ARTERIAL BLOOD GASES

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ARTERIAL BLOOD GASES
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ABGs Introduction

• ABGs are drawn and analyzed to determine the
  acid-base relationship of the blood and if the
  pt. is being well oxygenated
• Changes in the blood related to lung function are
  called respiratory and changes caused by renal
  action are called metabolic
• Arterial blood is used because it is a better
  indicator of activity in all parts of body
• pH, PaCO2, PaO2, are primary values

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pH

• Is a measurement of hydrogen ion activity and is
  expressed logarithmically
• As hydrogen ion concentration increases, pH
  decreases
• Normal pH range is 7.35 - 7.45, and if outside
  this range, have acid-base imbalance
• - lt 7.35 acidemia
• - gt 7.45 alkalemia
• Survival is unlikely if pH lt 6.8 or gt 8.0

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pH (contd)

• Buffers prevent extreme changes in the free
  Hydrogen ion concentration
• Four major buffer systems - hemoglobin -
  phosphate - bicarbonate - serum protein

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PaCO2

• PaCO2 directly reflects the adequacy of alveolar
  ventilation
• About 95 of CO2 in the blood is transported
  through buffering mechanisms in the rbc, but 5
  is dissolved in plasma
• This dissolved CO2 determines the blood partial
  pressure
• Normal range is 35-45 mmHg (torr) with a mean of
  40 mmHg

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PaCO2 (contd)

• Physiologic deadspace vs. anatomic deadspace
• Vd/Vt PaCO2 - PECO2/PaCO2
• Normal Vd/Vt 0.2 - 0.4

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PaCO2 - pH Relationship

• When acute changes in ventilation occur, a
  predictable relationship between pH and plasma
  carbonic acid results
• Normal PaCO2 40 mmHg
• - increase of 10 mmHg pH decr. 0.08 -
  decrease of 10 mmHg pH incr. 0.08
• Chronic ventilatory failure is when metabolic
  compensation of the acidemia has occurred, see
  high PaCO2 and normal pH

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O2 Transport and Utilization

• Person requires 250 ml of O2/min. at rest and
  exercise can incr. 20x
• Most of bodys O2 is combined with Hgb at 4 heme
  groups on the Hgb molecule
• - Hgb O2 oxyhemoglobin
• Carbaminohemoglobin CO2 Hgb
• Carboxyhemoglobin CO Hgb
• - competes with O2 , but has 200x the
  affinity for Hgb

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O2 Transport (contd)

• Normal Hgb. for male 14-16 g/100ml -female
  12-15 g/100ml
• PaO2 represents O2 dissolved in plasma
• - for each mmHg of pressure there is 0.003
  vol of dissolved O2
• O2 capacity amt. of O2 that can be max. bound
  to Hgb.
• - each gram of Hgb. can bind with 1.34 ml of
  O2

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O2 Transport (contd)

• O2 capacity 1.34 x Hgb. (PaO2 x 0.003)
• O2 content 1.34 x Hgb. x O2 Sat (PaO2 x
  0.003)
• O2 sat. (SaO2 or SpO2) varies directly with O2
  tension in accordance to the oxyhemoglobin
  dissociation curve
• Shape of dissoc. curve means the Hgb. holds on to
  O2 over a wide range at the upper end and lets go
  at the lower tensions

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O2 Transport (contd)

• Normal PaO2 80 -100 mmHg
• - varies with age - rule of thumb- PaO2
  105 - (.5 x age)
• - varies with altitude PaO2(corr.) PaO2
  at 760 x Act. Pb 760
• - varies with FIO2
• PaO2 O2 x 5

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Complications of Arterial Puncture

• Arteriospasm -artery surrounded by smoothe
  muscle, may see spasm which may cause
  occlusion
• Thrombi -possible dislodging of an
  atherosclerotic plaque or thrombus
• Emboli -may lead to occlusion of distal
  vessels

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Complications (contd)

• Infection
• Loss of blood flow and circulation -repeated
  punctures may compromise vessel integrity and
  circulation
• Hemorrhage

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Arterial Cannulation

• Refers to placement of indwelling arterial
  cannula for the immediate availability of
  arterial blood samples and continuous arterial
  pressure monitoring
• A-line is indicated when a pt. is, or may become,
  CP unstable , or needs serial blood gases and/or
  needs cont. arterial press. monitoring

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Complications of A-lines

• Necrosis and loss of tissue (small percentage)
• Infection (same incidence as venous)
• Thrombosis
• Hemorrhage
• Arteriospasm
• Emboli

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Equipment for A-line

• Arterial cannula
• 1 xylocaine, 25 guage needle, syringe
• Antiseptic swabs
• Continuous arterial keep-open setup consisting
  of -heparin -IV tubing set -500 ml
  NS -pressure bag -press. transducer -stopcocks
  
• -flushing device