Capnography in ICU

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Capnography in ICU
Shari McKeown, RRT
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Overview

• Mainstream sensor displays real-time, continuous
  carbon dioxide level throughout the respiratory
  cycle by measuring absorption of infrared light
  by CO2 molecules

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What does the waveform mean?
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Why does the CO2 level always slope upwards to
end-tidal?

• As expiration progresses, basal lung units empty
  last these contain a higher CO2 level (lower
  V/Q ratio)

• CO2 production continues throughout expiration,
  resulting in a higher CO2 at the end of the
  breath

www.capnography.com Bhavani Shankar Kodali MD
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What increases PETCO2?

• Increased CO2 Production
• Increased metabolic rate
• Fever
• Seizures
• Shivering
• Pain
• Bicarbonate infusion
• Increased delivery of CO2 to lungs
• Increased cardiac output
• Hypertension
• Reduced clearance of CO2 from lungs
• Hypoventilation
• Mainstem bronchus intubation (ETT in one lung)
• Partial airway obstruction

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What decreases PETCO2?

• Decreased CO2 Production
• Decreased metabolic rate
• Hypothermia
• Analgesia
• Sedation
• Decreased delivery of CO2 to lungs
• Decreased cardiac output
• Hypotension
• Hypovolemia
• Pulmonary Embolism
• Cardiac Arrest
• Rapid clearance of CO2 from lungs
• Hyperventilation
• No communication with alveolar gas
• Total airway obstruction
• Accidental tracheal extubation
• Apnea
• Increased alveolar deadspace
• High PEEP

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Cardiac Output

• Decreasing cardiac output will reduce pulmonary
  blood flow, causing a decrease in alveolar
  perfusion and increased alveolar deadspace
• A higher alveolar deadspace will result in lower
  ETCO2 values and higher Pa-ETCO2 gradient.
• Under conditions of constant lung ventilation,
  ETCO2 can be used as a monitor of pulmonary blood
  flow.

www.capnography.com Bhavani Shankar Kodali MD
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CPR

• During CPR, blood flow to the lungs is low and
  few alveoli are perfused
• Tidal volumes delivered with a resuscitation bag
  tend to be large, high deadspace results in
  PETCO2 is low
• If the blood flow to the lungs improves, more
  alveoli are perfused and PETCO2 will increase
• C02 presentation to the lungs is the major
  limiting determinant of PETCO2 and it has been
  found that PETCO2 correlates well with measured
  cardiac output during resuscitation
• Therefore PETCO2 can be used to judge the
  effectiveness of resuscitative attempts
• PETCO2 has a prognostic significance. It has been
  observed that non-survivors had lower PETCO2
  during CPR than survivors.

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How does PETCO2 correlate with PaCO2?

• Normal gradient of (a-ET)PCO2 is 2-5 mmHg, and
  will increase with age
• This is due to normal ventilation/perfusion (V/Q)
  mismatching throughout the lung
• An increased gradient reflects increased
  deadspace - alveoli that are ventilated but not
  perfused will have low CO2 when exhalation
  occurs, this results in a higher Pa-ETCO2
  gradient
• Pa-ETCO2 gradient will decrease in pregnancy
  reflecting the higher cardiac output and
  pulmonary perfusion in the pregnant patient
• PETCO2 should always be recorded when ABGs are
  taken to trend the Pa-ETCO2 gradient

Record hourly
Record when ABG drawn
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How can you use Pa-ETCO2 gradient for PEEP
titration?

• Pa-ETCO2 gradient is a good reflection of
  alveolar deadspace
• When V/Q is at its best (optimum PEEP) the
  Pa-ETCO2 gradient is low. Oxygenation should be
  optimal.
• As the level of PEEP is increased beyond this,
  alveolar deadspace increases, the Pa-ETPC02
  increases, and oxygenation worsens.
• Pa-ETC02 can be used as a sensitive indicator to
  titrate PEEP in patients with early ARDS or with
  alveolar edema

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What information can you get by looking at the
waveform?

• The shape of a capnogram is identical in all
  humans with healthy lungs. Any deviations in
  shape must be investigated to determine a
  physiological or a pathological cause of the
  abnormality

Normal waveform
www.capnography.com Bhavani Shankar Kodali MD
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Slanting of upstroke

• Occurs when there is obstruction to expiratory
  gas flow
• e.g. asthma, bronchospasm, obstructive pulmonary
  disease, and kinked endotracheal tube

Normal
Airway obstruction
www.capnography.com Bhavani Shankar Kodali MD
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Patient Efforts

• A sudden decrease during expiratory phase
  indicates spontaneous patient effort
• Waveform can be used to identify missed
  ventilator triggers that lead to
  patient-ventilator asynchrony

Normal
Patient Effort
www.capnography.com Bhavani Shankar Kodali MD
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Cardiac Oscillations

• Ripple during expiratory phase indicate small
  movements in alveolar gas
• Caused by cardiac or aortic pulsations against
  alveoli

Normal
Cardiac Oscillations
www.capnography.com Bhavani Shankar Kodali MD
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Heterogeneous Lung Pathology

• Lungs with differing compliance/resistances (e.g.
  single-lung transplant) will have different
  empyting rates, CO2 clearance times, and V/Q
  ratios
• May result in dual-peak or dual-slope waveforms

Normal
Heterogenous V/Q ratios
www.capnography.com Bhavani Shankar Kodali MD
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Waveform Trends

• Hypoventilation or patient fatigue (e.g. during
  CPAP trials) may result in gradual increase in
  ETCO2 over time (normal Pa-ETCO2)
• Sweep speed can be decreased to illustrate
  gradual trending

www.capnography.com Bhavani Shankar Kodali MD
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Waveform Trends

• Hyperventilation may result in gradual decrease
  in ETCO2 over time
• (normal Pa-ETCO2)
• This trend may also be caused by a patient with
  autopeep incomplete exhalation results in
  alveolar gas not reaching airway
• (increased Pa-ETCO2)

www.capnography.com Bhavani Shankar Kodali MD
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Clinical applications

• Estimate PaCo2
• Estimate alveolar deadspace
• Optimal PEEP setting
• Verify ETT placement
• Monitor adequacy of ventilation
• Evaluate weaning trial
• Monitor effectiveness of CPR
• Assess pulm blood flow
• Assess effectiveness of bronchodilators
• Detect patient/ventilator asynchrony
• Immediate alert to accidental extubation, large
  pulmonary embolism, apnea, circuit disconnection,
  leaks
• Trend metabolic rate

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Capnography in ICU
Shari McKeown, RRT