ventilator modes

Question 1

describe volume controlled ventilation (CMV or VCV)

Answer 1

• most common mode
• tidal volume is set and kept constant, delivered each breath
• rate can change minute ventilation
• gas delivered at a constant flow
• PiPs vary depending on number of factors (can result in barotrauma if get too high)

Question 2

what are some indications of VCV?

Answer 2

-best for patients with no respiratory effort and little expected change in airway resistance and intra thoracic pressure

Question 3

describe pressure controlled ventilation (PCV).

Answer 3

• tidal volume delivered is based on preset pressure target being reached
• mandatory rate and inspiration time are set
• inspiratory times are longer as the tidal volume delivered may take much of the ventilatory cycle
• tidal volume can vary breath to breath depending on airway resistance and other factors
• flow varies with each breath delivered
• *tidal volume set based on desired pressure not volume so that peak airway pressure is controlled
• expiration occurs when inspiratory time and airway pressure reached

Question 4

what results from PCV?

Answer 4

• tidal volumes are NOT guaranteed, so increased tidal volumes (volutrauma) can result if a drop in PiP occurs (laparoscopy)
• since inspiratory time is longer, delivered volume can reach and recruit collapsed airways

Question 5

what are some indications for PCV?

Answer 5

• useful when high PiPs are not appropriate
• LMAs, emphysema, neonates and children
• useful when low compliance is present (chest wont expand)
• laparoscopy, pregnancy, morbid obesity, ARDS, stiff chest, pulmonary fibrosis

Question 6

when using PCV, and compliance changes from low to high, how will tidal volume change?

Answer 6

Vt will increase with compliance

Question 7

what is volume guarantee (VG) with PCV?

Answer 7

VG allows to set Vt with PCV

Question 8

describe pressure support ventilation (PSV)

Answer 8

• when flow decreases expiration begins
• only inspiratory pressure is set
• respiratory rate is determined by the patient
• used with IMV and SIMV modes of ventilation
• pressure support approx. 5-10 cmH2O will overcome the negative inspiratory pressure resistance of ETT, circuit and filters
• adjust pressure support based on desired Vt and whether acceptable levels of ETCO2 can be maintained

Question 9

what is sensitivity trigger in PSV?

Answer 9

when pts. efforts reaches set inspiratory flow trigger (sensitivity based on L/min) the pressure support breath is initiated and delivered throughout the inspiration

• based on how much flow pulled across flow sensors; higher flow required means lower sensitivity
• *higher the sensitivity may detect any movement as breathing even physician stimulus, may need to lower sensitivity

Question 10

what are some indications of PSV?

Answer 10

• designed to augment Vt in spontaneously breathing patients (increase ventilation)
• to decrease work of breathing and increase patient comfort (negative resistance of ETT, LMA, filters and circuit)
• weak inspiratory effort (deep level of anesthetic; residual muscle paralysis)
• helps to “breathe pt. back” to wake up
• obstructed airway breathing

Question 11

describe synchronized intermittent mandatory ventilation (SIMV).

Answer 11

• synchronizes the patients efforts with ventilator
• senses negative pressure created by diaphragm then delivers full breath
• if pt. does not inspire within trigger window waiting time, vent delivers breath
• ensures sufficient mandatory breaths if pt. breathing efforts fall outside time frame of breath
• pt. breath does not compete with vent

Question 12

what are the advantages of SIMV?

Answer 12

• functions as a “backup” vent when pts. own respiratory rate decreases
• allows spontaneous patients breaths
• ideal when spontaneous breathing pt. is too deep to maintain normocarbia
• more pt. control over their own ventilation