Capnography in icu
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Capnography in ICU. Shari McKeown, RRT. Mainstream sensor displays real-time, continuous carbon dioxide level throughout the respiratory cycle by measuring absorption of infrared light by CO 2 molecules. Overview.

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Capnography in ICU

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Capnography in ICU

Shari McKeown, RRT

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


C-D alveolar gas, high CO2 with upward slope due to continuing CO2 production and emptying of all alveolar units

D end-tidal CO2. The highest CO2 value at end-expiration

A-B exhalation begins - anatomical and ETT deadspace, no CO2

B-C exhalation continues - anatomical deadspace mixed with alveolar gas, increasing CO2

D-E inspiration begins, CO2 rapidly drops

E-A inspired gas contains no CO2

What does the waveform mean?

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 Bhavani Shankar Kodali MD

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

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

  • Technical Errors

    • Circuit disconnection

    • Leaks

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. Bhavani Shankar Kodali MD


  • 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.

How does PETCO2correlate 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 ABG’s are taken to trend the Pa-ETCO2 gradient

Record hourly

Record when ABG drawn

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

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 Bhavani Shankar Kodali MD

Slanting of upstroke

  • Occurs when there is obstruction to expiratory gas flow

  • e.g. asthma, bronchospasm, obstructive pulmonary disease, and kinked endotracheal tube


Airway obstruction Bhavani Shankar Kodali MD

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


Patient Effort Bhavani Shankar Kodali MD

Cardiac Oscillations

  • Ripple during expiratory phase indicate small movements in alveolar gas

  • Caused by cardiac or aortic pulsations against alveoli


Cardiac Oscillations Bhavani Shankar Kodali MD

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


Heterogenous V/Q ratios Bhavani Shankar Kodali MD

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 Bhavani Shankar Kodali MD

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) Bhavani Shankar Kodali MD

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

Capnography in ICU

Shari McKeown, RRT

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