inflation of lungs flow controller

what must a ventilator maintain during inspiration

a constant pattern of flow, regardless of changes in patient compliance or resistance

what is the basic equation relating mouth pressure and flow

Pm = (flow × Raw) + PA

how can flow be expressed from this equation

Flow = (Pm − PA) / Raw

why does the Pm − PA gradient diminish across inspiration

due to time constants (compliance × resistance), inspiratory flow decays exponentially

at 3 time constants (3TC), what happens to Pm and PA

Pm = PA → deltaP = 0 → flow = 0

how does the ventilator control flow despite time constants

by using high internal ventilator resistance (Rvent)

what is the result of high Rvent

Pg ≠ Pm and time constants are extremely long (flow is controlled by ventilator)

how is 3 time constants calculated with Rvent

3TC = 3 × (compliance × (Rvent + Raw))

during ventilator-controlled flow, which pressure drives flow

driving pressure

what is the equation for flow under ventilator control

Flow = (Pg − PA) / (Rvent + Raw)

how is a time constant (TC) calculated in a ventilated patient

TC = compliance × (Rvent + Raw)

does flow decay according to lung time constants even with a flow controller

yes

why does flow not significantly decay during inspiration with a flow controller

Ti is much shorter than even one time constant, so flow doesn’t begin to decay

how does the ventilator maintain the set flow pattern

by continuously monitoring flow and adjusting internal resistance (Rvent) if the flow deviates

does the equation of motion change with a flow controller

no, it remains the same

what must the operator set when using a flow controller

a flow rate

what flow patterns can a flow controller maintain

square flow or decreasing flow

how does the ventilator respond to changes in lung characteristics

adjusts Rvent to maintain the set pattern; flow rate is unaffected

is the flow pattern maintained breath-to-breath

yes

what is the relationship between flow, volume, and time

Flow = volume / time

if flow is constant, what else is guaranteed

Vt and Ti are constant

what is the advantage of a flow controller over a pressure controller

Vt is guaranteed because it is a function of the set flow

how does the ventilator control flow in flow control mode

the ventilator controls the flow pattern, which is unaffected by changes in Cs or Raw

what happens to volume and inspiratory time if flow is controlled

Vt and Ti are constant and unaffected by lung characteristics

what is guaranteed in flow control mode

Vt

do changes in Cs or Raw affect tidal volume delivery in flow control

no

what happens to Pm in flow control mode

Pm increases across inspiration because the ventilator does not control pressure

what are Pmi and Pm

Pmi = initial mouth pressure; Pm = final mouth pressure

how is mouth pressure measured and displayed

the ventilator measures Pm throughout inspiration but only provides the highest value (Ppeak/PIP)

what is the equation for Pm

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

what is Pmi at the start of inspiration

Pmi = (flow × Raw) + PEEP (since alveolar volume = 0)

how does Pm change as volume is delivered

increases until the set volume is delivered

what flow patterns are available in flow control mode

square flow and decreasing flow

how does the ventilator deliver flow in a square flow pattern

maintains a constant flowrate from the beginning to the end of inspiration

how does Pm change in a square flow pattern

increases linearly from the start to the end of inspiration

which pressure is displayed as Ppeak/PIP

the Pm at the end of inspiration

how does alveolar volume and pressure change in a square flow pattern

volume and alveolar pressure increase linearly

how is Ti calculated in a square flow pattern

Ti = Vt / flowrate

how does the ventilator deliver flow in a decreasing flow pattern

PIFR is set, then flow decreases linearly across inspiration

is decreasing flow affected by patient compliance or resistance

no, it remains a constant pattern

what is the final flowrate in a decreasing flow pattern

either a percentage of PIFR or 5 L/min, depending on the ventilator

how do Pm and PA behave in a decreasing flow pattern

they do not increase linearly and do not have a parallel relationship

how is Ti determined in a decreasing flow pattern

using tidal volume and average flowrate

how is average flowrate calculated

average flow = (peak flow + final flow) / 2

what does the flow vs time waveform look like in decreasing flow

a straight descending line

what does the volume vs time waveform look like in decreasing flow

a slightly curved line, not straight

what does the pressure vs time waveform look like in decreasing flow

not straight lines; Pm and PA curves approach each other across inspiration

when can Ppeak/PIP occur in a decreasing flow pattern

either at the beginning of inspiration or at the end of inspiration