Capnography

Question 1

What are the three main events of respiration?

Answer 1

Cellular metabolism (O₂ consumption, CO₂ production)

Transport & diffusion between cells, blood, and alveoli

Ventilation between alveoli and atmosphere

Question 2

What does capnography depict?

Answer 2

Ventilation (movement of CO₂ from alveoli to atmosphere) and its interaction with perfusion and metabolism.

Question 3

Why is capnography clinically important?

Answer 3

It provides continuous, real-time monitoring of ventilation, confirms airway placement, and detects physiologic and equipment problems early

Question 4

What broad factors affect capnography?

Answer 4

Physiologic factors and equipment-related factors

Question 5

How does dead space affect EtCO₂?

Answer 5

↑ Dead space → lower EtCO₂ and wider PaCO₂–EtCO₂ gradient.

Question 6

How do ventilation–perfusion (V/Q) relationships affect EtCO₂?

Answer 6

V/Q mismatch (e.g., PE, shock) → reduced EtCO₂ due to less CO₂ delivery to alveoli.

Question 7

What are normal PaCO₂ and EtCO₂ values?

Answer 7

PaCO₂: ~35–45 mmHg

EtCO₂: ~35–45 mmHg (usually 2–5 mmHg lower than PaCO₂)

Question 8

What is the arterial–end-tidal CO₂ gradient?

Answer 8

PaCO₂ − EtCO₂; normally 2–5 mmHg.

Question 9

When does the PaCO₂–EtCO₂ gradient increase?

Answer 9

With increased dead space, low perfusion, or V/Q mismatch.

Question 10

How can equipment issues alter EtCO₂ readings?

Answer 10

Leaks, faulty valves, sampling errors, moisture, or kinks can distort waveform and values.

Question 11

What is capnography vs capnometry?

Answer 11

Capnography: Numeric + waveform

Capnometry: Numeric only

Question 12

Why is capnography more than EtCO₂?

Answer 12

The waveform reveals airway, ventilatory, and circuit problems beyond a single number.

Question 13

What is quantitative vs qualitative EtCO₂?

Answer 13

Quantitative: Numeric value (mmHg)

Qualitative: Color change (presence/absence of CO₂)

Question 14

What do EtCO₂ trends and histograms show?

Answer 14

Direction and magnitude of change over time—early warning of deterioration or improvement.

Question 15

How is CO₂ measured in capnography?

Answer 15

By infrared (IR) absorption

Question 16

What are colorimetric CO₂ detectors used for?

Answer 16

Rapid confirmation of tracheal vs oesophageal intubation

Question 17

What does capnography depict?

Answer 17

Respiration.

Question 18

Why does capnography give an excellent picture of respiration?

Answer 18

Because all three components of respiration—metabolism, transport, and ventilation—are required for CO₂ to appear in exhaled gas.

Question 19

How does metabolism affect capnography?

Answer 19

Cellular metabolism produces CO₂, which ultimately appears in exhaled breath

Question 20

How does transport affect capnography?

Answer 20

CO₂ must be transported via the bloodstream from tissues to the lungs.

Question 21

How does ventilation affect capnography?

Answer 21

Ventilation moves CO₂ from the alveoli to the atmosphere, where it is measured as EtCO₂.

Question 22

What happens to capnography if any one of the three components fails?

Answer 22

The EtCO₂ value or waveform changes, signalling a problem with respiration.

Question 23

Does capnography assess oxygenation?

Answer 23

No—it assesses ventilation and perfusion-related CO₂ elimination

Question 24

How should oxygenation be monitored to complete the respiratory picture?

Answer 24

With pulse oximetry (SpO₂).

Question 25

Key exam pearl combining monitors?

Answer 25

Capnography → ventilation & perfusion Pulse oximetry → oxygenation

Question 26

One-line takeaway?

Answer 26

Capnography shows how CO₂ is produced, transported, and exhaled—but SpO₂ is needed to assess oxygenation.

Question 27

How are factors affecting capnography broadly classified?

Answer 27

Physiologic factors (metabolism, transport, ventilation, V/Q).

Question 28

Which factors affect CO₂ production?

Answer 28

Substrate metabolism, drug therapy, and core temperature.

Question 29

How does increased metabolism affect EtCO₂?

Answer 29

Increases EtCO₂ (e.g. fever, sepsis, shivering).

Question 30

How do drugs affect CO₂ production?

Answer 30

Sedatives/paralytics ↓ CO₂ production Catecholamines ↑ CO₂ production

Question 31

How does core temperature affect EtCO₂?

Answer 31

Hyperthermia → ↑ EtCO₂ Hypothermia → ↓ EtCO₂

Question 32

What factors affect CO₂ transport to the lungs?

Answer 32

Cardiac output and pulmonary perfusion

Question 33

How does low cardiac output affect EtCO₂?

Answer 33

Decreases EtCO₂ due to reduced CO₂ delivery to alveoli.

Question 34

Why is EtCO₂ a marker of perfusion?

Answer 34

Because CO₂ must be transported by blood to be exhaled.

Question 35

What ventilation factors affect capnography?

Answer 35

Obstructive disease, restrictive disease, and respiratory rate.

Question 36

How does hypoventilation affect EtCO₂?

Answer 36

Increases EtCO₂.

Question 37

How does hyperventilation affect EtCO₂?

Answer 37

Decreases EtCO₂.

Question 38

How do obstructive lung diseases affect the capnogram?

Answer 38

Cause abnormal waveforms (e.g. prolonged upstroke, “shark-fin”).

Question 39

How do V/Q mismatches affect EtCO₂?

Answer 39

They usually reduce EtCO₂ by increasing dead space.

Question 40

Give examples of conditions causing low EtCO₂ via V/Q mismatch.

Answer 40

Pulmonary embolism, shock, cardiac arrest.

Question 41

One-line rule for EtCO₂ interpretation?

Answer 41

EtCO₂ reflects metabolism + circulation + ventilation—change in any one alters the reading.