Respiratory Flashcards
Indications for Mechanical Ventilation
- Airway Support - threatened airway, impaired airway reflexes
- Respiratory Support - high FiO2 required
- Ventilatory Support - deep sedation, regulate CO2
Delivery of non-invasive ventilation
- High flow nasal O2 - humidified O2, 40-60L/min FiO2 up to 1.0.
- CPAP
- BiPAP
Advantages of HFNO
- Humidification - prevent epithelial injury, help secretions
- PEEP
- Greater FiO2 - less entrainment of atmospheric gas
- Reduced CO2 dead space - washout
CPAP
- Continuous application of positive pressure throughout respiratory cycle
- Nasal, facemask, hood
- Better recruitment, improved V/Q
- Cheap, well tolerated
BiPAP
- Ventilatory support by difference between IPAP and EPAP
- Facmask
- ECOPD, prevent post-extubation respiratory failure.
PEEP
Pressure present in airway above atmospheric pressure at end expiration
Recruitment of alveoli and prevention of collapse (therefore reduced WOB)
Other physiological effects
- Improved pulmonary compliance
- Reduced venous return and preload
- Increased afterload and PVR
- Increased ICP by reduced venous drainage
Characteristics of ventilator modes
- Control - pressure or volume
- Cyclying - flow, time, pressure
- Trigger - patient, machine
- breath type - mandatory or spontaneous
- breath sequence - mandatory, intermittent mandatory, spontaneous
- synchronisation - synchronised, independent
- guarantee - TV, MV, pressure
PCV
- Pinsp, PEEP, Ti
- VT depends on compliance
- pressure rapidly delivered and held constant - square wave pattern
- flow decelerates
- pressure rapidly released in expiration
- gas equilibrates between varying alveolar time constants
VCV
- VT, PEEP, RR, flow pattern set
- constant inspiratory flow - gradual rise in Pinsp
Pressure support
Pinsp, PEEP, expiratory flow trigger set
Ventilator cycling
- Time - mandatory modes, insp and exp determined by time
- flow - insp and exp commenced after sensing change in flow (patient attempts breaths)
- pressure
Special modes
ASV - adjusts support based on patient requirements. support delivered in response to RR and effort
NAVA - neural assist - electronic activity from diaphragm, monitored via specialist NG tube
SIMV - time cycled mode mandatory breaths may be machine or patient triggered. pressure or volume controlled. spont ventilation permitted at varying parts of the cycle
APRV - Airway Pressure Release Ventilation
Advanced ventilatory mode
Maximises alveolar recruitment - used when alveolar recruitment felt to be possible
P-high, Thigh, Plow (0) and Tlow
CO2 release at Plow
breath spontaneously
extreme inverse ratio
Ads - recruitment, lung homogeneity, reducing in cyclical opening and closing of alveoli
disads - high local pressure, tachypnoea, rv dysfunction
Indications for proning
Critical care
- mod-severe ARDS Pf < 150 (< 48hrs into disease after IPPV optimised) PROSEVA trial
Theatre
- surgical access e.g. spine, posterior fossa
Contraindications
Absolute
- open chest, <24h post cardiac surgery, central ECMO cannulas, unstable spine
relative
- severe CVS instability, pregnancy, recent tracheostomy, significant trauma
Risks of proning
Instability
- airway dispalcement
- line displacement
- increased into abdominal pressure
- reflux and aspiration
Patient injury
- pressure sores
- brachial plexus
- periorbital oedema, chemises, blindness
Staff injury
How to prone
Pre-procedure
- MDT decision
- check with nurse in charge senior doctor re. timing
- ensure any procedures that require supine position carried out
- system assessment e.g. airway position, suction. 100% O2. stop non-essential infusions
- eye / nipple protection, remove anterior ecg stickers
- spigot and aspiration NG tube
Procedure
- minimum 1 airway and 2 each side
- pillows to chest, iliac crests, knees
- sheet over top and roll edges (Cornish pasty)
- slide away from ventilator
- turn to 90 degrees
- complete prone
post-procedure
- re-assess A-E
- check pressure areas
- complete positioning e.g. swimmers, revers trendelenburg
- 2-4 hourly head turn and arm change
- document
conscious proning
covid. if fio2 > 28% or more than basic resp support
position change every 1-2 hrs - fully prone, right recumbent, sitting up 30-60 degrees, left recumbent
Types of extra-corporeal life support
- VV-ECMO
- VA-ECMO
- AV-ECMO (rare)
- ECCO2R
- Cardiopulmonary bypass
- VAD
VV-ECMO concept
Gas exchange in native circulation failed
an oxygenator incorporated into extra-corporeal circuit
oxygenation determined by circuit flow rate and post-oxygenator oxygen content. increased by increasing flow rate
co2 diffuses out through oxygenator determined by sweep gas flow rate. CO2 more soluble, diffuses more readily and maintains diffusion gradient. clearance therefore related to sweep gas rather than blood flow
conventional ventilation reduced to rest settings and allow healing of pathology
VV oxygenator flows limited to 5-6l/min (cannula size)
VV-ECMO indications
despite optimum conventional management: including trial of proning:
- hypoxaemic resp failure PF < 80
- hypercapnia resp failure pH < 7.25
“potentially reversible severe resp failure”
- ventilatory support as bridge to lung transplant
specific clinical conditions include
- severe ARDS
- BPF
- severe asthma
- thoracic trauma
VV-ECMO contraindications
Absolute
- anticipated non-recovery without a viable plan to decannulate
relative
CNS - haemorrhage, severe debilitating pathology, significant njury
refractory or established MOF
Haem - bleeding, inability to anticoagulant
IPPV > 7 days with plat pressure > 30 and fio2 > 90
older age
O2 delivery in VV ECMO
O2 content minimum 240ml.min
O2 delivery minimum 300ml.min
Arterial O2 is mixture of venous blood removed and passed through oxygenator and blood passing through lungs and is typically 80-90%
increasing circuit flow increased circuit:native and therefore total O2 content
Issues on VV ECMO
recirculation - post oxygenator blood returning to oxygenator and not systemic circulation (high venous saturations) more common with single lumen dual cannula access
hypoxaemia - increased VO2 e.g. sepsis, recircualtion. DO2 / VO2 5:1 increase flow
CO2 removal - sweep gas 1-9l/min in 100% O2
Chatter - partial venous cannula occlusion e.g. rhythmic ventilation, cough, valsava
suck down - intravascular volume restriction or cannula misplacement abrupt reducing flow to 1l/min with full pump speed - haemolysis, air embolism
high Paco2 - may be increased metabolic state, circuit problem, monitoring problem. can increase sweep gas flow
ECMO emergencies
gas embolism - clamp return line, clamp drainage line, switch off pump, remove air using syringe at oxygenator port
accidental decannulation - turn off pump, compress site, major haemorrhage, emergency recannulation
circuit failure - change
VV-ECMO cannulas
Single lumen dual cannula - IJ - fem
bicaval dual lumen single cannula - RA / IVC
bifemoral
Dual lumen single cannula improved mobility, needs fluoroscopy / echo guidance
Haemodynamics on VV ecmo
hypoxia, hypercarbia, acidosis lead to elevated PA pressures, rv dysfunction. implication
- vv ECMO no direct haemodynamic support - need to manage conventionally
- improved oxygenation and co2 clearance improves PA pressures, rv function, lv function
- reduced thoracic pressures due to lower IPPV settings may also improve haemodynamics
Rest ventilator settings
recommended PEEP 10 Pinsp 25 rate 10 FiO2 as low as possible e.g. 30%
CESAR / EOLIA settings
weaning ecmo
reduced circuit flow or reduce sweep gas FiO2 from 1.0 - 0.21
over hours - days
assess ventilatory reserve
ECMO scoring systems
RESP - predicted survival on ecmo
Murray - predicted mortality without ecmo
RESP -22 to 16. Class I > 6 92% in-hospital survival. class V < -6 18% survival
Murray
- consolidation (quadrants on CXR)
- compliance
- P/F ratio
- PEEP
Severe > 2.5
ECMO trials
CESAR
- improved outcomes from severe potentially reversible resp failure in those transferred to ecmo centre (not with use of ecmo) vs conventional
- advocates transfer to specialist centre - Murray Score > 3 pH < 7.25 on optimum mx
EOLIA
- severe ARDS
- VV-ECMO vs VCV (28% crossover)
- no difference in 60d mortality.
What is ventilator weaning?
- slowly reducing ventilator support over time, increasing patients own ventilatory drive and extubation
- may be minutes - months
Simple: liberation from ventilator on first attempt (70%)
Difficult - 2-7 days after initial assessment
Prolonged - > 7 days