Capnography

Last Updated on by FRCEM Intermediate

IMPORTANT PATTERNS TO RECOGNISE

  • esophageal intubation (<6 waveforms of decreasing height)
  • right main bronchus intubation (ETCO2 can increase, decrease or stay the same, can also cause a bifid capnogram)
  • curare cleft (partially paralysed patient on mechanical ventilation)
  • cardiogenic oscillations
  • camel hump (seen in patients in lateral position)
  • Rebreathing capnogram of Mapleson D circuit
  • phase IV in pregnancy
  • Dilution of expiratory gases by the forward flow of fresh gases during the later part of expiration when expiratory flow rate decreases below the forward gas flow rate
  • sometimes see reverse phase 3 slope seen in patients with emphysema (alveolar destruction leads to rapid delivery of CO2 to airways)
  • Sticking inspiratory valve
  • expiratory valve malfunction
  • mandatory versus spontaneously triggered breaths
  • dual capnogram in lung transplants
  • air leak
  • malignant hypertherima
  • air / oxygen dilution during mask samplang of spontenously breathing patients

Use an algorithm for waveform capnography analysis.

  1. Look for presence of exhaled CO2 (Is a waveform present?)
  2. Inspiratory baseline (Is there rebreathing?)
  3. Expiratory upstroke (What is the shape i.e. steep, sloping, or prolonged?)
  4. Expiratory/alveolar plateau (Is it sloping, steep, or prolonged?)
  5. Inspiratory downstroke (Is it sloping, steep, or prolonged)

Ensure you evaluate the height, frequency, rhythm, baseline, and shape.

Hypoventilation – INCREASED ETCO2

  • Increased CO2 production (fever, NaHCO3 administration, tourniquet release, and overfeeding syndrome).
  • Pulmonary perfusion increase (increased cardiac output, increased blood pressure).
  • Alveolar ventilation decrease (hypoventilation, bronchial intubatio, partial airway obstruction, rebreathing).
  • Equipment malfunction (exhausted CO2 absorber, inadequate fresh gas flow, ventilator tubing leak, ventilator malfunction).

 

Hyperventilation – DECREASED ETCO2

  • Decreased CO2 production (hypothermia)
  • Pulmonary perfusion decrease (reduced cardiac output, hypotension, pulmonary embolism, cardiac arrest)
  • Alveolar ventilation increase (hyperventilation, apnea, total airway obstruction, extubation)
  • Apparatus malfunction (circuit disconnection, leak in sampling, ventilator malfunction)

Obstruction in the airway or breathing circuit

Rebreathing

The waveform with the baseline elevation, which is due to the inadequate exchange of CO2.

  • CO2 rebreathing (e.g. soda lime exhaustion)
  • Contamination of CO2 monitor (sudden elevation of base line and top line)
  • Inspiratory valve malfunction (elevation of the base line, prolongation of down stroke, prolongation of phase III)

Tapering of the ETCO2, suggestive of esophageal intubation

Try: remove the ETT-Improved view with videoscope and passes the ETT.

Sudden flat EtCO2 tracing

  • Kinked ET tube
  • Ventilator disconnection
  • Airway misplaced – extubation, oesophageal intubation
  • Capnograph not connected to circuit
  • Respiratory/Cardiac arrest
  • Apnoea test in brain death dead patient
  • Capnongraphy obstruction

Try : DOPES mnemonic (displacement, obstruction, PTX, equipment failure, breath stacking)

curare cleft

try: increase anesthesia

a sudden increase in ETCO2

  • ROSC during cardiac arrest
  • correction of ET tube obstruction

decreasing ETCO2

 

  • ET tube cuff leak
  • ET tube on hypopharnyx
  • Partial obstruction

CPR Assessment

 

  • Maintaining minimum of 10mmHg

Endobronchial intubation – bifid waveform

This bifid waveform represents the differential ventilation of two lungs. basically, as the ETT is positioned mainly in the right main bronchus, the airflow through the right lung is the best, and right-sided gas forms the first (brisk and steep) part of the waveform. Afterwards, one notices that there is a secondary transitional phase, which is the gas from the left lung escaping slowly up into the ETT. Of course, if the left lung was completely isolated (i.e. you have the cuff inflated in such a way as to block it totally) you would not see this waveform.

Bronchospasm – sawtooth slope

This is the classical sawtooth slope of the asthmatic patient. As the airway obstruction in the bronchi worsens, so the slope of the transitional phase becomes more gradual. Note that there is no distinct alpha angle; this means that the bronchial constriction is so severe that the dead space has not finished emptying by the time the next inspiration comes along. Thus, as bronchospasm is relieved, the alpha angle will again appear.

Mechanical airway obstruction


In the case of airway obstruction by some sort of fixed mechanical obstacle (eg. big gross tumour) the inspiratory flow will also be affected. Thus, both the transitional phase and the inspiratory phase will be affected, and most strikingly the inspiratory phase slope will become less steep, indicating that the obstruction cannot be overcome even by the powerful ventilator turbine.

Reversal of alveolar slope in emphysema

In emphysema, the alveolar slope will be reversed. The gas exchange surface is so poor, and the compliance of the lungs so abnormally increased, that the alveolar gas exchanges very rapidly. Thus, the part of the curve which represents arterial CO2 is the early peak, not the end-tidal value.

Thereafter, gas in the ventilator tubing diffuses backwards into the patient. There is an equilibration between the higher CO2 inside the patient and the lower CO2 in the ventilator circuit, with a resulting gradual drop of the total CO2 concentration in the capnograph-adjacent tubing.

This pattern can be seen in anaesthetic machines with rapid gas flow, where the ventilator contributes fresh gas to the tubing next to the capnometer. An alternative explanation is a pneumothorax with massive air leak – the air leak sucks CO2-rich air out of the capnometer, attracking fresh gas back through it.

Cardiac oscillations

This waveform represents the pulsation of an extra-large heart, transmitted to the lung parenchyma. The resulting changes in lung volume are enough to move a small amount of gas back and forth, creating “ventilation” of a sort. In some circumstances, this may be a feature of cardiomegaly

A high peak of the alveolar phase in poorly compliant lungs – Pig tail

This pattern is called a “pigtail” capnogram. It is typically seen in poor lung compliance, but it can also occur in pregnant women and obese patients. Essentially, the sudden peak of pre-inspiratory expired CO2 is due to sudden airway closure. It occurs when a poorly compliant lung (or a huge fat chest wall, or a big pregnant belly) crushes the last few milliliters of CO2-rich gas from the alveoli before the collapse of the lung parenchyma also occludes the small bronchi and puts an end to the escape of gas.

 

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