AMP 27: Mechanical Ventilation Flashcards
What are the two main types of respiratory failure?
oxygenation failure and ventilation failure
What does oxygenation depend on?
surface area available for gas exchange and preservation of the gas exchange barrier
What does ventilation depend on?
Fresh gas flow through alveoli removing CO2
What are the three main indications for mechanical ventilation?
- severe hypoxemia despite O2 therapy
- severe hypoventilation despite therapy
- excessive respiratory effort
What is the definition of “severe hypoxemia”?
cyanosis, PaO2 < 60 mm Hg, or SpO2 < 90%
What is the marker of “severe hypoventilation”?
PCO2 > 60 mm Hg
What does primarily control PCO2?
minute ventilation
What does “minute ventilation” mean and what are its determinants?
amount of fresh gas flow to alveoli in a minute, determined by respiratory rate and tidal volume
What are the three main ventilator breath patterns used?
- Assist control ventilation (A/C)
- Synchronized intermittent mandatory ventilation (SIMV)
- Continuous spontaneous ventilation
Describe A/C ventialtion
Assist/Control Ventilation
- all breaths are mandatory breaths (generated by the machine)
- minimum respiratory rate is set but patient can trigger additional breaths which are then generated by the machine, i.e., patient can increase the RR but all breaths are full ventilator breaths
What determines peak airway pressure in volume controled ventilation?
- tidal volume chosen
- compliance of the respiratory system
What determines the tidal volume in pressure controlled ventilation?
- level of airway pressure chosen
- compliance of the respiratory system
Describe SIMV
Synchronized Intermittent Mandatory Ventilation
- operator sets the number of full mandatory ventilator breaths
- patient can breathe spontaneously inbetween
- machine will try to synchronize mandatory and respiratory efforts
- generally used for patients with less severe disease than A/C
What are the two most common types of ventilator support for continuous spontaneous ventilation? Describe them
continuous positive airway pressure (CPAP)
- ventilator does not deliver breaths
- all breaths are spontaneous breaths/completely patient generated (rate, inspiratory time and TV all determined by patient)
pressure support ventilation (PSV)
- ventilator does not deliver breaths but tidal volume is augmented by the ventilator
- good for patients with adequate respiratory drive but inadequate ventilatory strength
- can be used alone or in conjunction with CPAP or SIMV
What is the reported normal tidal volume in dogs and cats?
10-15 ml/kg
When placing an animal on a ventilator, what is the maximum tidal volume that should be used initially
do not exceed 10 mL/kg to prevent lung injury from overdistension
What are common tidal volumes targeted in animals with severe lung disease?
6-8 mL/kg
When placing an animal on a ventilator what is a desirable initial peak airway pressure?
10-15 mm Hg
When placing an animal on a ventilator, what are reasonable trigger values to start with?
- an airway pressure drop of - 2 mm Hg
- a gas flow change of 2 L/min
What are the benefits of PEEP?
- improves oxygenation
- increased recruitment of collapsed alveoli
- may reduce ventilator-induced lung injury
What is the minimum PEEP set in most ventilator settings?
2 mm Hg to prevent atalectasis
Explain “breath stacking”. What is recommended to avoid this?
As respiratory rate increases, expiratory time is sacrificed to fit in the amount of breaths per minute. The animal is then not able to fully exhale before the start of the next inspiration, which creates an intrinsive positive end-expiratory pressure.
To avoid this an I:E ratio of at least 1:1 or higher is recommended. Common starting point is 1:2. If the respiratory rate is increase, the inspiratory time may need to be decreased to accomplish sufficient I:E ratio.
Fill in the blanks for initial ventilator settings, assuming the patient has normal lungs
Fill in the blanks for initial ventilator settings assuming the patient has lung disease
Explain how and why inspiratory pressure and tidal volumes are adjusted in an animal with lung disease?
- the inspiratory pressure needs to be higher due to decreased lung compliance, necessitating higher pressures for achieving same tidal volume for adeqaute ventilation
- the tidal volume needs to be lower since excessive distension is a major reason for ventilator-induced lung injury.
After connecting a patient to the ventilator, what are the first steps to assess safe ventilation?
- observe patient’s chest rise and whether it is appropriate
- auscultate both sides of the chest to make sure both sides of the lung are ventilated
- evaluate all monitoring available
- perform arterial blood gas once stable, or alternatively venous blood gas plus SpO2
What are the PaO2, SPO2 and PCO2 goals during ventilations?
- PaO2 80-1200 mm Hg
- SPO2 > 95%
- PCO2 35-55 mm Hg (35-40 in patients with brain disease)
How do you adjust the settings if a patient has a too high PaO2 (>120)?
- decrease FiO2 until < 60%
- once FiO2 < 60% start decreasing PEEP and peak airway pressure
How do you adjust the ventilator settings if the patient’s PaO2 is too low (< 80 mm Hg)?
- Increase the FiO2, increase to 100% if the PaO2/SPO2 drop is acute and/or severe.
- Increase PEEP and peak airway pressures to improve pulmonary oxygenation efficiency.
How do you adjust the ventilator settings in a patient with too high or too low PCO2?
- too high PCO2 → increase tidal volume and/or RR
- too low PCO2 → decrease tidal volume and/or RR
How does severe lung disease indirectly “protect” animals from ventilator-associated cardiovascular compromise?
animals with severe lung disease have such “stiff” lungs (decreased compliance) that usually only little of the positive pressure applied transmits to the cardiovascular system
What is the reported prevalance of VAP (ventilator associated pneumonia) in people?
10 - 48 %
What consitutes VAP (ventilator associated pneumonia)?
VAP is recognized when new pulmonary infiltrates develop ≥48 hours after beginning ventilation.
What are strategies that can be applied to reduce occurence of VAP?
- oral hygiene and frequent chlorhexidine mouth rinse
- using sterile technique when handling circuit, endotracheal, or tracheostomy tube
- reduce number of times circuit is being disconnected
- replace ventilator circuits only every 5 days
- personell to wear gloves and freuquently wash their hands
How frequently should the ventilator circuit be replaced?
Only every 5 days
earlier though if gross contanimation is observed
How can ventilator-induced lung injury (VILI) contribute to MODS?
pulmonary inflammation from VILI ⇒ inflammatory mediators ⇒ enter systemic circulation ⇒ contribute to systemic inflammatory response ⇒ organ damage
What are the 2 major mechanisms by which VILI occurs?
- Overextension of the lungs by excessive tidal volumes
- repetitive alveolar collapse from inadequate PEEP
When the blood gas goals cannot be achieved even with use of higher airway pressures or PEEP, what is a reasonable next step?
Change the patient’s blood gas goals, e.g., PaO2 > 60 mm Hg, PCO2 60-70 mm Hg
What are the general recommendations to avoid oxygen toxicity?
- FiO2 of 100% should not be adminsitered for longer than 12-24 hours
- For long term ventilation FiO2 should be < 60%
What is thought to be the major cause for ventilator induced penumothorax, and what can be applied to avoid this?
overdistention of the lungs, avoid high tidal volumes.
How would airway pressure and tidal volume each change respectively in a patient developing a pneumothorax under volume- or pressure-controlled ventilation
volume controlled ventilation ⇒ pressure would increase
pressure controlled ventilation ⇒ tidal volume would decrease
What are most common causes for an acute/rapid decline in oxygenation?
- pneumothorax
- machine malfunction
- disconnection of circuit
- loss of O2 supply
What are the most common causes for a gradual decline in oxygenation?
- progressing pneumonia
- ARDS
- VILI
List possible causes for hypercapnia in the ventilated patient:
- increased apparatus dead space (too much tubing)
- ET tube kink or obstruction
- incorrect circuit assembly (leaks, exhalation obstruction, etc.)
- pneumothorax
- increased pulmonary dead space (e.g., overdistension of alveoli or pulmonary embolism)
- inadequate tidal volumes or respiratory rate
- insufficient respiratory time
List possible causes for patient-ventilator-dsysynchrony
- Hypoxemia
- Hypercapnia
- Pneumothorax
- Hyperthermia
- inappropriate ventilator settings
- inadequate anesthesia depth
What are the physiologic goals a patient must achieve before disconnecting the ventilator?
- the original disease process is stable or improving
- the animal has a normal respiratory drive
- the animal does no longer require ventilator support for adequate gas exchange
- patient is oxygenating adequately (PaO2 to FiO2 ratio of at least 150 to 200)
What are common reasons to not wean an animal from the ventilator?
- requires > 60% FiO2 to oxygenate appropriately
- cardiovascular instability
- high peak inspired airway pressure (>25 cm H2O) needed
- high PEEP levels (>5 cm H2O) needed
During a spontaneous breathing trial, what are indications the trial has failed?
- hypoxemia
- hypercapnia
- hyperthermia
- tachycardia
- hypotension
- tachypnea
