2020B06 Draw and label a normal capnograph to show the phases of the respiratory cycle (30%).
List and briefly describe what information can be obtained from the capnograph (70%).

 

List:

·       Diagramme

·       Components

·       Airway & breathing

·       Circulation

·       Other

 

Diagramme:

 

Components:

1

Baseline

·         Should be near 0mmHg

2

Expiratory upstroke

·         Transition from anatomical dead space to alveolar gas

3

Alveolar plateau

·         Alveolar gas

4

Terminal upswing

·         Rarely present

*

etCO2

·         Approximates PaCO2

0

Inspiratory downstroke

·         Early inspiration

α

Angle

·         Transition between phases 2 and 3

β

Angle

·         Transition between phases 3 and 0

 

Airway & Breathing:

ETT placement

·         Trachea: normal waveform

·         Bronchus: ± bifid waveform (delayed expiration from other lung)

·         Oesophagus: small, vanishing sinusoidal waveform

·         Disconnection: no waveform

Ventilation

·         Mode of ventilation

o    Spontaneous: ↑slope phases 0 and 2, shortened phase 3

o    Mechanical: square-ish waveform, longer phase 3

·         Adequacy of ventilation:

o    VA ∝ 1/ etCO2

·         Alveolar dead space

o    ∝ difference between etCO2 to PaCO2 (from ABG)

o    Normal 2-5mmHg

·         Rebreathing: ↑phases 1 and 3

o    Important for Mapleson classification circuits

·         Bronchospasm: ↑slope phase 3 (and loss of α angle)

o    Increased heterogeneity of time constants

o    Earlier emptying of fast lung unit with low pCO2

o    Later emptying of slow lung units with high pCO2

o    Worse if short expiratory time

·         Emphysema: reversal of slope phase 3 (and ↑β angle)

o    ↑↑Alveolar volume, early completion of gas exchange

·         Small airway closure: terminal upswing = pigtail

o    Closure of lung units with low pCO2

o    Continued emptying of lung units with high pCO2

o    Seen in pregnancy, obesity, poor compliance (e.g. ARDS)

Equipment dysfunction

Tubing:

·         Circuit obstructed externally: ↓slope phases 0 and 2

·         Circuit disconnected: no waveform

Valves:

·         Insp valve stuck open: ↓slope phase 1

·         Insp valve stuck closed: no waveform

·         Exp valve stuck open: ↑phases 1 and 3

·         Exp valve stuck closed: ↓etCO2 (once ventilation fails)

Sampling line:

·         Leak -> ↓early phase 3, ↑terminal phase 3 (= dual plateau)

·         ↑Pressure at onset of inspiration reduces air entrainment

 

Circulation:

Cardiac output (RV)

·         Low output: ↓phase 3

·         Arrest without CPR: no waveform

·         Arrest with CPR: etCO2 >20mmHg indicates effective CPR

Cardiac oscillations

·         Small, regular, rapid sinusoidal waveforms

·         Seen in low frequency ventilation

·         Cardiac impulse causes back and forth motion between exhaled and fresh gas

 

Other:

Muscle relaxation

·         Spontaneous inspiration produces ‘curare cleft’ in phase 3

Sodalime exhaustion

·         ↑Phase 1 and 3 for a given MV

·         Normalises if ↑FGF

Metabolic rate

·         MR ∝ VCO2 ∝ height of Phase 3

o    ↑MR: sepsis, MH

o    ↓MR: hypothermia

 

 

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