Mechanical ventilation

รองศาสตราจารยน์ายแพทยเ์ฉลิม ลิ่วศรสีกลุหน่วยวชิาโรคระบบการหายใจ เวชบำาบดัวกิฤต และ ภมูแิพ้

ภาควชิาอายุรศาสตร ์คณะแพทยศาสตร ์มหาวทิยาลัยเชยีงใหม่

Ventilator settings

Breath Type: mode

2. Mandatory– Breaths that the ventilator delivers to the

patient at a set frequency, volume, flow– Ventilator controls start and/or stop (the

machine triggers and/or cycles the breath)– VCV, PCV

1. Spontaneous– Patient controls start (patient-triggered)

and stop (patient-cycled) [Ti, RR] and VT

– PSV, CPAP

Mandatory ventilation

Ventilator settings

PCV or VCV?

PCV vs VCVPCV VCV

Control setting

PIP, Ti, rise time VT, PIF, flow pattern

VT, flow Variable Constant

PIP Constant Variable

Cycle Time Volume

Distribution of ventilation

More uniform in variable RC lung

Less uniform in variable RC lung

PIF = peak insp. flow, PIP = peak insp. pressure, RC = time constant, Ti = insp. time, VT = tidal volume

VCV or PCV?

PCV: benefit

PCV or VCV for ARDS?

Esteban A. Chest 2000; 117: 1690-6.

PCV

VCV

In-hospital mortality

RCT, compared PCV vs square-wave flow VCV, limit Paw < 35 cmH2O

No difference in gas exchange and lung compliance

Low tidal volume ventilation (LTVV): ARMA study

ARDS Network. NEJM 2000; 342: 1301-8.

6 vs 12 ml/kg VT In-hospital death

39.8%

↓ 31%

(p=0.007)

VCV

If compliance decreases or airway resistance increases, the pressure increases to maintain Vt

VCV: pressure variable

11 22 33 44 55 66

SECSEC

11 22 33 44 55 66

PPawawcmHcmH2200

6060

-20-20

120120

120120

SECSEC

InspInsp

ExpExp

FlowFlowL/minL/min

Useful in patients with relatively normal lung + need constant minute ventilation (PaCO2) eg. IICP, met acidosis

VCVParameters

set

Peak inspiratory flow (PIF)

• Flow should be set to meet a patient’s inspiratory demand

• At least 4 x VE of the patient• 40 – 80 L/min• Which pattern? Square or decelerating

Flow shapes: VCV

Flow patternSquare (rectangular): short Ti, ↑Te, high Paw

• Obtain measurements of lung compliance and airway resistance

Decelerating: better distribution of ventilation, ↑ Ti, ↓PIP, and ↑ mean Paw (↑ oxygenation)

→ better for both OAD and low compliance (beware of ↓ Te)

Airway Pressure

C = VT / P [ P = PPlat – PEEP ]

R = P / V [ P = Ppeak – Pplat ]

Paw = PEEP + VT/C + RV

.

.

Waveform showing high airways resistance

This is an abnormal pressure-time waveform

time

pres

sure

Ppeak

Pres

Pplat

Pres

Scenario # 1

The increase in the peak airway pressure is drivenentirely by an increase in the airways resistance

pressure. Note the normal plateau pressure.

e.g. ET tubeblockage,

bronchospasm, secretion

Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP

time

flow

‘Square wave’flow pattern

Normal

Waveform showing increased airways resistance

Ppeak

Pplat

Pres

‘Square wave’ flow

pattern

Waveform showing decreased lung compliance

This is an abnormal pressure-time waveform

time

pres

sure

Pres

Pplat

Pres

Scenario # 2

The increase in the peak airway pressure is drivenby the decrease in the lung compliance.

Increased airways resistance is often also a part of this scenario.

e.g. pulm edemaPneumonia, PTX

Normal

time

flow

‘Square wave’flow pattern

Paw(peak)

Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP

Waveform showing decreased lung compliance

Ppeak

Pplat

Pres

‘Square wave’ flow

pattern

Waveform showing normal lung compliance and airway resistance

A patient with sudden onset of dyspnea and desaturation. No change in compliance

& resistance was observed. Dx = ?

time

pres

sure

Pres

Pplat

Pres

Scenario # 3

Paw(peak) = Flow x Resistance + Volume x 1/ Compliance

time

flow

‘Square wave’flow pattern

Paw(peak)

Pulmonary embolism

PCVParameters

set

Initial MV settings for PCV• Pressure setting: initiate pressure at 10

– 15 cm H2O → adjust to achieve desired VT

• If start with VCV: – Set at Pplat during VCV: adjust to achieve

desired VT or – Use PIP during VCV minus 5 cm H2O (PIP

– 5) as a starting point → adjust to achieve desired VT

Pitfall of PCV

• PCV settings in a patient (PBW = 45 kg) with severe CAP (bilateral alveolar infiltration)

• Are they appropriate?

PCVParameters

set

Volume inadequate

in PCV

Short Normal Long

How to set Ti in PCV?

1 2 3 4 5 6

SEC

1 2 3 4 5 6

VT

600 cc

120

120

SEC

.V

LPM

0

450 cc

Setting appropriate Ti in PCV

May start at 0.8-1.2 sec.

500 cc450 cc

Lost VT

1 2 3 4 5 6

SEC

1 2 3 4 5 6

VT

600 cc

120

120

SEC

.V

LPM

0

Setting appropriate Ti in PCV

PSVInitial settings: • PS level• FiO2 • CPAP• Trigger (type,

sensitivity)• Inspiratory rise

time• Expiratory flow

sensitivity (Esens)

Cautions: • Can’t be used in heavily sedated,

paralyzed, or comatose patients • Respiratory muscle fatigue if pressure

is set too low

PSV

• Spontaneous breathing trial (SBT): 5-7 cmH2O

• Adjusted to keep RR < 25-30 /min

Pressure-oriented ventilation: PCV, PSV (variable flow)

• Tidal volume is depending on:– Set target pressure– Ti– Respiratory system compliance– Airway resistance– Rise time– Patient effort

Initial flow in PCV, PSV

• Flow pattern: decelerating exponential• Flow change on demand• ‘Inspiratory rise time’• More inspiratory flow demand: rapid rise

time

PCV: Flow pattern variable

PSV

PCV & PSV: rise time

• Rise time = 50%

• Adjusted by monitor ventilator waveforms

Conclusion

VCV PCV PSV

Control VT, flow Pressure Pressure

Variable Pressure VT, flow VT, flow

Cycled Volume Time Flow

How much VT?• Range of 6 – 12 ml/kg IBW• 10 – 12 ml/kg IBW (normal lung

function)• 8 ml/kg IBW (obstructive lung disease)• 6 ml/kg IBW (ARDS) – can be as low

as 4 ml/kg

VT chosen should maintain a PPlat <30 cm H2O

Minute ventilation

• = RR x VT (L/min)• Normal = 5-10 L/min• Respiratory rate: normal 12-18 /min• Adjust RR and VT to keep PaCO2 and pH in

acceptable range• PaCO2 = k VCO2 / RR (VT – VD)

• (PaCO2 x RR x VT)1 = (PaCO2 x RR x VT)2

.

Examples• GBS on VCV: VT 450 ml, RR 16/min,

PaCO2 = 50

• What is the target PaCO2 for this patient?

• Ans 40• What is the target VE for this patient?• Ans 9 lit/min (RR x VT = 9,000)

I : E ratio• Mean anything?• Adjust PIF, VT, Ti, RR to keep enough Te

PEEP

• Initial set at 3 – 5 cmH2O• Restores FRC and physiological PEEP

that existed prior to intubation• Adjust to correct hypoxemia in diffuse

intrapulmonary Rt-to-Lt shunt– Mild ARDS: 5-8 cmH2O– Moderate-severe ARDS: 10-13 cmH2O

• Help to trigger MV in AE-COPD

FiO2

• Oxygenation failure: 1.0• Hypercapnic failure: 0.4

MV in asthma

• VE < 10 L/min, VT 6-10 ml/kg, RR 10-14

• TE > 4 sec., PIF 60-80 L/min (VCV)

Oddo M. Intensive Care Med 2006; 32: 501-10.

• Monitoring of hyperinflation by using Pplateau (<30 cmH2O) instead of measuring end-expiratory pause (PEEPi)

• Applied PEEP +

MV in COPD

• ↓ VE, VT may be 5-7 ml/kg (PCV < 30 cmH2O), ↓ RR

• Permissive hypercapnia (may allow pH down to pH 7.0-7.2, PaCO2 up to 90)

• High PIF (> 60 L/min), long Te• Add PEEP (80-85% of PEEPi) if PEEPi

loads patient’s effort to trigger the MV Mehrishi S. Hosp Physic 2004: 30-6. Budweiser S. Int J COPD 2008; 3: 605-18.

MacIntyre N. Proc Am Thorac Soc 2008; 5: 530-5.

Applied PEEP for COPD

Improve triggering

PEEPi = 10, applied PEEP =5, trigger = -2

Expiratory flow in COPD

ARDS: Berlin definitionTiming < 1 wk of a known clinical insult or new or

worsening respiratory symptoms

CXR or CT Bilateral opacities (not effusions, lobar/lung collapse, or nodules)

Origin of edema Not fully explained by cardiac failure or fluid overload (need objective assessment if no risk factor present)

Oxygenation Mild PaO2/FiO2 201-300 with PEEP/ CPAP > 5 cmH2O

Mod PaO2/FiO2 101-200 with PEEP > 5 cmH2O

Severe PaO2/FiO2 < 100 with PEEP > 5 cmH2O

The ARDS Definition Task Force. JAMA 2012;307:2526-33.

How to set low VT in ARDS?: modified from ARMA trial

• Initial VT of 6 ml/kg PBW (any ventilator mode), limit Pplat < 30 cmH2O

• PBW (♂) = 2.3 x (Ht in inches - 60) + 50• PBW (♀) = 2.3 x (Ht in inches - 60) + 45.5• Allow permissive hypercapnia• RR up to 35 /min (target pH 7.3-7.45)• VT can be adjusted to < 6 ml/kg (as low as 4 ml/kg)

if Pplat > 30 cmH2O• VT can be adjusted upto 8 ml/kg if severe dyspnea

(keep Pplat < 30 cmH2O)ARDS Network. NEJM 2000; 342: 1301-8.

MV protocol for ARDSSeverity

Mild

LTVV with low PEEP (<10

cmH2O)

Moderate to severe

LTVV with high PEEP (10 -15 cmH2O)

↑ SpO2 or ↓ PaCO2 or ↑ Crs

Adjust by assessment of recruitment potential

MV in normal lung

• Neuromuscular disease, CVA, IICP, metabolic acidosis

• VT 10-12 ml/kg• PEEP 3-5 cmH2O• RR 14-16 /min• Adjust to keep normal gas exchange

Don’t forget to double dose of medications !!!

Aerosol Rx during MV

Oxygen therapy

O2 therapy devices and estimated FiO2

Devices O2 flow (L/min)

Estimated FiO2

Cannula 2-6 0.21 + (0.04 x flow)

Simple mask

6 0.357 0.408 0.459 0.50

10 0.55

O2 therapy devices and estimated FiO2

Devices O2 flow (L/min) Estimated FiO2

Partial rebreathing mask

6 0.35

7 0.40

8 0.45

9 0.50

10 0.60

Non-rebreathing mask

> 10 0.95 + 5%

O2 delivery systems• Acute setting with FiO2 < 0.4 : Cannula

• Acute setting with FiO2 0.4-0.6 : Mask + reservoir bag (rebreathing mask)

• Acute setting with FiO2 > 0.6 : Non-rebreathing mask, NPPV, MV with PEEP

• Acute setting with chronic hypercapnia eg COPD : Cannula, Venturi mask

Thank you for your attention