Ventilation (ARDS/ventilators)
Ravindra Prasad, M.D., March 29, 1996
ARDS - diffuse (homogenous) lung injury vs. heterogeneous, with areas of relatively unaffected lung
CT: marked heterogeneity in first 10 days, areas of nl V/Q ratio using inert gas studies, compliance on lung corrected for aerated lung volume comparable to normal
“the functioning lung in ARDS is not so much stiff as it is small”
ventilation with “normal” volumes => inc. pressure
Barotrauma
Increased by:
• peak airway pressures > 40-50 cm H20 (although injury can be prevented if excessive inflation is limited; rats with chests strapped were OK ... “volutrauma”)
•peak alveolar pressures > 35-40 cm H20
•high mean pressure
•duration of ventilation
•etc.
Consequences:
Extra-alveolar gas: pneumothorax, subQ emphysema, subpleural air cysts, pneumomediastinum, pneumoperitoneum
Systemic gas embolism (if infiltrates block decompression of gas in bronchovascular sheath, and if vascular structures disrupted by necrosis)
Diffuse lung injury: indistinguishable from ARDS
Mechanical Ventilation
Volume-controlled (machine triggered) and
volume-assisted (patient triggered)
• flow limited (predetermined magnitude and pattern, eg sine, square, decelerating)
• pressure is the dependent variable
• used if peak airway pressures are of minor concern
• ventilation guarantee
• volume-controlled ventilation (VCV), volume-assist control ventilation (VACV), IMV, SIMV
Pressure-limited
• flow and volume are the dependent variables
• ventilator creates square wave of pressure
• flow pattern tends to be decelerating
2 types of breaths:
Pressure-controlled (machine triggered) and
pressure-assisted (patient triggered)
• off signal defined by set inspiratory time
• complete control of peak pressure
• good in cases of leakage (eg uncuffed tube, bronchopleural fistulas)
• pressure-control ventilation (PCV), pressure-limited
SIMV, pressure-limited IMV
Pressure-support
• always patient triggered
• off signal is a decrease in insp flow to a set level below initial insp flow
• pressure support (PS), SIMV/IMV + PS
• decelerating inspiratory flow
• pt determines frequency and insp time
• major concern is peak pressure, not ventilation guarantee
• can be used to wean
New modes
Pressure
regulated volume control (PRVC)
• decelerating flow pattern, constant pressure
• ventilation guarantee with lowest possible pressures
• can manipulate I:E easily
• pressure support changes with each breath to achieve set goal TV
Volume
support
• patients with limited breathing capacity, with intact respiratory drive
• advantages of pressure support with a ventilation guarantee (machine gives support if and only if pt unable to reach goal TV)
• support during ventilation, but only as much as pt requires for TV
• can be used to wean
• pt triggers each breath
• pressure is constant, flow is decelerating
• need back-up (PRVC) in case of apnea
SIMV (VC)
+ PS SIMV
(PC) + PS
Inverse
Ratio Ventilation
• 2 types:
Pressure-controlled or pressure-limited with decelerating insp flow
volume-cycled with either a square or decelerating wave pattern
• PC-IRV more common
• can increase mean alveolar pressure while keeping PIP <35 cm H2O => airways stented open more effectively than transient application of same peak pressure
• recruits more alveolar units than PEEP or conventional modes of ventilation at any given mean airway pressure
• can result in improved oxygenation at lower minute volume, peak airway pressure, and PEEP requirements.
Proportional
Assist Ventilation
• not commercially available
• ventilator simply amplifies pt effort (positive feedback) - you set factor of amplification
• pt has control of breathing pattern throughout (triggering and onward) => inc. comfort
• dec. likelihood of overventilation, because pt’s resp control system will downregulate resp if hypocapnia occurs
• requires active central drive
• positive feedback: potential for “run away”
Tracheal
gas insufflation
• decreases VD/VT
• under development
Airway
Pressure Release Ventilation
• CPAP with periodic (sudden) release of
valve => gas leaves lungs, ventilating
• maintains FRC
• recruits alveoli using lower peak and
mean airway pressures than used with conventional ventilation
• cardiac fnct theoretically impaired to a
lesser degree because pressures are lower than conventional modes
• pt can breathe spontaneously
• improves VD/VT
• speculative
High-frequency
ventilation
ARDS
• Goal for O2 sat ~90%
• Use traditional modes (volume-control)
• Add PEEP
• If PIP 40 cm H20, consider pressure-control, PRVC
• Adjust I:E ratio, consider inverse ratio ventilation
• Consider permissive hypercapnia vs. extracorporeal CO2 removal
• (may use support modes in addition if pt has respiratory drive)
References
Apostolakos, MJ et al. “New Modes of Mechanical Ventilation.” Clinical Pulmonary Medicine, Vol. 2, No. 2, March 1995, pp. 121-128.
MacIntyre, NR. “Clinically Available New Strategies for Mechanical Ventilatory Support.” Chest, Vol. 104, No. 2, August 1993, pp. 560-565.
Marcy, TW. “Barotrauma: Detection, Recognition, and Management.” Chest 1993, Vol. 104, No. 2, August 1993, pp. 578-584.
Siemens-Elema AB. System SV 300. SV 300 Ventilatory modes: How do they work - How are they used? Sweden, September 1994.
Tharratt, RS et al. “Pressure Controlled Inverse Ratio Ventilation in Severe Adult Respiratory Failure.” Chest, Vol. 4, No. 4, October 1988, pp. 755-762.