Ventilators Flashcards
Respiratory rate
8-12 bpm
Vt
5-7mL/kg of IDEAL body weight
MV
- Vt x RR
Flow rate
5 x MV
I:E ratio
Normally 1:2
-increase expiratory time for patients with increased compliance (COPD)
Inspiratory Time
(Vt / Inspiratory flow) x 60
Expiratory Time
60 seconds / RR = time for one breath cycle.
Time for breath cycle - inspiratory time = expiratory time
What is pressure variable
- pressure is impedance to flow.
- this occurs in the breathing circuit or patients airway (trachea, bronchi, bronchioles, alveoli, ETT)
- back pressure from the impedance is from resistance and compliance from chest wall and lungs
- expresses in cmH2O or mmHg or kPa
What do we want the ETCO2 at?
30-33 mmHg
If your ETCO2 is too high, how do you adjust ventilator?
- increase MV by increasing Vt or RR
* * increase Vt first to recruit alveoli
Volume control ventilation
- delivers a set Vt to the patient
- anesthesia provider sets MV or Vt and RR
- time initiated
- volume limited
- loses 4-5cm H2O to circuit compliance
- patient paralyzed
- high PIP can limit inspiratory time and Vt delivery
What is the flow rate like in volume control
- the flow rate is constant
- too low of flows will result in partial Vt delivery
- too high of flows will results in inspiratory pause
What are two disadvantages of volume control?
- high PIP
- ventilator dyssynchrony
Pressure control ventilation
- delivers set pressure to patient
- anesthesia providers sets a max inspiratory pressure above PEEP
- ventilator increases the pressure to set level at start of inspiration time and maintains this pressure until exhalation begins
- patient paralyzed*
What is the flow rate like in pressure control?
-flow rate is highest at the beginning of inspiration and then decreases
What is tidal volume like in this mode?
- varies with resistance and compliance and vent dyssynchrony.
- If resistance increases Vt decreases
- if compliance decreases Vt decreases
- *important that a respirometer be used
Respirometer
- senses tidal volume in the expiratory limb of ventilator
- converts gas flow into electrical pulses
- exhaled Vt expected to measure: Vt set on vent + FGF - Vt lost in vent
- alarm will ring if apnea is present
What does increasing PEEP do in PCV
-increasing PEEP decreases Vt
When do we use PCV
- whenever we want to avoid high airway pressures
- often with LMA
- this reduces ventilator-induced lung injury
- obese patients, lung injury, single lung ventilation, ETT cuff leak
IMV (intermittent mandatory volume)
- ventilator delivers preset Vt at preset rate, permitting spontaneous unassisted breaths
- delivers through separate parallel circuit
- used for weaning
- not synchronized with patient
SIMV (synchronized intermittent mechanical ventilation)
- mix of mandatory breaths and assisted breaths;
- synchronizes mandatory breaths with beginning of spontaneous breath
- contains trigger window on monitor
- if patient does not trigger, a mechanical breath is delivered
- if patient triggers, patient determines Vt delivered
- mandatory Vt and RR is needed
- I:E ratio not required
- not every spontaneous breath is assisted
AC (Assist Control)
-mix of mandatory and assisted breaths
-once triggered every breath is treated the same: given a preset Vt or pressure delivery
Trigger = time, pressure, volume
Control = volume or pressure
Cycling = time
-guarantees MV and requires little respiratory effort
-can have breath stacking and does not protect against hyperventilation
PSV (pressure support ventilation)
- applies positive pressure to airway
- patient’s inspiratory effort generates negative pressure or flow in the inspiratory limb of the ventilator
- delivers small pressure support
- results in larger Vt than the patient would take on his/her own
- for maintenance or emergence of patients who are SV
- patient can become apneic
High Frequency (Jet) Ventilation
- very low Vt with very high rates
- usually seen when shocking kidney stones (ESWL)
- I:E usually 1:3
- 100-200 breaths per minute
- driving pressure 15-30psi
CPAP (continuous positive airway pressure)
- positive pressure maintained during inspiration and expiration
- can be done via mask
- used when trying to recruit alveoli
- often used just to open alveoli of top lung during surgery while dependent lung is fully ventilated
Changes to what four variables affect Vt?
- fresh gas flows
- bellows
- RR
- I:E ratio
Relationship: FGF and Vt
- direct
- increase FGF increase Vt, MV, and PIP
Relationship: RR and Vt
- indirect
- increase RR, decrease Vt
- decrease RR, increase Vt
Relationship: I:E ratio and Vt
- direct
- increase I:E ratio, increase Vt
- decrease I:E ratio, decrease Vt
Relationship: Bellows Height to Vt
- direct
- increase bellows height, increase Vt
- decrease bellows height, decrease Vt
In an anesthesia machine that couples FGF with Vt, what is the true Vt delivered to patient?
-Vt programmed + FGF on inspiration - circuit compliance
How to find FGF on inspiration
1)convert fresh gases from L/min to mL/min
(1L/min air + 3L/min O2 = 4000mL/min FGF)
2)use I:E ratio to find fraction of mL/min in inspiration
(I:E of 1:2 means 1/3 is inspired)
3)multiply inspired fraction of FGF and total FGF in mL/min
-1/3 * 4000mL/min = 1,333mL/min
4)divide inspired FGF by RR
-1,333 / 10 bpm = 133mL
5)add inspired FGF total to bellows total
-500mL + 133mL = 633mL
Resistance
The force that acts opposite to the relative motion of an object
- [P (airway) - P (alveolar)] / Gas Flow Rate
- ETT, trachea, bronchi, bronchioles, alveoli
Compliance
Delta volume / Delta pressure
- measures elasticity of lungs and chest wall
- influenced by: muscle tone, degree of lung inflation, alveolar surface tension, amount of interstitial lung water, and pulmonary fibrosis
Dynamic Compliance
- means “movement”
- the compliance of the lung/chest wall during air movement
- the pressure required to inflate the lung to a given volume is a function of airway resistance AND the tendency of the lung/chest to collapse
Static Compliance
- not moving
- measures lung compliance when there is no airflow
- the pressure required to keep the lung inflated is a function of the tendency of the lung/chest to collapse
- there is no airflow, so no resistance to overcome
O2 content formula
(Hbg x SaO2 x 1.34-1.39) + (PaO2 x 0.0031)
O2 delivery formula
(CO x O2 content)
Each time you increase FiO2 by ______ your SaO2 increases by _____
10%
50mmHg
Vent Alarms
- low pressure (disconnect alarm)
- sub atmospheric pressure alarm (neg pressure alarm)
- sustained/continuing pressure alarm
- high PIP alarm
- low oxygen supply alarm
- ventilator inability to deliver set Vt
Low pressure alarm
- detects a drop in circuit pressure
- check for disconnections
Sub atmospheric pressure alarm
- patient trying to breathe against vent
- switch to PSV, bag, or deepen sedation
Sustained/continuing pressure alarm
~ 15cm H20 for more than 20 seconds
- check for things sitting on circuit
High PIP alarm (set at 30cmH2o)
- occlusion, kinked tube, accumulating fluid in lungs, mucus plug
Low oxygen supply alarm
- low oxygen coming into the machine
Monitors
- ETCO2
- respirometer = Vt and PAP
- oxygen analyzer = calibrate at 21%
- VIGILENCE. = sweep check
ICU vs OR vent
- delivers higher Vt and PIP
- does not have CO2 rebreathing no absorber
- ## support more modes
