Preoxygenation

Question 1

How many ml of O2 is carried per 1 gram of Hb

Answer 1

1.34 mls of O2 –> This is Hufner’s constant

Question 2

What % of the blood in circulation is arterial?

Answer 2

20%

Question 3

What is the total content of O2 in 5L of blood

Answer 3

20% of blood is arterial blood containing 200mlO2/L blood

80% is venous containing 150 mlO2/L blood

Maths –> 800mLO2 in total blood volume (5L)

This is the blood O2 reservoir

Question 4

Why is the FRC deemed the reservoir volume for O2

Answer 4

The lungs come to rest at the FRC during apnoea

Question 5

What is the FRC O2 reservoir

Answer 5

1500 ml x 0.14 mLO2

Question 6

Give the values for the lung volumes and capacities for a 70kg male

Answer 6

VT = 500 ml
IRV = 3000 ml
ERV = 1500 ml
RV = 1000 ml

TLC = 6000 ml
VC = 5000 ml
FRC = 2500 ml
IC = 3500 ml

Question 7

Define functional residual capacity (FRC)

Answer 7

This is the lung volume following a normal (unforced) expiration and is the sum of the residual volume and the expiratory reserve volume. This is the volume at which the lung remains during apnoea.

Question 8

Name the factors that may reduce the FRC

Answer 8

Position - supine
Obesity
Pregnancy
Induction of anaesthesia

Question 9

How does general anaesthesia reduce FRC

Answer 9

GA –> reduced respiratory muscle tone.

FRC depends on balance of outward (chest wall, intercostal and diaphragmatic muscle tone) and inward (lung elasticity) forces. Any factor that reduces the outward force or increases the inward force will result in reduced FRC. During anaesthesia intercostal and diaphragmatic muscle tone are reduced leading to a reduction in the FRC.

Question 10

What is the normal FRC in a healthy patient standing and how does this change during GA

Answer 10

2500ml

During GA –> FRC can reduce to less than 2000 ml (with additional risk factors < 1000 ml)

Question 11

What is the composition of gas in the FRC

Answer 11

It is the same as the alveolar gas content as FRC occurs at the end of expiration.
O2 - 14%
CO2 - 5%
N2 - 75%
H2O vapour - 6%

Question 12

What is another name for preoxygenation

Answer 12

Denitrogenation - replacing N2 in lungs with O2 so as to provide a larger reservoir of O2 during intubation.

Question 13

How does the blood reservoir of oxygen changed during preoxygenation

Answer 13

Minimally as Hb already 97% saturated at existing FiO2

Question 14

What is the total oxygen reservoir in a healthy anaesthetized patient breathing room air and and what is the theoretical time until desaturation

Answer 14

300 ml O2 (FRC) + 800 mlO2 (blood)

= 1100 mlO2

O2 uptake = 250 ml/min

In theory: 4 minutes of O2 reserve

Question 15

Why does hypoxia occur fasted than the theoretical 4 minute reserves would provide?

Answer 15

Hypoxia develops so quickly because the saturation measured is the value in ARTERIAL blood only (does not include 800 ml blood O2 reservoir). Arterial blood relies on the 300 ml O2 in the FRC alone which is depleted very quickly during apnoea.

Question 16

Compare the time to hypoxia in a patient who has not undergone preoxygenation versus one that has

Answer 16

Preoxygenation can achieve 80 - 85% O2 in the alveoli (or FRC) as 5% CO@ and 6% H2O vapour is present.

Rate of O2 consumption: 250 ml/min

Non-preoxygenated patient:
14% of 1900ml (FRC) = 266 ml O2 (1 minute reservoir)

Pre-oxygenated patient:
80% of 1900ml (FRC) = 1520 ml O2 (6 minute reservoir)

Question 17

Describe how to preoxygenate

Answer 17

Tidal breathing for 3 minutes

1. Circle system (FGF = 6 L/min = 1 VE)
2. Bain system (FGF = 9L/min = 1.5 VE)

OR

4 x Vital capacity breaths within 1 minute
FGF required 15 L/min to prevent reservoir bag collapse during VC breaths.

Question 18

List 5 situations when pre-oxygenation is vital

Answer 18

1. Predicted difficult airway (Fixed C-spine)
2. Decreased FRC (Obese/Pregnant)
3. Increase VO2 (Children/Obese/Pregnant)
4. Hypoxia –> serious consequences (IHD)
5. Existing hypoxia (e.g respiratory illness)

Question 19

Pro tips for preoxygenation x 3

Answer 19

1. Fowlers pre-oxygenation
2. IV midazolam in mask phobic patients
3. Anxious patient hyperventilating (reservoir bag empty) –> O2 flush with patient warning about noise

Question 20

What happens when the O2 flush button is pressed?

Answer 20

When the anesthesia provider presses the flush button (Figure 2B), the pin lifts the ball valve off the valve seat and allows oxygen to flow into a common gas outlet at a rate of 35-75 L/min. … Released oxygen bypasses the flowmeters and vaporizers and enters directly into the fresh gas common outlet.

Question 21

What action should be taken if the patient moves the facemask during pre-oxygenation

Answer 21

The patient breaths in N2 and the process must be restarted.

Question 22

Why is a higher flow rate needed when preoxygenating with a Bain system than with a circle system?

Answer 22

Unlike the Bain system, a circle system does not rely on fresh gas flow to prevent rebreathing. Providing the fresh gas flow exceeds minute volume, the inspired gas mixture in a circle system is only fresh gas and therefore has an oxygen concentration of 100%.

Question 23

In the event of accidental oesophageal intubation, how could preoxygenation influence recognition of the problem?

Answer 23

Preoxygenation prolongs the period before desaturation occurs, and therefore hypoxia may be a late sign of oesophageal intubation.

Question 24

Why might preoxygenation be useful at the end of anaesthesia?

Answer 24

Complications can arise at the end of anaesthesia and preoxygenation provides the same benefits as at induction.

Question 25

What is the target fro preoxygenation

Answer 25

ET O2 of 85%