Preoxygenation Flashcards
How many ml of O2 is carried per 1 gram of Hb
1.34 mls of O2 –> This is Hufner’s constant
What % of the blood in circulation is arterial?
20%
What is the total content of O2 in 5L of blood
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
Why is the FRC deemed the reservoir volume for O2
The lungs come to rest at the FRC during apnoea
What is the FRC O2 reservoir
1500 ml x 0.14 mLO2
Give the values for the lung volumes and capacities for a 70kg male
VT = 500 ml IRV = 3000 ml ERV = 1500 ml RV = 1000 ml
TLC = 6000 ml VC = 5000 ml FRC = 2500 ml IC = 3500 ml
Define functional residual capacity (FRC)
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.
Name the factors that may reduce the FRC
Position - supine
Obesity
Pregnancy
Induction of anaesthesia
How does general anaesthesia reduce FRC
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.
What is the normal FRC in a healthy patient standing and how does this change during GA
2500ml
During GA –> FRC can reduce to less than 2000 ml (with additional risk factors < 1000 ml)
What is the composition of gas in the FRC
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%
What is another name for preoxygenation
Denitrogenation - replacing N2 in lungs with O2 so as to provide a larger reservoir of O2 during intubation.
How does the blood reservoir of oxygen changed during preoxygenation
Minimally as Hb already 97% saturated at existing FiO2
What is the total oxygen reservoir in a healthy anaesthetized patient breathing room air and and what is the theoretical time until desaturation
300 ml O2 (FRC) + 800 mlO2 (blood)
= 1100 mlO2
O2 uptake = 250 ml/min
In theory: 4 minutes of O2 reserve
Why does hypoxia occur fasted than the theoretical 4 minute reserves would provide?
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.
Compare the time to hypoxia in a patient who has not undergone preoxygenation versus one that has
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)
Describe how to preoxygenate
Tidal breathing for 3 minutes
- Circle system (FGF = 6 L/min = 1 VE)
- 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.
List 5 situations when pre-oxygenation is vital
- Predicted difficult airway (Fixed C-spine)
- Decreased FRC (Obese/Pregnant)
- Increase VO2 (Children/Obese/Pregnant)
- Hypoxia –> serious consequences (IHD)
- Existing hypoxia (e.g respiratory illness)
Pro tips for preoxygenation x 3
- Fowlers pre-oxygenation
- IV midazolam in mask phobic patients
- Anxious patient hyperventilating (reservoir bag empty) –> O2 flush with patient warning about noise
What happens when the O2 flush button is pressed?
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.
What action should be taken if the patient moves the facemask during pre-oxygenation
The patient breaths in N2 and the process must be restarted.
Why is a higher flow rate needed when preoxygenating with a Bain system than with a circle system?
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%.
In the event of accidental oesophageal intubation, how could preoxygenation influence recognition of the problem?
Preoxygenation prolongs the period before desaturation occurs, and therefore hypoxia may be a late sign of oesophageal intubation.
Why might preoxygenation be useful at the end of anaesthesia?
Complications can arise at the end of anaesthesia and preoxygenation provides the same benefits as at induction.
What is the target fro preoxygenation
ET O2 of 85%
