Resp Flashcards
driving pressure
ΔP = VT/CRS (resp system compliance)
Bedside - Pplat - PEEP
providing lung-protective ventilatory strategy that is adapted to the size of the aerated lung
Has been assessed in retropective analysis of RCT patients
Aim for driving pressure of <15
transpulmonary pressure definition
TPP is the difference between the alveolar pressure (Palv) and pleural pressure (Ppl).
TPP is the net distending pressure applied to the lung.
How to measure alveolar pressure (Palv)
difficult to measure instantaneously during flow, but equalises to airway pressure at states of zero flow with airway occluded.
Classically measured as inspiratory pause pressure after complete tidal volume.
How to measure pleural pressure (Ppl).
estimated from oesphageal pressure (Pes.) with a thin wall latex
oesophageal ballon inserted via the NG or OG route.
Its measurement is prone to error;
- Malposition – gastric (one of third balloon placements in study below challenging)
- Positioning: supine vs erect (addition of mediastinal weight)
- Assumption that pleural pressures even through the chest
- Extrinsic factors – obesity, rising intra-abdominal pressure
Rationale for using transpulmonary pressure
the effects of chest wall compliance are negated and a true measure of lung distension is obtained. This may allow the safe tolerance of higher plateau pressures.
May have a role in obesity, raised intra-abdominal pressure and air trapping.
More accurate prevention of ventilator associated lung injury may be obtained by using TPP, e.g.:
Limit recruitment maneuvers to TPP 25 cmH2O
Setting PEEP to TPP 0-10 cmH2O
Limiting volutrauma by setting VT to a TPP 25 cmH2O
Determination of respiratory muscle work in spontaneous ventilation
Assessment of ventilator dys-synchrony
Estimation of auto-PEEP in spontaneously breathing patients
POssible causes of a patient “not being able to get enough air” ie causes of vent asynchrony
Patient factors
- Airway / trache – blocked, displaced or too small diameter
- Respiratory e.g. pneumonia, PE, PTX
- Cardiac – ongoing ischaemia, cardiac failure, fluid overload
- Neuromuscular – weakness, fatigue
- Sepsis
- Metabolic
- Central – increased respiratory drive, pain, agitation
Ventilator factors
- Unsuitable mode
- Triggering threshold too high
- Inspiratory flow rate too low
- Prolonged inspiratory time
- Inappropriate cycling
- Inadequate pressure support
- Inadequately set tidal volume
- Ventilator malfunction
Methods of determining PEEP
Use the ARDSNet PEEP/FiO2 escalation tables (setting the PEEP according to the severity of the oxygenation failure)
Titrate PEEP according to maximum compliance, i.e. set the PEEP which achieves the highest static compliance
Set the PEEP using the lower inflection point of the pressure volume curve (the point that indicates the pressure at which alveolar recruitment is maximal.
The measurement requires paralysis and - ideally - serial static measurements
Use a staircase recruitment (or derecruitment) manoeuvre to find the lowest PEEP at which the maximal oxygenation is maintained.
Using a PA catheter, titrate PEEP to achieve the smallest intrapulmonary shunt
Titrate PEEP according to the transpulmonary pressure
- Transpulmonary pressure = (Pplat - Pes)
- The ideal TPP is 0-10 in end-expiration and no more than 25 in inspiration
Using electrical impedance tomography, titrate PEEP to achieve the highest electrical impedance in the thorax (i.e. the greatest amount of aerated lung)
Sequential CT scans to visually determine a PEEP at which the greatest volume of lung is recruited during end-expiration
Patient-Ventilator Dyssynchrony definition
when the patient’s demands are not met by the ventilator,
Due to issues including;
(1) timing of inspiration
(2) adequate inspiratory flow for demand
(3) timing of the switch to expiration
(4) duration of inspiration
Indications for NIV
Strong indications -
Cardiogenic pulmonary oedema: improves survival, decreases rate of intubation (Cochrane review)
COPD: halves mortality when compared to invasive ventilation (Cochrane review)
Obesity hypoventilation sydrome: mainstay of chronic maintenance and rescue for acute respiratory failure (Carrillo et al, 2012)
Rib fractures and chest trauma: reduced mortality, intubation rate and infections (Chiumillo et al, 2013)
Weak indications -
Asthma: no mortality benefit, but prevents intubation, decreases ICU stay and imrpoves delivery of nebulised drugs (Lim et al, 2012)
Weaning COPD patients from invasive ventilation: improves mortality, reduces VAP risk (Cochrane review)
Elective extubation of patients without respiratory failure: Cooperative hypercapneic high-risk patients may benefit (Ferrer et al, 2006)
Ventilation for cystic fibrosis patients awaiting lung transplant: based on small-scale observational studies (Bright-Thomas et al, 2013)
Community-acquired pneumonia: useful in patients with pre-existing cardiac or respiratory disease (Carrillo et al, 2012)
Post-operative respiratory failure- “prophylactic NIV” - little data in support of this (Jaber et al, 2012)
Lung infection in the neutropenic patient: improves survival when compared to intubation (one small trial)
Limitations of therapy: if the patient requires intubation but is “not for “ intubation; NIV provides comfort (Azoulay et al, 2010)
Complications of NIV
Mask intolerance, agitation and claustrophobia Increased need for sedation Delay of intubation Aspiration Poor clearance of secretions Hypotension of hypovolemic patients Facial pressure areas Raised intracranial pressure Aerophagy (swallowing air) Damage to facial, nasal and oesophageal surgical sites or traumatic injuries, leading to surgical emphysema, pneumothorax and pneumomediastinum
mechanisms by which an ICU ventilator may cycle from inspiration to expiration
Time cycled.
Once the time programmed for inspiration (inspiratory flow time plus inspiratory pause time) is completed, the ventilator automatically cycles to expiration. This occurs independent of any patient effort or other variables.
Flow cycled.
Once flow has decreased to a pre-determined minimum value, (eg 25% maximum flow rate), the ventilator cycles to expiration. In lungs with poor compliance, the cycling threshold will be reached more quickly, resulting in a shorter time for inspiration and a smaller tidal volume. Used more in spontaneous modes
Pressure cycled.
Once a set pressure is reached, the ventilator will cycle to expiration. Non-compliant lungs will have smaller tidal volumes than compliant lungs. The most common application for this mode is as an alarm setting as a safety feature to prevent sustained or excessive high pressures.
Volume cycled
Once a set volume is reached, the ventilator will cycle to expiration (or inspiratory pause).
Dapsone
Pneumocystis prophylaxis in those with sulphonamide allergy
Side effects-
- methamoglobinaemia
- haemolytic anaemia
- agrnulocytosis
Methamoglobinaemia
Use a co-oximeter to measure oxygen saturation - the pulse oximeter will read about 82%.
Increase the oxygen carrying capacity of blood by transfusion of PRBCs
Aim for a high PaO2
Infuse methylene blue to reduce all the Fe3+ back into Fe2+
Infuse glucose - it is essential for the hexose monophosphate shunt, which produces the NADPH required for methylene blue to be effective
Types of ventilator associated lung injury
Barotrauma (due to disruption of Basement membrane, seen when transpulmonary pressure >50)
Volutrauma (also BM)
Atelectotrauma (why PEEP helpful)
Macroscopic shear injury - at junction of good and bad lung
Biotrauma - upregulation of pulmonary cytokine production
Oxygen toxicity - destruction of alveolar cells
Causes of wheeze
Extrathoracic causes
- Anaphylaxis
- Vocal cord paralysis
- Laryngeal stenosis
- Goiter with thoracic inlet obstruction
- Anxiety with hyperventilation
Intrathoracic central airway causes
- Tracheal stenosis
- Mediastinal tumours
- Hyperdynamic airway collapse due to tracehomalacia
- Mucus plugs
- Thoracic aortic aneurysm
- Foreign body inhalation
Intrathoracic lower airway causes
- Bronchitis or bronchiolitis
- COPD
- Pulmonary oedema - “cardiac asthma”
- Airway distortion due to mechanical causes, eg. bronchial mass, bronchiectasis, pneumothorax
- Exposure to inhaled irritant or corrosive agent, and this includes the aspiration of gastric contents
