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
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