2020B Question 06
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%).
Examiner Report
The main domains expected to be covered in this were:
- An accurate, labeled diagram of a capnograph, showing correctly labeled axes with the phases of the cycle clearly labeled
- A number of different pieces of information that can be obtained, with some description of the value of this information
- Evidence from this of a recognition that the capnograph is used as a monitor of both the patient and equipment
Common problems encountered were:
- Poor diagrams
Very few diagrams presented actually resembled a capnograph trace, with many rigid square waves being drawn. Axes were often unlabeled and many candidates took very little care to accurately demonstrate the phases. Identification of what is happening in the cycle beyond a simple ‘inspiration/expiration’ divide was expected. Books differ in how these are labeled and latitude was allowed here. - Many candidates attempted to argue that information from a capnograph can be directly used to calculate dead space either by substituting numbers into a Bohr equation or by using Fowler’s method, neither of which is the case
- Very few candidates listed use of a capnogram to confirm ETT position, or circuit patency and/or disconnections, and many could not give more than two examples of altered capnograph traces
The best resources for this question are: Miller’s Anaesthesia, ninth edition, Chapter 41: Respiratory Monitoring; Nunn and Lumb’s Applied Respiratory Physiology, ninth edition, Chapter 9: Carbon Dioxide.
Model Answer
Structure:
- Diagram
- Components
- Airway & breathing
- Circulation
- Other
Diagram
The capnograph waveform consists of four components:
- Baseline
Inspiration and early dead-space expiration (containing no CO2). - Alveolar exhalation
- Alveolar plateau
Highest point is defined as ETCO2. - Inspiration
Airway & Breathing
| Factor | Detail |
|---|---|
| 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 - Spontaneous: ↑ Slope phases 0 and 2, shortened phase 3 - Mechanical: Square-ish waveform, longer phase 3 - Adequacy of ventilation: - - Alveolar dead space - ∝ Difference between EtCO2 to PaCO2 (from ABG) - Normal 2-5mmHg - Rebreathing: ↑ Phases 1 and 3 - Important for Mapleson classification circuits - Bronchospasm: ↑ Slope phase 3 (and loss of α angle) - Increased heterogeneity of time constants - Earlier emptying of fast lung unit with low pCO2 - Later emptying of slow lung units with high PCO2 - Worse if short expiratory time - Emphysema: Reversal of slope phase 3 (and ↑ β angle) - ↑↑ Alveolar volume, early completion of gas exchange - Small airway closure: Terminal upswing = pigtail - Closure of lung units with low pCO2 - Continued emptying of lung units with high pCO2 - 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
| Factor | Detail |
|---|---|
| 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
| Factor | Detail |
|---|---|
| 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 - ↑ MR: Sepsis, MH - ↓ MR: Hypothermia |