Oxygen Therapy Flashcards
what is the percentage of oxygen in the air
21%
why does the oxygen decrease from the atmosphere into the lung
- mixing with dead space gases
- water vapour
describe the oxygen cascade
- Oxygen starts as dry atmospheric gas at 21 kPa
- it becomes humidified tracheal gas at 19.8 kPa
- alveolar gas it turns to 14 kPa due to mixing with dead space
- in arterial gas it goes down to 13.3 kPa
- in capillary blood it is 6-7 kPa
- and in the mitochondria it is 1-5 kPa
What does the oxygen bind to in haemoglobin
haem
Describe how oxygen binds to haemoglobin
- its difficult for the first oxygen to bind to the haemoglobin
- once that first oxygen molecule binds there is a conformation change and then oxygen binds to the haemoglobin more easily
describe what the oxygen haemoglobin dissociation curve shows you
- shows the change in saturation with the change in partial pressure of oxygen
- sigmoid shape due to the binding of oxygen to haemoglobin - first oxygen molecule is hard to bind but the conformation change makes it easier to bind
What shifts the oxygen dissociation curve right
- high carbon dioxide
- increase in temperature
- pH - increase in acid (lower pH)
- exercise
- 2,3- DPG
What shift the oxygen dissociation curve to the left
- lower CO2
- higher pH
- lower temperature
What happens when the oxygen dissociation curve is shifted to the right
- oxygen release is easier
What are the two ways in which oxygen is delivered in a hospital setting
- wall oxygen
- cylinder oxygen
how do you classify what oxygen supply patients should receive
- Critical patients
- most patients
- selected COPD patients
What should the oxygen saturation not be below
- below 90/94
If patients have an oxygen saturation above …
94% then you don’t need to deliver oxygen
what is the target oxygen stats for patients with COPD
- 88-92%
- these patients are at risk of hypercapnia respiratory failure
What is the target stats for all other patients
- greater than 94%
How should you administer oxygen in patients with COPD
- venturi masks in patients with COPD
How do you measure oxygen saturations
- pulse oximetry
What are the BTS oxygen prescribing guidelines
- Critical illness requires high levels of oxygen supplementation
- serious illness requires moderate level of oxygen if the patient is hyperaemic
- COPD and other conditions require controlled or low-dose oxygen therapy
- conditions for which patients should be monitored closely but oxygen therapy is not required unless the patient is hypoxaemic
What are the dangers of oxygen
- free radicalisation - oxygen toxicity
- flammable - explosive
- loss of hypoxic drive
- cause vasoconstriction
Name the 4 oxygen delivery devices
- venturi mask
- face mask
- nasal cannulae
- resovoir bag with face mask
Who is the nasal cannulae suitable for
- for most patients with type 1 or type II respiratory failure
How much oxygen can the nasal cannulae give
- 2-5L a minute gives approximately 24-50% FIO2
- range as its not a fixed performance device - breath through your mouth
What does the FIO2 depend on in nasal cannulae
- oxygen flow rate
- patients minute volume
- inspiratory flow
- pattern of breathing
What are the benefits of nasal cannulae
- comfortable and easily tolerated
- no re-breathing
- low cost
Who do you use a simple face mask for
Type 1 respiratory failure
What is type 1 respiratory failure
hypoxia less than 8 kPa but no hypercapnia
What is type 2 respiratory failure
Hypoxia and hypercapnia
Describe what flow the simple face masks gives
- variable oxygen concentration between 35% and 60%
- flow rate is 5-10L/min
- flow must be at least 5L/min to avoid carbon dioxide build up
In simple face masks what must the flow rate be above
- flow must be at least 5L/min to avoid carbon dioxide build up
What are the benefit and disadvantages of a simple face mask
+
- low cost
- uncomfortable
Who do you use humidified oxygen in
- reasonable to use humidified oxygen for patients who require moderate to high flow oxygen systems for more than 24 hours or who report upper airway discomfort due to dryness
Why do we humidify oxygen in the body
- stops drying out the mucoua if this happened it would bleed
- helps with secretions and prevents mucous plugging
who do you use a high concentration reservoir mask in
- non re-breathing reservoir mask
- critical illness/trauma patients
- post-cardiac or respiratory arrest
What is the oxygen concentration in a high concentration reservoir mask
- oxygen concentrations between 70-80% or above
- highest concentration of oxygen in non invasive delivery
Who do you use a high flow nasal cannulae for
- critical illness - non compliant with face masks
- effective longer term use if high concentrations of oxygen are required
describe the flow and oxygen concentration in a high flow nasal cannulae
- humidified and warmed for increased tolerance
- oxygen concentrations range between 20-100%
- flow rate is up to 60L/min
- can provide PEEP 7.4cm H20 at 60L/min
What is a PEEP
Positive end expiratory pressure (PEEP), is a pressure applied by the ventilator at the end of each breath to ensure that the alveoli are not so prone to collapse
- KEEPS ALVEOLI OPEN WHEN YOU EXPIRE
What is a normal PEEP
5cm of water
What is a venturi mask used for
COPD
describe how venturi masks work
- colour coded
- deliver constant oxygen concentration
- increasing flow does not increase oxygen concentration
In an venturi mask increasing flow does not….
increase oxygen concentration
describe the difference between a 24% and a 28% Venturi mask
For 24% Venturi mask, the typical oxygen flow of 2 l/min gives a total gas flow of 51 l/min
For 28% Venturi mask, 4 l/min oxygen flow, gives a total gas flow of 44 l/min
What is the worry with COPD patients and oxygen
- potential CO2 retention leading to narcosis and respiratory arrest
What percentage of COPD patients are CO2 retainers
5-15%
describe the pathophysiology in regards to COPD retainer patients
- if too much oxygen is given patients loose their drive to breathe and can become hypercapnia and may be at risk of respiratory arrest
what is worse hypoxia or hypercapnia
hypoxia
How long does it take for a COPD patient to develop significant hypercapnia?
- in an acute setting with a COPD patient you should give the oxygen as hypoxia is worse
- takes 15 minutes to become hypercapnia
what is the recommended treatment for COPD patients
- initially on assessing acutely unwell patient - use 15L non rebreathe mask
- target stats of 88-92%
- use a venturi mask to titrate oxygen
- take a baseline ABG and repeat it 30 minutes after starting oxygen therapy
How do you measure shifts to the right on the oxygen haemoglobin dissociation curve
- use P50 marker
what is the normal for
- PO2
- PCO2
- HCO3-
- standard HCO3-
- base deficit
- PO2 = 10.6-13.3 kPa (80-100mmHg)
- HCO3- (22-26mmol/l)
- PCO2 ( 4.8-6.1 kPa) (36-46mmHg)
- standard HCO3- = 22-26mmol/l
- base deficit +- 2.5
Why do you use a non breathe face mask
- higher concentration of oxygen - gives you 60-80 % whereas a rebreather face mask gives you 40-60%
