Mechanical Ventillation Flashcards
Patient Causes for high airway pressure (volume control modes) / low tidal volume (pressure control modes)
Patient
- bronchospasm
- reduced lung compliance
- pulmonary oedema
- ARDS
- collapse
- reduced pleural compliance (e.g., pneumothorax)
- reduced chest wall compliance (e.g., massive ascites)
- ventillator dyssynchrony
Equipment Causes for high airway pressure (volume control modes) / low tidal volume (pressure control modes)
ETT
- kinking
- blocked
Circuit
- condesation in tubing
- kinking
- wet filter
Ventillator
- inappropriate settings
- malfunction
Approach to hypotensive patient immediately post intubation (three initial steps)
- Give fluid
- Disconnect from circuit and hand bag
- Consider needle thoracostomy
Causes of hypotension immediately post intubation
- Drugs
- Gas trapping
- Pneumothorax
Three immediate steps for hypoxic, intubated patient
- Confirm adequate pulse oximetry waveform
- Increase FiO2 to 1.0
- Confirm ventillation
- tube fogging
- end-tidal CO2
- chest wall movements
- auscultate for air entry
Approach to hypoxic, intubated patient with poor ventillation (e.g., chest wall not moving, low tidal volumes/high pressures)
- Confirm adequate pulse oximetry waveform
- Increase FiO2 to 1.0
- Confirm ventillation
- tube fogging
- end-tidal CO2
- chest wall movements
- auscultate for air entry
- Disconnect from circuit and manually ventillate
- Assess ease of manual ventillation
- Difficult
- ETT/patient problem
- Easy
- Ventillator problem
Approach to hyoxic intubated patient with adequate ventillation
- Confirm adequate pulse oximetry waveform
- Increase FiO2 to 1.0
- Confirm ventillation
- tube fogging
- end-tidal CO2
- chest wall movements
- auscultate for air entry
- Assess patient for:
- pneumothorax
- collapse
- pulmonary oedema
- bronchospasm
- Treat and adjust ventillator
Equation for total airway pressure (inspiratory)
Total airway pressure = airway pressure + alveolar pressure
Total airway pressure = flow x resistance + volume/compliance + PEEP
Define inspiratory airway pressure
The pressure during inspiration caused by airway resistance to flow and alveolar compliance
What determines mean alveolar pressure?
- Tidal volume OR inspiratory pressure
- Inspiratory time
- PEEP
How can mechanical ventillation be adjusted to improve oxygenation?
- Increase FiO2
- Increased mean alveolar pressure
- increase inspiratory time
- increase tidal volume OR inspiratory pressure
- increase PEEP
List adverse effects of mechanical ventillation
Barotrauma
Gas trapping
Oxygen toxicity
Cardiovascular depression
What factors contribute to barotrauma
High tidal volume
High maximum alveolar pressure
High shear forces
List five barotrauma related injuries
- Pneumothorax
- Pneumomediastinum
- Pneumopericardium
- Surgical emphysema
- Acute lung injury
Strategies for increasing carbon dioxide elimination
- increase tidal volume
- increase respiratory rate
- decrease dead space
What two factors is the arterial pCO2 dependent upon?
- Alveolar ventillation
- CO2 production
What determines alveolar ventillation?
Alveolar ventillation (minute ventillation) = respiratory rate x (tidal volume - dead space)
List three cardiovascular effects of positive pressure ventillation
- Reduced venous return (preload)
- Reduced afterload
- Reduced myocardial oxygen consumption
What factors reduce venous return (preload) in positive pressure ventillation?
- high inspiratory pressure
- high PEEP
- prolonged inspiratory time
Describe how afterload is reduced during positive pressure ventillation
Afterload = ventricular wall tension
Ventricular wall tension = ( transmural pressure x radius ) / ( 2 x wall thickness)
Transmural pressure = intraventricular pressure - intrapleural pressure
Positive pressure ventillation increases intrapleural pressure, thereby reducing transmural pressure.
Define compliance
It expresses distensibility; that is, the tendency of a chamber to increase in volume when exposed to a given distending pressure
State the equation describing static thoracic compliance
Cstat = VT / [Pplateau - PEEP(tot)]
( Compliance = ∆volume / ∆pressure )
What are the four targets for the ARDSnet lung protective ventillation strategy?
- Tidal volume 6mL/kg (predicted body weight)
- Plateau pressure ≤ 30mmH2O
- pH 7.3 to 7.45 (permissive hypercapnia)
- I:E ratio ≤ 1
What is the effect of positive pressure ventillation on a volume depleted patient?
Reduced cardiac output (impaired filling due to raised intrathoracic pressure)
What is the effect of positive pressure on the failing heart?
Increased cardiac output (reduced afterload effect dominant, preload likely to be excessive)
Define closing pressure
The transpulmonary pressure at which distal airspaces begin to collapse
Give two examples of disease states increasing closing pressure
COPD
ARDS
What are the two possible alveolar effects of increasing PEEP?
- Alveolar recruitment
- Alveolar overdistension
(Dependent on recruitable lung volume)
What two observations can indicate PEEP is promoting alveolar recruitment?
- increased lung compliance
- improved oxygenation (e.g., A-a gradient; PAO2/FiO2 ratio)
List contraindications to non-invasive ventillation
A — unprotected airway, obstruction, oesophageal/max.facs surgery
B — untreated pneumothorax, (impending) respiratory arrest
C — shock
D — altered mental status (agitation vs. sedation), head trauma/surgery (esp. base of skull fracture — risk of pneumocephalus)
List indications for mechanical ventillation (A to E then other)
A – protection and patency
B – respiratory failure (hypercapnic or hypoxic)
C – minimise oxygen consumption and optimize oxygen delivery (e.g. sepsis)
D – unresponsive to pain, terminate seizure, prevent secondary brain injury
E — temperature control (e.g. serotonin syndrome)
Other — safety for transport (e.g. psychosis), humanitarian reasons
Complications of NIV
- air swallowing with abdominal distension -> vomiting and aspiration
- hypotension (if hypovolaemic)
- raised ICP
- increased intraocular pressure
- claustrophobia/anxiety
- agitation
- pressure ulcers/necrosis (nasal bridge)
- facial or ocular abrasions
What is the rationale for protective lung ventillation in ARDS?
- low tidal volume ventilation reduces ventilator-associated lung injury (VALI)
- volutrauma (hyperinflation and shearing injury)
- barotrauma (alveolar rupture and pneumothorax)
- biotrauma (release of inflammatory mediators)
- hypercapnia may also have directly beneficial effects in ARDS
- clear evidence for benefit in ARDS in animals and humans
What are the four targets for the ARDSnet lung protective ventillation strategy?
- Tidal volume 6mL/kg (predicted body weight)
- Plateau pressure ≤ 30mmH2O
- pH 7.3 to 7.45 (permissive hypercapnia)
- I:E ratio ≤ 1
What are the three pathophysiological processes in ventillator-associated lung injury (VALI)?
- volutrauma (hyperinflation and shearing injury)
- barotrauma (alveolar rupture and sequelae)
- biotrauma (release of inflammatory mediators)
