Respiratory Support (Source: Revision Notes)

Question 1

What is the estimated FiO2 of a Huson mask at 5-6 litres?

Answer 1

0.4

Question 2

How does the patients minute volume affect the FiO2 delivered by an oxygen mask?

Answer 2

With normal work of breathing and resp rate, the proportion of oxygen flow relative to entrained air will be relatively high and therefore the fio2 will be relatively high
If work of breathing and respiratory rate are increased the proportion of oxygen flow to entrained air will fall and so too will the FiO2

Question 3

How does a venturi mask result in a fixed FiO2?

Answer 3

The flow of O2 is forced through a fixed aperture leading to acceleration of flow and entrainmentof a fixed proportion of room air; the FIo2 is therefore fixed and independent of respiratory effort

Question 4

What FiO2 is typically delivered by simple nasal cannulae?

Answer 4

2-4 litres per minute will equate to an FiO2 of 0.24 - 0.35, although this will vary with respiratory effort

Question 5

Describe high flow nasal cannulae?

Answer 5

High flow nasal cannulae utilise the Venturi effect and are capable of delivering up to 60L of flow per minute, with an FiO2 of between 0.21 and 1.0
They can humidify and warm inspired gas
They may produce a degree of positive end expiratory pressure, particularly to the soft tissues of the nasopharynx

Question 6

What is peak pressure?

Answer 6

Maximum airway pressure measured in the respiratory cycle. Usually taken to represent pressures applied to the large airways (and is therefore influenced by airway resistance)

Question 7

What is plateau pressure?

Answer 7

Airway pressure measured during an inspiratory pause. Usually taken to represent the pressure applied to alveoli.

Question 8

What is rise time?

Answer 8

The proportion of Tinsp taken to reach target pressure (or volume)

Question 9

What is Tinsp?

Answer 9

Time in seconds spent in inspiration

Question 10

What does ‘control’ refer to on a ventilator?

Answer 10

The target that the ventilator seeks to achieve. Either
Volume - the operator determines the volume to be delivered; Paw is determined by resistance and compliance.
Pressure - operator determines the pressure. resistance, compliance, and Tinsp determine Vt.

Question 11

What does ‘cycle’ refer to on a ventilator?

Answer 11

The variable that terminates inspiratory phase and allows expiration. Can be set to:

1. Time - cycling occurs after a designated time period (Tinsp)
2. Flow - cycling occurs when the gas flow decreases to a designated proportion of the peak inspiratory flow (usually at 25%)
3. Volume - cycling occurs when a designated volume of gas has been delivered
4. Limit - inspiratory phase is terminated if alarm limits (pressure or volume) are reached.

Question 12

What is ‘trigger’ on a ventilator and it’s possible variables?

Answer 12

The variable that initiates inspiration

1. Time: inspiration occurs after a designated time period
2. Pressure: Fall in pressure within the ventilator circuit triggers inspiration
3. Flow: alteration in the flow through the circuit (modern ICU circuit with continuous flow of gas, respiratory effort causes a decrease in flow, thereby triggering breath
4. Diaphragmatic neural activity - NAVA

Question 13

What are the different flow patterns possible on a ventilator?

Answer 13

1. Constant (square wave) - flow rate increases rapidly and remains constant until the taget variable has been achieved (typical of some volume controlled modes)
2. Decelerating flow - typical of pressure controlled modes (and more recently of volume-controlled modes), flow falls as alveolar pressure increases. May lead to improved distribution of gas throughout alveoli with differing time constants. The degree of deceleration may be altered in some ventilators by controlling the ramp.
3. Sinusoidal flow - typical of spontaneous unassisted breathing

Question 14

In a ventilated patient, what are the determinants of oxygenations?

Answer 14

1. FiO2
2. Mean airway pressure. This is determined by PEEP and the I:E ratio (the greater the proportion of the resp cycle spent in inspiration the greater the mean airway pressure)

Question 15

In a ventilated patient what is CO2 clearance determined by?

Answer 15

Minute volume - frequency, tidal volume and volume of dead space

Question 16

What are the adverse effects of mechanical ventilation?

Answer 16

1. Anaesthetic and sedation related
2. Airway related - damage to local structures, loss of airway
3. Haemodynamic - effect of PEEP - may decrease preload on increase RV afterload
4. Ventilator induced lung injury.
   - volutrauma
   - baratrauma (avoid Pplat >30)
   - atelectotrauma - theoretically reduced by PEEP
   - biotrauma - injury to the alveolar membrane triggers up regulation of cytokines resulting in a systemic inflammatory repsonse and multi-organ failure
   - oxygen toxicity
5. VAP

Question 17

What are the benefits of NIV (vs mechanical ventilation)?

Answer 17

Safe
Avoids intubation associated complications
Decreases work of breathing
Increase in mean airway pressure

Question 18

What are the contra-indications to non-invasive ventilation?

Answer 18

Need for intubation
Facial or upper airway injury
Excess secretions
Multi-organ failure
Agitation
Upper GI haemorrhage

Question 19

For which conditions are there good evidence to support the use of NIV?

Answer 19

1. Exacerbation of COPD - NICE guidelines recommend as first-line for T2RF secondary to exac COPD which fails to respond to medical therapy. Benefits less clear if there is a pnuemonia as well, and it’s much more likely to fail.
2. Cardiogenic pulmonary oedema - NIV improves mortality and rate of intubation. Theoretical benefits include decrease WOB, decreased preload and decreased afterload.
3. Trauma - in patients with blunt chest trauma and pulmonary contusions
4. Post-extubation - as a prophylactic adjunct but not a rescue therapy.

Question 20

What are the indications for invasive ventilation

Answer 20

1. Airway compromise - trauma, tumour, infection, bleeding, decreased GCS, burns
2. Hypoxia or hypercapnia
3. Significant haemodynamic instability (reduces O2 consumption, reduces preload and reduces afterload)
4. Decreased conscious level, raised ICP, refractory seizures
5. Facilitation of procedures and transfer

Question 21

What is a recruitment maneouvre?

Answer 21

A deliberate, transient increase in intrathoracic pressure utilised in the management of ARDS with a view to improving oxygenation and/or compliance
It’s based on the principle that:
-a number of lung units within the ARDS lung are collapsed
-re-opening of these units would improve oxygenation and compliance
-these units have a critical opening pressure, which, if exceeded, will result in re-aeration

Question 22

Describe different techniques to provide a recruitment maneouvre

Answer 22

1. Sigh breath - a large Vt or high Pinsp are supplied for 1 breath
2. Sustained inflation - ariway pressures are increased to supranormal levels (e.g. 40cmH2O) for a period of time (e.g. 40secs)
3. Extended sigh - increase PEEP over a period of time - usually 2 mins - with a resultant increase in peak pressure and therefore recruitment
4. incremental PEEP - stepwise increase up to Ppeak 45cmH2O then gradually step down

Question 23

What are the risks associated with recruitment maneouvres?

Answer 23

Barotrauma

Haemodynamic instability

Question 24

What are the advantages of prone ventilation?

Answer 24

Ventilation - more homogenous distribution of ventilation due to improvement in thoraco-abdominal compliance. Alveolar pressure more evenly distributed - vulnerable lung unit less likely to collapse on expiration. The dependent heart does not compress posterior lung units. Improved alveolar recruitment, better drainage of secretions.
Perfusion - more homogenous distribution of perfusion - in semi-recombent position perfusion and atelectasis are greatest at the bases - proning diverts perfusion to better aerated regions. Possible reduction in extravascular lung water.

Question 25

What is high-frequency oscillatory ventilation (HFOV)?

Answer 25

A continuous flow of gas, maintained at a designated positive pressure (mean airway pressure) by an adjustable expiratory valve. The mean airway pressure and FiO2 are the primary determinants of oxygenation. A piston or diaphragm oscillates the airway pressure around the mean at very high frequency (4-15Hz), leading to very small 'tidal volumes', significantly less than anatomical dead space

Question 26

What are the potential benefits of HFOV?

Answer 26

Protective strategy - lower Vt decreases risk of volutrauma. Lower peak pressure, therefore lower risk of barotrauma. Open lung therefore less risk of atelectotrauma. Improved oxygenation - recruitment of larger number of aerated lung units. Increased FRC.

Question 27

What are the disadvantages of HFOV?

Answer 27

Resp- less effective CO2 clearance, is settings inappropriate. Risk of dynamic hyperinflation.Unable to measure MV, ETCO2, FiO2 effecively, cooling and ring of inspired gases. CV - increased vagal activity, decreased venous return and cardiac output. Neuro - increased sedation requirements, potential increase in intracranial pressure.

Question 28

Does the evidence support the use of HFOV?

Answer 28

2 large randomised control trials - OSCAR and OSCILLATE. OSCAR - demonstrated no improvement in outcomes OSCILLATE demonstrated worse outcomes

Question 29

What are the mechanisms of gas transport in HFOV?

Answer 29

Convection - some degree of bulk flow in proximal airways Molecular diffusion - Movement of gaseous molecules from area of high concentration to low concentration Co-axial flow - bias flow within the ventilator circuit continues into airways with inwards flow occurring in middle and co-axial outwards flow occurring in the periphery. Taylor dispersion - interplay between convective forces and molecular diffusion Pendulluft ventilation - exchange between adjacent lung units of differing time constraints Cardiogenic mixing - agitation of lung units adjacent to heart and great vessels enhances molecular diffusion

Question 30

What is APRV ventilation?

Answer 30

Airway pressure release ventilation It applies a continuous airway pressure identical to CPAP and adds a time-cycled release phase to a lower set pressure The tidal volume (or release volume) depends on the difference between the set high and low pressures, the Flow and the compliance of the respiratory system. It provides a means of maintaining high mean airway pressure (thereby enhancing oxygenation) whilst minimising ventilator associated lung injury.

Question 31

What is NAVA?

Answer 31

Neurally adjusted ventilatory assist A specialised NGT with a set of electrodes detects the patients neural breathing demand for a breath by monitoring the electrical signal of the diaphragm (Edi). Edi above a designated level will trigger the ventilator, the level of support provided is proportional to the magnitude of Edi detected

Question 32

What are the indications for ECMO?

Answer 32

In hypoxic respiratory failure where risk of mortality is > 80% (and consider when >50%) 80% mortality is identified where PaO2/FiO2 <80 of FiO2 >90% and Murray score 3-4 50% mortality identified by PaO2/FiO2 < 20 on FiO2 >90% CO2 retention due to asthma or permissive hypercapnia with Paco2 > 10.6 Severe air leak syndromes

Question 33

What are the contraindications for ECMO?

Answer 33

None are absolute Conditions known to have a poor outcome are relative contra-indications - mechanical ventilation at high settings - FiO2 >90%, plateau pressure > 30cm H2O for > 7 days Major pharmacological immunosuppression CNS haemorrhage thats recent or expanding

Question 34

Which study investigated use of ECMO in severe respiratory failure?

Answer 34

CESAR Pts were randomised to transfer to a specialist resp failure centre for ECMO or standard treatment Significant decrease in mortality in the intervention group, although not all patients received ECMO Therefore transfer to a specialist centre may be a key component of the intervention

Question 35

What are the key components of an ECMO circuit?

Answer 35

A venous access cannula - femoral or jugular A centrifugal pump An oxygenator - a polymethylpentane membrane that allows transfer of gas from a fresh gas flow to blood A fresh gas supply A return cannula - for isolated respiratory support this is venous - with the tip in the RA. for cardiac and respi the retun cannula is inserted via the femoral artery into the descending aorta.

Question 36

In VV ECMO what is the prmary determinant of systemic oxygenation?

Answer 36

ECMO blood flow relative to cardiac output The closer the ECMO blood flow to CO, the greater the systemic oxygenation If circuit flow is maximised but oxygenation inadequate, measures may be taken to reduce cardiac output, including beta blockers, hypothermia and sedation

Question 37

In VV ECMO what is CO2 clearance related to?

Answer 37

ECMO sweep gas flow

Question 38

What are the risks associated with extracorporeal support?

Answer 38

Need for transfer to a specialist centre Cannulation - haemorrhage, vascular injury, cardiac injury, hypotension and arrythmias on initiation Extracorporeal circuit - infection, air embolus, exsanguination, activation of clotting cascade Coagulation - thrombocytopemia, hypofibrinogenaemia, acquired Von Wilebrand deficiency, thrombus formation within the circuit or cannulated vessels Decreased pharmacokinetics due to increased volume of distribution and sequestration of drug onto extracoporeal membrane, risk of under dosing antibiotics