BASIC Flashcards
Define respiratory failure
The inability for the respiratory system to meet the metabolic demands of the body by supplying oxygen and removing carbon dioxide
Type 1 - hypoxic - PaO2 <60
Type 2 - hypercapnic - PaCO2 >50
What are the 4 major pathophysiological mechanisms of respiratory failure?
- Low inspired partial pressure of oxygen
- Hypoventilation
- V/Q mismatch
- Diffusion abnormality
What is the Aa gradient, what is it used for?
The gradient between the partial pressure of O2 in the alveolus (PAO2) vs the arterial blood (PaO2)
Used to determine whether a shunt or diffusion abnormality is present
should be<20mmHg - affected by age and FiO2
normal implies hypoxameia is caused by hypercapnea
How is pAO2 calculated?
Alveolar gas equation:
pAO2 = FiO2 (PB-SVPH2O) - PaCO2/R+F
PB= barometric pressure 760mmHg at sea level SVPH2O= saturated vapour pressure of water 47mmHg at 37 degrees R= resp quotient carbon dioxide made from O2 consumed (usually 0.8) F= correction factor (minor and ignored)
pAO2 = (FiO2 x 713) - (PaCO2 x 1.25)
What are the causes of hypoventilation?
Resp Centre: head injury, encephalopathy, anaesthesia/drugs
Nerves: cord injury, GBS, MND
NMJ: MG, neuromuscular blockade
Muscles: myopathy, muscular dystrophy, malnutrition
Wall: kyphoscoliosis, ank spond, pleural fibrosis
lungs: APO, pneumonia, haemorrhage, ARDS
What are the causes of VQ mismatch?
V Pneumonia APO Atelectasis Haemorrhage
P
PE
Low cardiac output (and all associated causes eg Hypovolaemia)
high intrathoracic pressures
Anatomical shunting - Eisenmenger, TOF
What is the oxygen delivery equation?
O2 delivery = CO x Oxygen content
DO2 = CO x Hb x SaO2 x 1.34 + free O2 (PaO2x0.003)
What should be available for emergency airway equipment?
- O2
- Mask
- Airways - OPA/NPA/LMA
- Laryngoscopes
- ETTs
- Monitors (ECG/SaO2/ETCO2)
- Emerg drugs
- Suction
- self inflating bag mask
What size LMA would you use for a female? large male?
3 for female, 5 for large male
Sizes of Macintosh blades?
2,3,4
Sizes of ETT for male/female? normal depth?
8-9 for male, 7-8 for female
22-23cm depth
complications of ETT placement?
Meds
- hypotension
- anaphylaxis
- arrhythmias
Placement
- trauma (dental/soft tissue)
- incorrect placement (endobronchial > PTx)
- HTN/Tachycardia
- raised ICP
- hypoxaemia
- aspiration/laryngospasm
longterm
- laryngeal/tracheal stenosis
How is airway pressure calculated? Alveolar pressure?
Airway Pressure = Flow x Resistance + (volume/complicance) + PEEP
Alveolar pressure = (volume/complicance) + PEEP
Flow = Volume/time
How can shunting be reduced by mechanical ventilation?
reopening alveoli, keeping them open with PEEP, prolonging inspiration (more even distribution of ventilation)
How is alveolar ventilation calculated?
Alv Vent = RR x (Tidal Volume - Dead Space Volume)
How can O2 be improved using mechanical ventilation? CO2?
O2 improved by:
- increasing FiO2, PEEP, insp time
CO2 removal improved by:
- Increasing TV, RR (MV)
- Decreasing deadspace
What is gas trapping? what value increases with gas trapping?
- occurs with insufficient time for alveoli to empty before next breath (worse with airway obstruction Asthma/COPD). Causes progressive hyperinflation and rise in intrinsic PEEP
- can result in barotrauma and cardiovascular compromise from high ITP
What is set in Volume preset assist control?
TV + RR
same breath delivered whether pt or vent triggers
pt can breath above the prescribed rate
uncomfortable, needs sedation
risk of barotrauma with fall in compliance
difficult to set flow trigger correctlty
What is set in Pressure preset assist control?
Insp P + RR
because flow falls to zero if the breath is long enough and stays there as long as you apply pressure an insp hold is built in to each breath and improves oxygenation
pt can breath above the prescribed rate
uncomfortable, needs sedation
risk of changes in tidal volume with change in compliance
What is set in Pressure support?
insp pressure
only delivered when pt triggers a breath
(on newer vents will switch to backup if no triggers)
swaps to exp when the insp flow falls to a preset level
need some support to overcome added work of breathing through ETT and demand valve of ventilator
What is set in Synchronized intermittent mandatory ventilation (SIMV) ?
RR and either insp P or V (more common)
usually combined with pressure support
set number of mandatory breaths and pt can take additional PS breaths between
if pt breaths in SIMV period will deliver a mandatory breath
if pt breaths in spont period will PS
these periods are determined by the set RR
What are reasonable starting settings for FiO2, RR, TV, insp P, I:E, PEEP, triggering?
FiO2 - 100 then adjust to PaO2 or SaO2
RR- 12 (higher if high metabolic rate)
TV- 6-8mL/kg PREDICTED BW
Insp P - set to acheive a good TV, sum of PEEP and insp P should not be >30cmH2O
I:E - 1:2 is normal, higher (1:1) is better for oxygenation but worse for gas trapping
PEEP - 5cmH2O (unless asthma or COPD and not spont breathing then 0)
Triggering - flow or pressure. flow leads to more synchrony but is more sensitive. usually start with pressure -2cmH2O or mod sensitive flow
Describe the how the respiratory cycle time, and inspiratory time are calculated
Respiratory cycle time = 60/RR
Resp cycle time = insp time + exp time
exp time is not set
insp time = insp flow time + insp pause time
therefore you can only set 2 of insp time, insp flow time and insp pause time
remember flow = volume / time
Which lab value should be used to titrate minute ventilation?
In general pH should be used to titrate MV and not CO2 as the pathophysiological consequences of raised CO2 are mediated by acidosis.
Note raised ICP is an exception to this
What are causes of high airway pressure in volume preset modes or low TV in pressure preset modes?
Ventilator: inapp setting, malfunction
Circuit: kinking, pooling of water, wet filter
ETT: kinked, obstructed with sputum/blood, endobronchial
Pt: bronchospasm, decreased lung compliance (APO, consolidation, atelectasis), decreased pleural compliance PTx, decreased wall compliance (abdominal distension), dysynchrony (cough/hiccup)
What is the easiest way to determine if there is a problem with the vent/circuit/ETT/pt causing alarms on the vent?
Disconnect and manually ventilate
How can the ventilator be used to estimate alveolar pressure?
Insp pause pressure
Airway pressure = flow*resistence + Alveolar pressure
so if flow = 0 , Airway P = Alv P
Which pressure (airway or alveolar) is important for lung injury? haemodynamic effects?
Alveolar is what is important for both
How is Total PEEP calculated? how is it estimated using the ventilator?
PEEP Total = Intrinsic PEEP + Extrinsic PEEP
Extrinsic PEEP = PEEP applied (set on vent)
intrinsic PEEP = result of gas trapping
Estimate PEEP total with exp pause
Differentials and management for hypotension post-initiation of IPPV
DDx
- hypovolaemia (decreased vasc return due to increased ITP)
- Anaesthetic drugs
- Gas trapping from over-enthusiastic ventilating
- Tension PTx
First two are most common - give fluids, if this doesnt work disconnect from vent - this will resolve gas trapping, if that fails consider PTx
Describe the optimal ventilation strategy for ARDS with rationale
- High PEEP, low tidal volumes (6-8ml/hg predicted), plateau P <30cmH2O
- as PEEP increase so should FiO2
In ARDS there are dependent areas of lung with low compliance and relatively normal areas of lung with normal compliance. Ventilating this normally with cause barotrauma to the normal areas and cause rapid opening/closing of the alveoli in the dependent areas increasing shear injury. using high PEEP stents open alveoli and low tidal volumes prevents recurrent shear injury.
Describe the general ventilation strategy for unilateral lung disease
- low TV, low pressure as with ARDS
- increasing insp time increases gas distribution across lung
- Ventilating in lateral position with unaffected lung dependent (down) increases perfusion to normal lung and decreases its compliance (making it more similar to the bad lung) and makes it easier to ventilate
Describe the general ventilation strategy for Asthma
- Airway resistance and therefore pressure will be high but alveolar pressure will stay normal
- Volume control as Pressure control will deliver low TV as pressure is high
- maximize exp time to minimize gas trapping (this will shorten insp time and therefore increase insp flow time and airway pressure but alveolar pressure will be unaffected). Lowest RR you can to maximize exp time
- monitor plateau pressure and PEEP total (aim <10cmH2O)