Surgical Critical Care - Airway

Question 1

How is the airway assessed clinically?

Answer 1

**Look **- accessory muscles, foreign bodies, facial/airway injury, cyanosis
**Listen **- stridor, wheeze, gurgling, snoring
Feel- chest wall movements, airflow from nose and mouth

Question 2

How is the head tilt, chin lift performed?

Answer 2

Head tilt - one hand on the forehead and the other on the occipital protuberance to tilt the head back gently
Chin lift - use fingers of one hand placed under the mandible in the midline to lift the chin upward

Question 3

How is the jaw thrust performed?

Answer 3

The index and middle fingers are placed behind the angle of the mandible bilaterally, upward pressure is then applied to lift the mandible and open the airway, the thumbs can be used to open the mouth slightly

Question 4

What is the oxygen cascade?

Answer 4

Described the incremental drops in pO2 from atmosphere to arterial circulation
- Atmosphere 21.0 kPa
- Tracheal 19.8 kPa
- Alveolar 14.0 kPa
- Arterial 13.3 kPa

Question 5

How is oxygen transported in the body?

Answer 5

99% bound to Hb
1% dissolved in solution

Question 6

What is Henry’s Law?

Answer 6

Law stating gas content of a solution is equal to product of the solubility and the partial pressure of the gas

Question 7

What is Hb composed of?

Answer 7

Globular protein consisting a haem component and a globin chain. Haem is formed from Fe2+ and a protoporphyrin ring. Globin is formed from 2xalpha, 2xbeta and a 2,3-biphosphoglycerate (2,3-BPG) molecule in adults. The complex can bind up to 4 Oxygen molecules.

Question 8

What molecules can bind Hb under normal circumstances?

Answer 8

Oxygen - up to 4
CO2 - binds globin chain
Protons - bind amino, carboxyl and imidazole groups within the globin chain
2,3-BPG - byproduct of red cell metabolism

Question 9

Where are the main sites of haematopoeisis?

Answer 9

Yolk sac - first few weeks of gestation
Bone marrow - from first few weeks after birth
Liver and Spleen - most important sites up until 7 months gestation (adult can revert to these sites in pathological states - ‘extramedullary’ haematopoiesis

Question 10

Average life span of a red blood cell

Answer 10

120 days - after which it is broken down by the reticuloendothelial system

Question 11

What accounts for the shape of the oxygen dissociation curve?

Answer 11

sigmoidal reflects the progressive nature with which oxygen binds Hb
termed “cooperative” binding, where one oxygen facilitates binding of the next

Question 13

What is the Bohr effect?

Answer 13

A shift in the oxygen dissociation curve to the right, signifying a reduction of the oxygen affinity of Hb
Therefore, greater tendency to offload oxygen into tissues

Caused by:
- increased temperature
- increased acidity
- increased 2,3-BPG (caused by hypoxia)
- increased CO2

Question 14

How does the fetal Hb differ?

Answer 14

Gamma subunit instead of beta, causing an increased affinity for oxygen and left shifted dissociation curve

Question 15

How much oxygen can Hb bind?

Answer 15

Each Hb can bind 1.34ml of Oxygen

Therefore, 1.34 x [Hb] = blood’s maximum oxygen carrying capacity

Question 16

What is the FiCO2 of atmospheric air?

Answer 16

0.00035, since 0.035% of atmosphere is made up of CO2

Question 17

What are the changes in the CO2 cascade?

Answer 17

Atmospheric air = 0.03kPa
Alveolar air = 5.30 kPa
Arterial blood = 5.30 kPa
Venous blood = 6.10 kPa
Exhaled air = 4.00 kPa

Question 18

How is CO2 transported around the body?

Answer 18

Bicarbonate ions = 85-90%
Carbamino compounds = 5-10%
Dissolved in solution = 5%

Question 19

How does CO2 transport differ between arterial and venous blood?

Answer 19

Arterial - less carbamino and more bicarbonate carriage

Question 20

How does CO2 become a bicarbonate ion?

Answer 20

CO2 + H2O = H2CO3 = H+ + HCO3-

Catalyst: carbonic anhydrase

Question 21

What happens to the increased H+ formed as part of the CO2 + H2O = H2CO3 = H+ + HCO3- reaction?

Answer 21

“mopped up” by buffer systems
E.g. bind Hb imidazole and polypeptide groups causing Bohr effect and reduced affinity for oxygen leading to oxygen unloading at tissues

Question 22

Describe the chloride shift?

Answer 22

Bicarbonate exits red cells in exchange for chloride ions to maintain electrochemical neutrality

Question 23

How does the shape of the oxygen and carbon dioxide dissociation curves differ?

Answer 23

O2 - sigmoidal
CO2 - curvilinear

Question 24

What is the Haldane effect?

Answer 24

Describes the changes in the affinity of blood for CO2 as PaO2 changes. As PaO2 increases the affinity of blood for CO2 decreases. Seen as a downward shift of the CO2 dissociation curve.

Question 25

What equation defines the relationship between PaO2 and PaCO2?

Answer 25

Alveolar gas equation
PaO2 = PiO2 - PaCO2 / R

R = respiratory exchange ratio, usually 0.8
PiO2 = inspired PO2

Question 26

What is the danger of over-oxygenation in CO2 retainers?

Answer 26

May induce apnoea
1. loss of hypoxic drive - if chronically has high CO2 then they may rely on that for respiratory drive
2. loss of pulmonary vasoconstriction - deteriorating V/Q mismatch

Question 27

Problems with oxygen therapy

Answer 27

1. Absoption atelectasis - collapse of alveoli following oxygen absorption, as there is a lack of Nitrogen which normally splints open the airway
2. Pulmonary toxicity - O2 irritates respiratory mucosa causing loss of surfactant and progressive fibrosis
3. Risk of fires and explosion especially in smoking population

Question 28

What is pulse oximetry and what does it measure?

Answer 28

Non-invasive and continuous method of assessing arterial oxygen saturation and pulse rate using a probe on the finger or earlobe

Does not assess ventilation, measure total oxygen content of blood or PaO2.

Question 29

How does pulse oximetry work?

Answer 29

Principles of spectrophotometry - emits red LED (660nm) and infared (940nm) wavelengths, then detects light absorbed by saturated and unsaturated Hb using a photodetector, the ratio of these determine the reading

Question 30

Disadvantages of pulse oximetry

Answer 30

1. Diminished accuracy below 70%
2. Delay of 20s in most probes
3. Poor signal in states of shock and light pollution
4. Affected by pigment e.g. nail varnish, bilirubin
5. Affected by abnormal pulsations as a result of arrhythmia or valvular disease

Question 31

What is methaemoglobin?

Answer 31

Hb with ferric ion Fe3+ (instead of ferrous Fe2+)
Reduced affinity for oxygen, may be treated using a reducing agent such as methylene blue

Question 32

How can ventilation be assessed?

Answer 32

Ventilation is the ability to expel CO2
This can be assessed using capnography

Question 33

How is a nasopharyngeal airway inserted?

Answer 33

1. Check for nostril patency
2. Prepare a lubricated tube +/- stopper e.g. safety pin
3. Insert bevel side first along the floor of the nasal cavity, perpendicular to the orrifice towards the ear
4. Reassess breathing - “look, listen and feel”

Question 34

How is a oropharyngeal airway inserted?

Answer 34

1. Open the mouth using basic airway manouvres
2. Suction debris from oropharynx
3. Insert OPA upside down along the hard palate
4. Then, rotate 180 degrees
5. Insert until some resistance felt and the bite guard is at the level of the incisors
6. Reassess breathing - “look, listen and feel”

Question 35

What is the anatomical location of a surgical airway?

Answer 35

Median cricothyroid ligament, which forms the thickened anterior segment of the cricothyroid membrane

Question 37

How is jet insufflation performed and what are the main considerations?

Answer 37

Needle is passed directly through median cricothyroid ligament to deliver oxygen
Patient is oxygenated but not ventilated, leading to progressive hypercarbia
Therefore, this methid is a temporary measure (<45 minutes) permitting time to establish a definitive airway