Anesthesia Ventilators Flashcards
What are some of the harmful and unnatural effects of positive pressure ventilation?
Cyclic recruitment and de-recruitment of collapsed alveoli; repetitive shear stress destroying cellular structures; inspiratory flow is directed to less resistant areas (path of least resistance) or areas that remain open resulting in overinflated alveoli
What effects does general anesthesia have on the lung?
Decreased functional residual capacity, decreased lung compliance, increased airway resistance leading to airway closure and atelectasis
4 causes of VILI?
Volutrauma, barotrauma, atelectrauma, biotrauma
What effect can PPV have on circulation?
Impeded preload/venous return leading to decreased cardiac output, distended lungs and cardiac septal shift, and organ ischemia due to decreased cardiac output (renal, liver, GI - decreased UOP/sodium excretion)
Do large tidal volume and high FiO2 improve gas exchange and prevent atelectasis?
NO
What are some principles of protective mechanical ventilation?
Large TV can cause acute lung injury; spontaneous ventilation preserves lung function (therefore, mild hypercarbia can be allowed [50-55]); PEEP helps keep alveoli open (needed every paralysis case)
Why are bellows ventilators preferred over piston ventilators?
Because the anesthetist can visualize the bellows moving during ventilation and can note the patient’s inspiratory effort during spontaneous ventilation by watching the bellows (a safety issue)
What is compliance?
A change in volume for a given change in pressure
At what stage of respiration is compliance highest and lowest in the lung?
Compliance is the highest at the beginning of inspiration and lower near the end of inspiration. Lung volume change happens more rapidly at the beginning of inspiration and slows toward the end.
What kind of ventilators use a decelerating flow pattern?
A variable flow ventilator
How much pressure is needed to achieve a flow rate of 1 L/sec in the lungs?
2 cm H20
How much pressure does it take to distend the lungs to 500 mL?
3 cm H2O
How is static lung compliance measured?
Plateau pressure
How is dynamic lung compliance measured?
Peak pressure
Ascending bellows remain descended during a ________.
disconnect
Weight of ascending bellows adds _ to_ cmH2O PEEP
2; 3
_____ ______ ____ can cause small amounts of tidal volume to slowly ____ with each cycle and the bellows will gradually descend until flat.
Small circuit leaks; leak
Why are descending bellows considered to be less safe than ascending bellows?
During disconnect, the bellows will fill due to gravity and disconnect may not be detected.
Initial ventilator settings (RR, TV, PiP, FiO2; I:E ratio)
8-12 breaths/min; 6-8 mL/kg ideal body weight; <35-40 cm H2O; 40-50%; 1:2
Without _ to _ of ____, TV of 6-8 ml/kg may be too low
4; 5; PEEP
PEEP is not recommended with severe lung disease because…
diseased lung has weak areas in the alveoli that could be damaged
What factors can lead to a difference in set and delivered tidal volume?
Circuit compliance (5 ml/cm H2O); gas sampling (2.5 ml/sec), gas compression (3%)
What is the normal I:E ratio for spontaneous breathing?
1:1.5
Why does expiration time usually have to be set to be longer than inspiratory time?
Because expiration is passive with ventilation
The higher the I:E ratio, the ________ the inspiratory time.
greater
The higher the I:E, the ________ the inspiratory pressures.
lower
What is inverse ratio ventilation used for?
(2:1) allow longer inspiratory times under lower pressures to reach and recruit collapsed alveoli.
What is the most common ventilation mode?
Volume controlled ventilation (CMV or VCV)
What is set and varies with VCV?
Set - TV, rate, flow
Varies - PiP
What is set/variable with pressure controlled ventilation?
Fixed - Inspiratory pressure target, rate, inspiration time (longer than VCV)
Varies - tidal volume varies depending on resistance, flow varies and decelerates near end inspiration
What is a benefit and a danger of pressure controlled ventilation (PCV)?
Since inspiratory time is longer, the volume delivered can reach and recruit collapsed airways.
A drop in resistance (such as after laparoscopy) can result in increased tidal volumes and volutrauma.
With what indications is PCV useful?
LMA, emphysema, neonates, children (high PiPs not appropriate)
Laparoscopy, pregnancy, morbid obesity, ARDS (low compliance present)
How to volume and pressure flow waveforms differ?
Volume control ventilation has a plateau at the top of the waveform due to constant flow inspiration; pressure control ventilation has a decelerating flow waveform which shows a decline in pressure toward the end of inspiration rather than a plateau.
How does volume guarantee increase the safety of pressure control ventilation?
Set volume prevents volutrauma with a sudden increase of compliance such as at the end of a laparoscopic surgery when insufflation is discontinued.
What is the purpose of pressure support ventilation?
To overcome the negative inspiratory pressure resistance of the ETT, circuit, and filter, allowing the patient to attempt to take their own breaths (at around 5-10 cmH2O); when set inspiratory trigger (flow) is detected, a breath is delivered.
Adjust pressure support based on desired ______ ______ and whether acceptable levels of _____ can be maintained.
tidal volume; ETCO2
A trigger can be based on _______, ________, ______ or ______ preset value.
pressure; volume; flow; time
Which trigger variable is independent of patient effort?
Time
Which 3 trigger variables are based on a change in present values and based on patient effort?
Volume, flow, or pressure
Which variable is set for pressure support ventilation?
Inspiratory pressure
Pressure support ventilation can be used alone or with which 2 modes of ventiation?
IMV and SIMV
What is SIMV?
Synchronized Intermittent Mandatory Ventilation; eliminates competition between patient effort and ventilator by allowing patient a certain time window in which to initiate a breath. If the patient does not, the ventilator will deliver a breath to prevent apnea. If the patient does trigger a breath, the ventilator can assist with pressure support.
How can you distinguish a patient-triggered breath on an SIMV pressure waveform?
The patient-triggered breath will show a small “triangle” of negative pressure before positive pressure is delivered
What is A/C ventilation?
Assist control ventilation will deliver a volume-controlled breath when it senses the patient’s negative-pressure trigger
