Step 3:

    If the blood is alkalotic or acidotic, we now need todetermine if it is caused primarily by a respiratory or metabolicproblem.

    A. Assess the PaCO2 level.

    Remember that with a respiratory problem, as the pHdecreases below 7.35, the PaCO2 should rise. If the pH risesabove 7.45, the PaCO2 should fall.

    Compare the pH and the PaCO2 values. If pH and PaCO2are indeed moving in opposite directions, then the problem is

    primarily respiratory in nature.

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    Steps on Dissecting Acid Base Disorder

    y Step 3:

    B. Assess the HCO3 value. Recall that with a

    metabolic problem, normally as the pH increases,the HCO3 should also increase. Likewise, as the pHdecreases, so should the HCO3.

    Compare the two values. If they are moving inthesamedirection,thenthe problemisprimarilymetabolic in nature.

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    CASE 1

    y Jane Doe is a 45-year-old female admitted to thenursing unit with a severe asthma attack. She has

    been experiencing increasing shortness of breath

    since admission three hours ago.y Her arterial blood gas result is as follows:

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    y Followthe steps:

    1. Clinical Prediction

    2. Assess the pH.

    3. Assess the PaCO2.

    4. Assess the HCO3.

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    Acidosisis present(decreasedpH) withthe PaCO2 being increased,reflecting a primaryrespiratory problem.

    Forthis patient, weneedto improvetheventilationstatus byproviding oxygen therapy, mechanical ventilation, pulmonary toilet or byadministering bronchodilators.

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    CASE 2

    y John Doe is a 55-year-old male admitted to yournursing unit with a recurring bowel obstruction. Hehas been experiencing intractable vomiting for the

    last several hours despite the use of antiemetics.

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    y Followthe steps:

    1. Clinical Prediction

    2. Assess the pH.

    3. Assess the PaCO2.

    4. Assess the HCO3.

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    Alkalosisis present(increasedpH) withthe HCO3 increased,reflecting a primarymetabolic problem. Treatmentofthis patientmightincludetheadministration ofIVfluids and measures to reduce the excessbase.

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    Compensation

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    Compensation

    y body attempts to restore the normal blood pH duringan acid-base disorder

    y compensation can be metabolic or respiratory in

    origin

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    Respiratory Compensation

    y Respiratory compensation

    rapid process in which ventilation is adjusted to alter the pCO2in response to a primary alteration in the [HCO3

    -].

    Increased HCO3- levels will stimulate hypoventilation and a

    subsequent rise in pCO2.

    Decreased HCO3- levels will produce the opposite effect.

    begins within seconds and can reach maximum effectivenesswithin 12-24 hours

    Respiratory compensationcannevercompletely regainanormal bloodpH. Otherfactorsinvolvedinventilationcontrol,especially theneedfor oxygen, willnotallowforfullcompensation.

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    Compensation

    y Metabolic compensation is a slower process Occurs in two phases

    First phase involves intracellular non-bicarbonate buffers responsible for the first phase, which occurs immediately

    Second phase kidneys are responsible begins within hours, but takes 2 to 5 days to reach maximal

    effectiveness kidneys achieve compensation by altering net bicarbonate

    reabsoprtion and net acid excretion into the urine. Given enough time (up to 4 weeks) metabolic compensation may

    be able to return the blood pH to normal in chronic respiratorydisorders.2

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    y Metabolic disturbance lungs compensate

    y Respiratory disturbance kidney regulate

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    CASE 3

    y Mrs. L is a thin, elderly-looking 61 year old COPDpatient. She has an ABG done as part of her routinecare in pulmonary clinic.

    y Her ABG shows the following results

    pH : 7.37

    CO2: 63

    pO2 : 58 HCO3: 35

    SaO2: 89%

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    y Followthe steps:

    1. Clinical Prediction

    2. Assess the pH.

    3. Assess the PaCO2.

    4. Assess the HCO3.

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    Clinical Prediction

    y Thin

    Decreased muscle mass leading to poor respiratory musclefunction

    y

    COP

    D Interstitial damage/ bronchoconstriction decreased gas

    exchanges increase CO2 retention

    Interstitial damage/bronchoconstriction decreased gasexchnages decrease o2 entry

    RESPIRATORY ACIDOSIS

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    1. pH - acidic

    2. Assess the PaCO2 - increased

    3. Assess the HCO3 - increased

    y pH : 7.37

    y CO2: 63

    y HCO3: 35 Simple compensated respiratory acidosis

    Full compensation is evidenced by the normal pH in spte of heracid/base disorder

    This is her baseline and doesnt require treatment

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    Actual Patient

    y JL 35y/o female came in to the ER due to shortness of breathy Desired Fio2

    A= pCO2/ 0.8 B = (713 x del FiO2/100) A C = p02/B Des FiO2 = (des PO2/C) + A/ 713 x 100

    Del FiO2 NC = (# LPM x 4) + 20 FM = (# LPM 1) x 10

    y Mechanical ventilator with the following settings: TV: 450, FiO2:60, PF:60and BUR:20. Dormicum was discontinued.

    y Arterial blood gas pH 7.64

    pCO2 29.8 pO2 109.1 HCO3 21.6mechanical ventilation settings were adjusted to FiO2:40%, Peak flow 45 and BUR:14.