Capnography

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Capnography

• NURA 833
• Fall 2005
• Staci Rivas, CRNA, MSN

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CAPNOGRAPHY

• Term capnography comes from the greek work
  KAPNOS, meaning smoke.
• Anesthesia contex inspired and exspired gases
  sampled at the Y connector, mask or nasal
  cannula.
• Gives insight into alterations in ventilation,
  cardiac output, distribution of pulmonary blood
  flow and metabolic activity.

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2 Techniques for Monitoring ETCO2

• 2 methods for obtaining gas sample of analysis
• Mainstream
• Sidestream
• Mainstream (Flow-through or In-line)
• Adapter placed in the breathing circuit
• No gas is removed from the airway
• Adds bulk to the breathing system
• Electronics are vulnerable to mechanical damage

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Mainstream Analyzer
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Sidestream Analyzer
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Sidestream Analyzer

• Sidestream (aspiration)
• Aspirate gas from an airway sampling site and
  transport the gas sample through a tube to a
  remote CO2 analyzer
• Provides ability to analyze multiple gases
• Can use in non-intubated patients
• Potential for disconnect or leak giving false
  readings
• Withdrawls 50 to 500ml/min of gas from breathing
  circuit (most common is 150-200ml/min)
• Water vapor from circuit condenses on its way to
  monitor
• A water trap is usually interposed between the
  sample line and analyzer to protect optical
  equipment

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Location of Sensor

• The location of CO2 sensor greatly affects the
  measurement
• Measurement made further from the alveolus can
  become mixed with fresh gas causing a dilution of
  CO2 values and rounding of capnogram

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Measurement Techniques

• Infrared analyzer
• Photo acoustic spectrometer
• Raman spectrometer
• Mass spectrometer

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How ETCO2 Works

• ETCO2 monitoring determines the CO2 concentration
  of exhaled gas
• Photo detector measures the amount of infrared
  light absorbed by airway gas during inspiration
  and expiration
• CO2 molecules absorb specific wavelengths of
  infrared light energy
• Light absorption increases directly with CO2
  concentration
• A monitor converts this data to a CO2 value and a
  corresponding waveform (capnogram)

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The Capnogram Waveform

• Is divided into four distinct phases

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The Capnogram Waveform

• The first phase (A-B) represents exhalation of
  anatomic deadspace
• Normally devoid of carbon dioxide
• The second phase (B-C) is present as sharp
  upstroke
• Determined by the evenness of ventilation and
  alveolar emptying
• End of Phase II signifies usual end of expiration

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The Capnogram Waveform

• The third phase (C-D) reflects exhalation of
  alveolar gas
• Considered expiratory pause
• Point D designates end-tidal CO2 concentration
• The forth phase (D-E) reflects the beginning of
  inspiration
• Normally will be gases lacking carbon dioxide and
  approach zero

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• Connections on sample tubing loose

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Increasing ETCO2

• Hypoventilation (decrease RR or TV)
• Increase in metabolic rate
• Increase in body temperature
• Malignant hyperthermia
• Release of tourniquet
• Absorption of CO2 from peritoneal insufflation
• Sudden increase in blood pressure

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Decreasing ETCO2

• Gradual
• Hyperventilation (increase RR or TV)
• Decrease in metabolic rate
• Decrease in body temperature
• Rapid
• Embolism (air or thrombus)
• Sudden hypotension
• Circulatory arrest

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Increase in Inspired CO2 (Rise in Baseline)

• CO2 absorbent exhausted
• Faulty expiratory valve
• Calibration error in monitor
• Water in analyzer

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Loss of Plateau /Sloping of ETCO2 Waveform

• Obstruction of expiration (asthma, COPD,
  bronchospasm)
• A-a gradient is increased
• No plateau is reached prior to next inspiration
• Kinked endotracheal tube

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Cleft in Phase III of Waveform

• Patient is inspiring during exhalation phase of
  mechanical ventilation
• Muscle relaxant plasma levels subsiding
• PaCO2 increasing cause spontaneous respiration
• Increasing pain
• In older texts referred to as Curare cleft

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Cardiogenic Oscillations

• Caused by beating of heart against lungs
• May be seen more as relaxant wears off and tone
  returns to chest, abdominal walls and diaphragm
• More common in pediatrics because heart takes up
  relatively more space in the chest