4. shear stress
Alveolar Interdependence
Structural integrity ↑
High resistance to VILI
Atelectrauma & Stress Raiser
Repeated Opening (Recruitment) & Closing (Derecruitment)
Surfactant deactivation
↓
Alveolar and alveolar ducts collapsed
↓
Loss of alveolar interdependency
↓
Lung instability with
repetitive alveolar collapsing and expansion
↓
Stress-riser : cause great stress to patent alveoli
(collapsed alveoli that are adjacent to open alveoli)
Transpulmonary pressure of
only 30 cmH2O could result in
a stress of 140 cmH2O being
exerted between two adjacent
alveoli as one expands and
the other unstable unit
remains collapsed
5. PEEP (Positive End Expiratory Pressure)
Continous positive pressure throughout all of ventilation
Used for all ventilation mode & all patient
Protect lung from atelectrauma, Recruit alveoli (FRC↑, PaO2↑)
PEEP range (generally) : 5 ~ 24 cmH2O
Minimum PEEP
- PEEP level needed to achieve PaO2≥60mmHg, SaO2≥90% with FiO2<0.6
- Standard practice : Start with minimum value of 5cmH2O
Optimal PEEP
- PEEP that maximizes oxygen delivery (DO2)
- DO2 = C.O × CaO2
MV Setting : PEEP
6. Benefit
Alveolar recruitment ↑
Increased functional residual capacity (FRC↑)
Prevention of alveolar collapse (end-expiratory)
Improve lung compliance
Improve shunt fraction
Improve gas exchange
PEEP의 cardiovascular Effect (Mean Airway pressure↑)
Normovolemic 환자 : low PEEP (5~10cmH2O) 적용시 C.O 감소되는 경우는 드물며 high PEEP ( > 15cmH2O) 적용시에는 CO 감소 가능
LV dysfunction 환자
- LV overload 환자에게 venous return 감소는 length-tension relationship을 호전시켜 stroke volume이 증가할 수 있음
- Intrathracic pressure의 상승은 transmural LV pressure를 감소시켜(주먹으로 심장을 쥐어짜는 효과) LV afterload를 감소시킴
- Hypoxemia로 인한 LV dysfx의 경우 PEEP 적용을 통한 myocardial oxygention의 상승과 performance 향상을 기대
정상 심기능을 가진 경우에는 심기능이 주로 전부하에 영향을 받음, 혈액량 감소시 RV 전부하 감소의 영향 → 저혈압
중증심부전의 경우 용적은 증가된 상태여서 주로 후부하에 영향 받음, LV 전부하 및 후부하 모두 감소 → LV fx 향상 기대
Risk
Venous return↓ & Cardiac output↓
Barotrauma risk↑
Regional hyperinflation and overdistension
MV Setting : PEEP
7. Optimal PEEP Setting : Method
1. ARDSnet PEEP Table 2 . Esophageal probe &
Transpulmonary pressure
3 . Lung Compliance &
Pressure-Volume curve
4 . Stress Index
8. Optimal PEEP Setting : Method
1. ARDSnet PEEP Table 2 . Esophageal probe &
Transpulmonary pressure
3 . Lung Compliance &
Pressure-Volume curve
4 . Stress Index
21. Lung compliance & Optimal PEEP
PEEP을 2 cmH2O 씩 증가시키면서 Compliance를 측정
하여 Compliance가 최대치가 되는 지점에서의 PEEP을
Setting한다
P-V curve의 LIP 지점을 활용하여 LIP + 2 cmH20 지점을 PEEP으로
setting한다
Lung recruitment maneuver를 실시할 경우, 호기시의 LDEF 지점을
활용하여 LDEF + 2 cmH2O 지점을 PEEP으로 Setting한다
23. For the optimal PEEP level, PEEP is increased in increaments of 2cmH2O (PEEP을 2cmH2O 씩 증가시키면서 compliance 등의 변화를 확인)
Blood pressure, Cardiac output, PaO2, Lung compliance, Plateau pressure, Mixed venous oxygen saturation are assessed
PEEP is increased increamentally until there is decline in O2 delivery (DO2) → PEEP is adjusted down to the previous level (“Best” PEEP)
Before determining PEEP using this approach, patient’s hemodynamic status must be stabilized
Lung compliance & Optimal PEEP
26. LIP (lower inflection point)
UIP (upper inflection point)
UIP (upper inflection point)
LIP (lower inflection point)
Optimal PEEP : Pressure – Volume curve
Traditional : Pressure at which alveolar overstretching occurred
Recent : could be end of recruitment ± lung over distension
(If recruitment continues above UIP, regional overdistension is marked)
Traditional : Pressure at which collapsed alveoli reopened
Recent : Start of recruitment of alveoli with similar opening pressures (influenced by chest wall)
LIP presence on P-V curve simply indicates homogeneously injured lung and need for recruitment
LIP absence on P-V curve indicates heterogeneously injured lung and PEEP induced recruitment & overdistension
27. Low flow required to reduce resistance effect
Slow-flow curve (< 6 L/min) may identify the lower inflection point for purposes of setting PEEP
Flow of less than 6 L/min is recommeneded for identifying LIP from a slow-flow pressure-volume curve
Start at low PEEP to identify LIP
In about 25% of patients, LIP cannot be identified from the pressure-volume curve
Optimal PEEP : Slow- flow Pressure – Volume curve
28. Slow-flow pressure-volume curve with use of a set rate of 5 breaths/min,
I:E ratio of 1.5:1, and VT of 500 ml LIP is approximately 8 cm H2O
The respiratory cycle time is 12 seconds (cycle time = 60/f = 60/5 = 12 seconds)
An I:E ratio of 1.5:1 results in an inspiratory time of 4.8 seconds
Inspiratory flow is VT/TI = 0.5 L/4.8 sec = 0.104 L/sec, or approximately 6 L/min
Make the slow-flow PV curve by manually
Ventilator have a automated tool for performing a "Low Flow" maneuver Ventilator does not have a automated tool
Optimal PEEP : Slow- flow Pressure – Volume curve
31. The energy applied to the lung in inspiration is not recovered
in expiration because alveolar recruitment is an energy-dissipating
process (Energy for alveolar recruitment d/t effect of surface
tension force)
Hysteresis
Pressure – Volume curve & Hysteresis
32. LIP (lower inflection point)
UIP (upper inflection point)
LDEF (lower deflection point)
Optimal PEEP : LIP + 2 cmH2O
Lung Recruitment → Opitmal PEEP : LDEF + 2~3 cmH2O
Optimal PEEP : Pressure – Volume curve
P-V curve contour (LIP presence) may
simply indicate the presence of homogenous
or heterogenous lung disease and the
potential for recruitment
LIP 는 inspiration시에 alveolar의 reopening과
연관된 pressure를 의미함
Alveolar의 closure는 expiration시에 이루어지며,
LIP를 호기말 alveolar closure의 방지 목적을 위한
optimal PEEP 적용에 활용하는 것은 제한점이 있다
P-V curve의 기울기 = Compliance (ΔV/ΔP) LEDF : probably indicates required PEEP to
maintain recruitment
33. Lung Recruitment Maneuver & PEEP
Lung recruitment maneuvers
1. Sustained inflation
: Prolonged high continuous positive airway pressure
30-40 cmH2O for 30-40s
2 . Pressure controlled ventilation
: Stepwise increased in PEEP
3 . Extensive Sighs
: CPAP 45 cmH2O, 45/16 and 1:2 for 120s
Other approaches
- Prone position
- Variable ventilation (APRV, HFOV)
Tobin MJ: Principes and Practice of mechanical ventilation
35. Optimal PEEP Setting : Method
1. ARDSnet PEEP Table 2 . Esophageal probe &
Transpulmonary pressure
3 . Lung Compliance &
Pressure-Volume curve
4 . Stress Index
36. Optimal PEEP : Stress Index
Volume control mode & Constant flow pattern에서 Stress Index를 적용함
Flow가 일정하면 기도 저항에 의해 발생하는 압력이 일정하기 때문에 (PAW = Flow × airway Resistance)
Pressure–time curve의 모양의 변화는 inspiration 동안의 lung의 compliance 변화를 의미하게 됨
In normal lung, the pressure–time curve will be linear, reflecting constant compliance
In normal lung, Stress Index will be 0.9 ~ 1.1
37. Optimal PEEP : Stress Index
Stress Index guides clinicians in avoiding lung stress and
protecting the lung
38. Baseline Ventilation
VT = 6 ml/kg PBW
PEEP = 5 cmH2O
Measure Stress Index
During low VT ventilation
Adjust PEEP to achieve 0.9
< Stress Index < 1.1
0.9 < Stress Index < 1.1
PEEP unchanged
1.1 < Stress Index
Decrease PEEP
Stress Index < 0.9
Increase PEEP
Optimal PEEP : Stress Index
39. Total PEEP = Extrinsic PEEP + Intrinsic PEEP
Total PEEP ↑ :
Venous Return↓, LV filling pressure↓, C.O ↓
Extrinsic PEEP (PEEPE)
- Pressure applied externally to proximal airway
- Present throuout respiratory cycle
Intrinsic PEEP
Lungs don’t fully empty at end-expiration
- d/t Airway resistance↑, Compliance↓, Minute ventilation↑, Expiratory time↓
Gas trapped at end expiration
- Additional pressure remaining in alveoli at end expiration
- Must have active end-expiratory flow
- Further add to difficulty in triggering
- Inability to trigger
- Raise the baseline pressure (Driving pressure↓)
Increase WOB to trigger a ventilator breath (if pressure-triggering is used)
- Tachypnea, Dys-synchrony
Poor ventilation & Oxygenation (d/t lower Driving pressure)
Higher airway pressure : PNX risk↑, Hemodynamic effect (BP↓)
Setting PEEPE in COPD : Set ~ 85% of PEEPI (Auto-PEEP) : WOB↓
(환자의 흡기 노력이 모두 trigger 될 수 있을 정도의 PEEPE을 세팅함)
Intrinsic PEEP (PEEPI) (esp. obstructive airway disease, COPD)
