inflation of lungs pressure controller

what are the four functions of the ventilator

inspiration, inspiration to exhalation, exhalation, exhalation to inspiration

how can Pm be expressed using flow, resistance, volume, compliance, and PEEP

Pm = (flow × Raw) + (volume / Cs + PEEP)

what is the primary goal of the ventilator during inspiration

to maintain adequate minute ventilation for the patient

which three variables can the ventilator control according to the equation of motion

pressure, flow, and volume

how many variables can the ventilator control at a time

one; the remaining two become dependent variables

which patient-specific factors influence the equation of motion but are not controlled by the ventilator

Cs and Raw

can time serve as a control variable in ventilation

in some cases

what is the main goal of a pressure controller ventilator

to generate a set pressure that does not vary from the beginning to the end of inspiration

what is the term for the pressure generated by the ventilator

driving pressure (Pg)

how does the ventilator transmit pressure to the airway opening

the internal resistance of the ventilator is negligible, so Pg is transmitted undiminished to the airway opening (Pm)

how does Pm behave during inspiration in pressure control

Pm is constant throughout inspiration

what is the basic relationship between flow, pressure, and resistance

Flow = detaP / Raw

what is the equation of motion

Pm = (flow × Raw) + (volume / Cs + PEEP)

how can flow be expressed from the equation of motion

Flow = (Pm − (volume / Cs + PEEP)) / Raw

how does flow vary in pressure control ventilation

flow varies directly with the gradient between Pm and PA, and inversely with Raw

what happens to flow as the alveoli fill and PA rises

the gradient (Pm − PA) decreases, so flow gradually decreases

when does flow stop in pressure control ventilation

when PA = Pm, typically occurring after 3 time constants

what do time constants represent in ventilation

the amount of time needed for the lungs to inflate or deflate

how is a single time constant (TC) calculated

TC = compliance × resistance

how many time constants are required for the lungs to completely fill and empty

5 TC for complete equilibrium

why is 3TC often used in practice instead of 5TC

at 3TC, 95% of the volume is inhaled/exhaled, leaving only 5% deltaP, which is negligible

what percentage of volume is inhaled/exhaled at 1 TC

63% (37% left)

what percentage of volume is inhaled/exhaled at 2 TC

86% (14% left)

what percentage of volume is inhaled/exhaled at 3TC

95% (5% left)

what is the relationship between alveolar pressure and tidal volume

PA = Vt / Cs → Vt = Cs × PA

when does PA equal Pm in pressure control

after 3 time constants have occurred

how does Pm affect volume delivered

larger Pm means a greater volume delivered

what happens after 3 time constants in pressure control ventilation

deltaP = 0, so, PA = Pm, flow decays to zero, maximum volume at that Pm is delivered

what happens to flow in 3TC

flow decays to zero because deltaP = 0

can the ventilator determine patient-specific time constants

no

how does the ventilator end inspiration in pressure control

based on the set Ti

what must Ti be to allow flow to decay to zero

Ti must be greater than 3TC

what happens if Ti less than 3TC

inspiration ends before flow decays to zero; Pm is greater than PA, and maximum volume is not delivered

how does the RT ensure proper Ti

by analyzing the flow vs time curve to relate Ti to 3TC

how do you calculate 3 time constants (3TC) for a patient

3TC = 3 × (C × Raw)

how do you calculate peak inspiratory flow rate (PIFR) in pressure control

PIFR = (Pm − PA) / Raw

how does a change in compliance affect flow in pressure control

fow pattern decays faster if compliance decreases and slower if compliance increases (due to changes in time constants)

how does a change in compliance affect tidal volume

compliance and tidal volume have a direct relationship

how does a change in compliance affect PA

PA still rises toward Pm, but the rate at which it rises is determined by the time constant

how does compliance affect time constants

it has a direct relationship

what is the equation for flow in pressure control ventilation

Flow = deltaP / Raw

is the initial flow rate affected by changes in compliance

no

how does compliance affect TC and flow decay

decreased compliance leads to a shorter TC and faster flow decay; increased compliance leads to a longer TC and slower flow decay

how does decreased compliance affect tidal volume

tidal volume decreases because equilibrium between Pm and PA is reached faster and flow decays more quickly

how does increased compliance affect tidal volume

tidal volume increases because flow decays more slowly and more volume is delivered

how does a change in Raw affect Pm in pressure control

Pm is unchanged; the ventilator maintains the set driving pressure

how does Raw affect TC and flow decay

decreased resistance leads to a shorter TC and faster flow decay; increased resistance leads to a longer TC and slower flow decay

how does Raw affect PA reaching Pm

increased Raw leads to PA taking longer to reach Pm which decreases Raw leading to PA reaches Pm faster

how does Raw affect Vt

if Ti greater than 3 TC, Vt is unchanged; if Ti is less than 3 TC, Vt decreases