Dr Abid Ali Rizvi

Lost in a Maze

User manual is superficial, with poor translation from German.

Nothing useful is available online about this machine.

Experts outside Germany are not very familiar.

Lack of interest in the end users.

HFO: What’s different in Sophie ?

Amplitude and Posc

Vo and MVo of HFO

HF I:E Ratio

Optimum Frequency

Pmax and Insp

Sigh breaths

Flow limit

The HFO seal

Amplitude, DP, & Posc: Notice the units in which the parameters are displayed.

Amplitude (DP) cmH2O

FYI: 1mbar = 1.012 cmH2O

Pressure of Oscillation (Posc) in cmH2O

• Amplitude (%): Sophie expresses the depth of

oscillations as a percentage swing (peak to

trough) around MAP.

• Its range is : 0-100% .

• Posc is the actual pressure gradient (P in

cmH2O), generated by the piston to create the

desired percentage swings around the MAP.

Diagram

Peak PRESSURE

Trough Pressure

MAP (Paw)

In the previous slide

• MAP is 13 cmH2O, Amplitude is 12%, & Posc is 21 cmH2O

• Requested Swing around MAP = 3.12 cmH2O

[12% of 13 = 1.56] + [12% of 13 = 1.56] = 3.12 cmH2O

• But actual oscillation generated are: Peak = 24 cmH2O & Trough = 3 cmH2O

• P of [24 – 3] = 21 is required in this case to actually

oscillate the MAP of 13 by 3.12 cmH2O in each cycle.

Above MAP Below

MAP

Factors influencing the Posc [DP] magnitude (for the

same set Amplitude): Impedance [Z]

Impedance is the total opposition that a circuit

(tubing, ET and airways) has against a passing

oscillatory current (of air flow).

Resistance [R]

+ Elastance [E]

+ Inertance [I]

A high impedance circuit will result in high

Posc [DP], but low gas flow & tidal volume.

[Z]

Factors influencing the Posc [DP] magnitude (for the

same set Amplitude): Compliance [C]

Stiffer the lungs, the DP generated will be high;

Transmission of the DP to alveoli is when [C] is

[C] of chest wall or abdomen also contributes in DP size.

Level of water in the humidifier bottle inversely alters the compliance the ventilator tubing (circuit).

Over-distended lungs have poor compliance.

Factors influencing the Posc [DP] magnitude (for the

same set Amplitude %): Frequency [F]

Impedance of the ET & airways es at higher [F].

DP size therefore increases at higher frequencies.

Transmission of DP from ET to alveoli is also ed

with increasing frequencies.

This damping of DP (with increasing Frequency)

across the ET occurs mainly with I:E ratio of 1:2

rather than 1:1.

Factors influencing the Posc [DP] magnitude

(for the same set Amplitude): MAP [Paw]

When MAP is reduced, DP will also decrease

to keep the percentage swing (Amplitude)

same.

Over-distended lungs (due to high Paw) have

lower compliance (even if healthy), therefore,

they will generate high DP

When Posc (P) is found too large

especially on a low MAP.

• risk of de-recruitment &/or gas trapping

during expiratory phase especially if the trough

pressure is reaching sub-zero values.

• Weaker parts of distal most bronchioles will

collapse and create choke points, resulting in

incomplete alveolar emptying.

• This effect is exaggerated when airway resistance

is high in combination with surfactant deficiency

(e.g. Meconium Aspiration).

P Trough reaching sub-zero values

Setting the alarm limits for Posc:

• 30% above and below the current value is recommended so that we can get a clue if ET resistance or lung compliance has changed significantly.

Vo: [Tidal Volume of Oscillation] in mL

• By dialing an amplitude, the machine

generates Posc (DP) which in turn yields a tidal

volume for each oscillatory cycle.

• Vo is the basis of bulk convection which is

the major component of the 6 gas exchange

mechanisms in HFO ventilation.

Convection

Convection & Diffusion

Diffusion

Direct ventilation of close by alveoli

Pendelluft

Diffusion

Turbulence

Radial Mixing

Collateral ventilation

Asymmetric velocity profiles

in inspiration and expiration

Cardiogenic oscillations

Insp

iratory flo

w

Exp

irat

ory

flo

w

Mixing by asymmetric velocity

profiles is also convection

(flow) dependent.

Vo: [Tidal Volume of Oscillation] in mL

• Recommended Vo for effective HFO ventilation is 2-2.5 ml/kg, which is roughly the anatomical dead space volume in Term Neonates.

• If Vo is < 50-75% of the anatomical dead space, there is generally a deterioration of gas transport efficiency, even if the HFO frequency is proportionately increased to maintain the minute volume.

1) Patient is 1.5 kg, so Tidal Volume

per cycle is 7÷1.5 = 4.7 mL/kg.

2) This baby is not getting any benefit

of HFOV, in fact he is on super-IMV.

3) This is the advantage of keeping an

eye on Vos.

Tidal Volume and Lung Injury go Hand in Glove.

ET size decides the way Frequency changes the Vo (tidal vol. in mL)

Frequency

Vo

(tidal vo

l. in m

L)

STIFF LUNG

NORMAL LUNG

OVER COMPLIANT LUNG

ET 2

.5

COMPLIANCE NORMAL

3.5 ET

3.0 ET

2.5 ET

MAP 10, DP 50, Insp 33%

Minute volume of Oscillation (MVo) in L/min

• It is the total amount of air exhaled back into the HFO in one minute.

• Measured directly using the flow sensor at the ET end of the circuit.

• [Vosc] x [Frequency]

• This parameter is used to calculate:

– DCO2 (“Gas Exchange Coefficient”).

– Pressure cost of ventilation.

What is DCO2 in HFO ?

• A mathematical model which predicts the chances of normal pCO2

• DCO2 per Kg of 60-80 pCO2 < 6.7 in ~80% cases.

• DCO2 = [Tidal Vol]2 X Frequency

= [Minute Vol]2 Frequency

Minute volume of Oscillation (MVo)

Sophie

MV

o in

L/min

Frequency

MVo in HFOV is independent of frequency in Sophie, Sensormedics 3100A and SLE 5000

3100A SLE 5000

FREQUENCY IN HERTZ 6 7 8 9 10 11 12 13 14 15

3.0

2

.5

2.0

1

.5 1

.0

0.5

Babylog8000

Infrasonic

ET 2.5, MAP 15, DP 30, F 6-15 Hz

It pays to pay attention on I:E Ratio

1. Machine’s default setting is at 40% which may increase the risk of barotrauma, cardiac compression and gas trapping [Traditional

teaching]

Inspiratory share (HF I-E ratio) should be

kept at 33% (1:2).

HF I:E %: Set Versus Delivered

This is NOT the HFO I:E Ratio

HFO I:E Ratio [ %], keep it 33%

Inspiratory time: 33% [1:2 ratio]

• It is an extrapolation of the I:E Ratio of 1:2 of the

conventional ventilation.

• With I:E Ratio of 1:2, delivered intra-alveolar

pressure is significantly lower than the set MAP.

• Protection from barotrauma, but at the expense of

optimum alveolar recruitment.

• There is no difference in the incidence of gas

trapping with either 1:2 or 1:1 Ratio.

Inspiratory time: 50%[I:E Ratio 1:1]

• Delivers MAP to the alveoli without damping.

• That means intra-alveolar pressure is actually

same as the displayed MAP.

• Good for opening the lungs but barotrauma risk.

• Although Vo (tidal volume) delivered is higher at

I:E Ratio of 1:1, gas trapping does not occur.

• Resulting higher MAP in distal lung units splints

them open more efficiently.

• This lowers the airway resistance in expiration

phase, allowing complete emptying of the alveoli.

To Avoid Air Leaks

• Be very careful if using high MAP &/or huge DP in neonates, when HFO is operating at/close to I:E Ratio of 1:1

• This combination is seen in:

– SLE 5000 ALWAYS.

– Sophie OFTEN.

– SensorMedics NEVER.

Pa = Mean alveolar pressure

Pao = Mean pressure at airway opening (ET)

Effect of I/E ratio on mean alveolar pressure during high-frequency oscillatory ventilation. J. J. Pillow Journal of Applied Physiology July 1999. An in vitro assessment of gas trapping during high frequency oscillation. Jaana A Leipälä, Anne Greenough et al 2005 Physiol. Meas. 26 329 Matching Ventilatory Support Strategies to Respiratory Pathophysiology. Anne Greenough, Steven M. Donn. Clin Perinatol 34 (2007) Elsevier

Dräger, SLE 5000 and Sophie support I:E Ratio of 1:1 (Insp Time: 50%)

INSPIRATORY TIME %

Pd

iff = (Pa-P

ao)

Analyze

This

Higher frequencies

(13-15Hz)

Short Insp. Time 33%

(I:E ratio 1:2)

Large Amplitude Demands

Adequate time may not be available for the piston to

complete the forward stroke displacement.

The piston then will not be completing its full

deflection in the inspiration for generating the required

peak pressure.

How do these machines tackle the problem of insufficient forward stroke time

Sensormedics 3100A:

• Insp. time % is never compromised by the machine.

• Machine may lower the MAP &/or the P delivered.

Stephan Sophie:

• Will not compromise on MAP and Amplitude.

• Machine increases the insp time % automatically to deliver the P.

SLE 5000:

• Has a fixed I:E Ratio of 1:1,[it has no dial for I:E Ratio]

What happened here?

Blue is square pressure wave of Sensormedics 3100A Red is the quasi-sine wave in Sophie Green is a pure sine wave, for comparison.

Active expiration has still not started.

Forward flow of gas can therefore occur in this phase.

Fidelity of I:E ratio, set vs. measured

• Insp time% we dial in Sophie, is the setting up the

electronic timer’s signal duration for the forward

piston displacement.

• It does not reflect the resultant duration of the

forward air flow, which is measured in real-time by

the pneumotachograph.

• If the impedance of the airways/ET is low, the actual

forward air flow may not cease as soon as the piston

stops the inspiratory stroke.

Pmax and Insp

• Press Insp Hold Button for inline bagging (without disconnection)

• Breath will be given at the set Pmax, and for the duration of the Insp ( its Ti in sec).

• Its like a manual conventional breath for boosting alveolar recruitment.

• To be effective, the Ti should be around 1 sec and Pmax should be close to the Posc peak.

Pmax and Insp

Adding regular sigh breaths to HFO

• Set HFO from the IMV menu instead of CPAP menu.

• Useful in lungs with major atelectasis.

• Contraindicated in presence of established /impending barotrauma.

• This mode is used for few hrs, once the atelectasis is resolved, sigh breaths do not offer any additional advantage.

• MAP is displayed as PEEP when adding sigh breaths in HFO.

• Rate of sigh breaths is generally 1-5 per min; and Ti is kept close to 1 sec.

photo

HFO Seal

• Nurses plug it when HFO is started.

• It seals (disables) a mechanical safety valve through which baby can breath room air, if the ventilator fails completely.

• This valve will also open (and disturb the HFOV of baby) when ever a negative pressure of minus 6 is generated in ET by the spontaneous breaths or by deep troughs in delta P (high amplitude setting).

Flow limit

• Recommended setting is at 2 liters.

• If the difference between inspiratory and

expiratory flow rate is more than 2L/min, then it

machine will consider it as ET disconnection

or dislodgement and give an alarm.

• If the ET is very leaky, than set this parameter at

a higher limit, to prevent false alarm.

Importance of heat & humidification in HFO

• Since the minute volume in oscillator is up to

10 times the conventional ventilation, the heat

and humidity loss from the upper airway can

be massive if not taken care of.

• Gas temp should be 36.5-37.5O C.

• Humidity of the inhaled gases:

Ideal 100%

Minimum 70%