Ventilation: Positive and Negative Pressure

Negative and Positive Pressure Ventilation

• Negative pressure ventilation involves alveolar pressure being less than atmospheric pressure
• Iron lungs and chest cuirass are examples of negative pressure ventilation
• Negative pressure ventilation does not usually require an artificial airway
• Positive pressure ventilation involves airway pressure being greater than alveolar pressure
• A pressure gradient determines volume in positive pressure ventilation

Mechanical Breath Terminology

• Mechanical breath variables include control, trigger, and cycle
• Breath sequence can be continuous mandatory, intermittent mandatory, or continuous spontaneous
• Types of control or target schemes are set point, servo, adaptive, and optimal

Mechanical Breath Variables

• The control variable is the mechanism to deliver a breath, such as pressure-controlled or volume-controlled ventilation
• The trigger variable is the mechanism to start inspiration, such as pressure or flow trigger by patient, or time trigger by ventilator
• The cycle variable is the mechanism to end inspiration, such as volume-cycled, pressure-cycled, flow-cycled, or time-cycled

Breath Sequence

• Continuous mandatory ventilation controls all breaths by the ventilator, with no spontaneous breaths allowed, e.g., CMV
• Intermittent mandatory ventilation provides a set number of mandatory breaths, with spontaneous breaths allowed between mandatory breaths, e.g., SIMV
• Continuous spontaneous ventilation involves all breaths being spontaneous with assistance, e.g., pressure support ventilation (PSV) or without assistance, e.g., continuous positive airway pressure or CPAP

Type of Control or Target Scheme (Ventilator Goals)

• Set point control involves the ventilator reaching a targeted goal, such as setting pressure for pressure-controlled ventilation
• Servo control involves the ventilator adjusting its output to suit the patient's variable, like proportional assist ventilation adjusting pressure to meet flow demand
• Adaptive control involves the ventilator adjusting a set point to reach a different targeted set point, such as pressure-regulated volume control adjusting pressure
• Optimal control involves the ventilator using a mathematical model to alter set points to achieve a target goal, like adaptive support ventilation altering frequency, tidal volume, and pressure

Simple vs Closed-Loop Systems

• A simple operating mode example is volume-controlled ventilation
• In a simple operating mode, setting a tidal volume delivers that set tidal volume at a constant flow
• A closed-loop system example is pressure support ventilation (PSV)
• Closed-loop systems achieve a sustained pressure plateau at variable flow that adjusts to patient needs
• With variable flow, closed-loop systems respond to changes in compliance and airflow resistance

Spontaneous Breathing

• Frequency and tidal volume are determined by the patient during spontaneous breathing

Positive End-Expiratory Pressure (PEEP)

• Positive End-Expiratory Pressure is end-expiratory pressure above 0 cm H2O
• Indications for PEEP include refractory hypoxemia due to intrapulmonary shunting, decreased FRC, and decreased compliance
• PEEP can be used to compensate for auto-PEEP
• PEEP reduces the threshold for alveolar opening and enhances gas diffusion and oxygenation
• Complications of PEEP include decreased venous return, barotrauma, increased intracranial pressure, and alterations of renal functions and water metabolism

Physiology of PEEP (PEEP Effects)

• PEEP decreases the pressure threshold for alveolar inflation
• It Increases FRC
• Improves ventilation
• Increase Ventilation/Perfusion ratio
• Improves oxygenation
• Decreases work of breathing

Continuous Positive Airway Pressure (CPAP)

• CPAP is similar to PEEP
• CPAP involves all breaths being spontaneous, with no mechanical breaths
• To use CPAP, patients must have adequate and sustainable spontaneous breathing, PaCO2

Bilevel Positive Airway Pressure (BiPAP)

• BiPAP provides independent positive airway pressure at both inspiration and expiration levels
• Indications for BiPAP include preventing intubation, chronic ventilatory failure, restrictive chest wall disease, neuromuscular disease, and nocturnal hypoventilation
• Initial BiPAP settings involve IPAP at 8 cm H2O
• Initial BiPAP settings EPAP at 4 cm H2O.
• Adjustments to BiPAP are made via adjustments to the pressure difference between IPAP and EPAP
• Increasing the IPAP level by 2 cm H2O increments can regulate ventilation
• Increasing the EPAP level by 2 cm H2O increments can regulate oxygenation

Controlled Mandatory Ventilation (CMV)

• Controlled mandatory ventilation is a control mode of continuous mandatory ventilation
• CMV utilizes a preset tidal volume
• CMV is time-triggered
• Patient trigger and spontaneous breathing are not possible in CMV
• Patient monitoring is extremely important with CMV
• Indications for CMV include tetanus, seizures, complete rest, crushed chest injury, and paradoxical chest wall movement
• Complications of CMV include issues related to prolonged sedation, apnea, hypoxia, and rapid disuse atrophy of respiratory muscles

Assist/Control (AC)

• Assist is patient-triggered, involving a negative pressure deflection below baseline before inspiration
• Control is time-triggered, with no negative deflection
• Indications for AC include full ventilatory support and minimizing atelectasis
• Advantages of AC include reduced work of breathing and patient's ability to increase minute ventilation
• Complications of AC include high respiratory frequency in respiratory center dysfunction
• Alveolar hyperventilation can also be a complication of AC

Intermittent Mandatory Ventilation (IMV)

• IMV involves a mandatory ventilator frequency with precise time interval between mechanical breaths
• Spontaneous breathing is allowed between mandatory breaths in IMV
• Air trapping or breath stacking can occur when mechanical breaths occur before complete exhalation of spontaneous breaths

Synchronized Intermittent Mandatory Ventilation (SIMV)

• SIMV involves a mandatory frequency with slightly variable time interval between mechanical breaths
• Spontaneous breathing is allowed between mandatory breaths in SIMV, like IMV
• Breath stacking is reduced or avoided because mechanical breaths are synchronized with spontaneous breaths
• Indications and advantages of SIMV include providing partial ventilatory support, maintaining respiratory muscle strength, avoiding muscle atrophy, reducing V/Q mismatch, and facilitating weaning
• A complication of SIMV includes muscle fatigue and weaning failure without pressure support

Mandatory Minute Ventilation (MMV)

• MMV is used to maintain a minimum minute ventilation, and to prevent hypoventilation and hypercapnia
• MMV involves a preset minute ventilation
• Ventilator frequency changes automatically to achieve preset minute ventilation
• Hamilton Veolar changes pressure support level instead of frequency
• The function of MMV may be averted by a rapid shallow breathing (RSB) pattern
• MMV can result in adequate minute ventilation
• MMV can also cause high VD/VT due to RSB

Mechanics of Mandatory Minute Ventilation (MMV)

• The ventilator adjusts the mandatory frequency the keep an appropriate minute volume
• The trigger mechanisms increases the ventilator frequency if the preset threshold has not been met
• All mandatory breasts are volume controlled while a patient controls their own spontaneous frequency and volume

Pressure Support Ventilation (PSV)

• PSV involves a preset pressure plateau (pressure-limited)
• PSV is only active during spontaneous breaths (patient-triggered), and not during mechanical breaths
• Tidal volume is determined by the patient
• PSV ends when inspiratory flow drops to a predetermined level (flow-cycled)
• Indications for PSV include low spontaneous tidal volume, high spontaneous frequency, and increased work of breathing
• Titration of PSV involves targeting a VT of 10 to 15 mL/kg
• Spontaneous frequency target is <25/min

Characterics of Pressure Support Ventilation (PSV)

• Pressure-supported breaths are considered spontaneous
• Pressure-supported breaths are patient-triggered
• Pressure-supported breaths are technically flow-cycled by a minimum spontaneous inspiratory flow threshold

Adaptive Support Ventilation (ASV)

• ASV is a dual control mode, with a variable mandatory frequency and pressure support level to match a patient's breathing pattern
• ASV requires patient's body weight and desired minute ventilation
• Estimated minute ventilation is 100 mL/min/kg (adults) or 200 mL/min/kg (children)
• Adjustments can range from 20% to 200% of predetermined setting
• ASV provides optimal frequency based on lowest total work of breathing

Proportional Assist Ventilation (PAV)

• PAV is the same as proportional pressure support
• PAV provides variable pressure to provide pressure support (PS)
• Pressure support is proportional to patient's pulmonary characteristics (elastance and airflow resistance)
• PAV accounts for Ventilatory demand (flow or volume)
• Flow assist is used to overcome airflow resistance
• Volume assist is used to overcome restrictive lung defects
• Overshoot of pressure support may occur during PAV with a sudden decrease in airflow resistance
• Overshoot of pressure support may occur duirng PAV with a sudden improvement of high lung elastance (low lung compliance)

Volume-Assured Pressure Support (VAPS)

• VAPS is used to pressure augmentation, volume-assisted pressure support
• VAPS incorporates PSV with conventional volume-assisted ventilation (VAV)
• VAPS involves a Preset tidal volume and pressure support (PS)
• PS is set to yield a VT less than preset VT
• VAPS provides stable tidal volume in patients with irregular breathing pattern
• VAPS may be time- or patient-triggered
• Once breath is triggered, PS is activated to deliver a pressure-limited breath
• Delivered volume = preset volume (pressure-limited breath)
• If delivered volume is less than preset volume, there is a switch from pressure-limited breath to volume-limited breath
• This results in longer I-time until preset volume is delivered
• It is important to monitor patient for air trapping due to I:E ratio change

Pressure-Regulated Volume Control (PRVC)

• PRVC is the same as adaptive pressure control, autoflow, adaptive pressure ventilation, volume control +, volume target volume control, and pressure-controlled volume guarantee
• PRVC provides volume-controlled breaths with lowest pressure possible by altering the inspiratory flow and I-time
• PIP is increased in response to increased airflow resistance or decreased compliance
• PRVC keeps lowest pressure possible by decreasing inspiratory flow
• Decreased flow = decreased resistance = decreased pressure
• Automode feature occurs with Siemens 300A
• Automode incorporates PRVC and volume support
• Time-cycles if no patient-triggering effort (12, 8, and 5 sec in adult, pediatric, and neonate modes)
• PRVC becomes active, preset volume, variable PIP up to pressure limit
• Volume support if 2 consecutive breaths are present
• Volume support becomes active, patient-triggered, pressure-limited, flow-cycled

PRVC

• Used when wanting to set volume, not as helpful with DKA
• PIP is increased in response to increased airflow resistance or decreased compliance
• PRVC reduces inspiratory flow and lowers pressures as much as possible
• Decreased flow increases inspiratory time

Adaptive Pressure Control (APC)

• APC is pressure-controlled ventilation that uses closed-loop control of the pressure setting to deliver a minimum volume
• Reduced patient effort triggers a higher inflation pressure
• Increased patient effort triggers a lower inflation pressure
• Closed-loop control cannot distinguish between improved compliance and increased patient effort
• Increased patient effort due to pain and anxiety may lead to an unwanted reduction in inflating pressure

Volume Ventilation Plus (VV+)

• VV+ features two dual mode volume-targeted breath types
• Volume control plus involves these qualities; Preset VT and I-time, test breath to determine relative compliance, variable pressures to maintain stable VT, variable flow to meet or maintain flow demand, and active exhalation valve for spontaneous breathing and venting excessive pressure
• It also features a volume support which can control VT and create increased patient compliance
• It is often used in post-anesthesia recovery
• It relies on a preset VT with a variable pressure support kevel to maintain target VT
• Less patient effort results in higher pressure support
• More patient effort results in lower pressure support
• Ventilator frequency and minute ventilation are determined by the triggering effort of the patient
• I-time is determined by the patient respiratory demand

Pressure-Controlled Ventilation (PCV)

• Indications for PCV include ARDS and excessive airway pressures during volume-controlled ventilation
• PCV provides preset pressure but variable volume due to changing compliance or airflow resistance
• Pressure plateau is created at the beginning of inspiration and maintained for a preset inspiratory time and frequency
• PCV may be patient-triggered for additional breaths

Airway Pressure Release Ventilation (APRV)

• Indications for APRV include ARDS and excessive airway pressures during volume-controlled ventilation
• APRV uses two CPAP or pressure levels
• High pressure (Phigh or PINSP) and low pressure (Plow or PEEP)
• The patient spends most of the time at Phigh
• The patient spends less than 1.5 sec at Plow
• Spontaneous breathing is allowed at both levels
• Inspiration involves pressure change from Plow to Phigh
• Expiration involves pressure change from Phigh to Plow
• In APRV, delivered volume is determined by the pressure gradient (Phigh and Plow)
• Pressure gradient must be monitored closely
• Delivered volume becomes excessive when compliance is increased or airflow resistance is decreased
• Delivered volume becomes inadequate when compliance is decreased or airflow resistance is increased
• Patient-ventilator dyssynchrony occurs due to spontaneous inspiration during pressure release
• Patient-ventilator dyssynchrony also occurs due to spontaneous expiration during pressure increase

Biphasic Positive Airway Pressure (Biphasic PAP)

• Biphasic PAP is the same as Bilevel, BIPAP, Bi-Vent, BiPhasic, PCV+, and DuoPAP
• Biphasic PAP is similar to APRV
• Biphasic PAP features two CPAP or pressure levels
• High pressure (Phigh or PINSP) where a patient spends less time
• Low pressure (Plow or PEEP) where a patient spends more time

Inverse Ratio Ventilation (IRV)

• Indications for IRV include intrapulmonary shunting, V/Q mismatch, and deadspace ventilation
• I:E ratio for IRV ranges from 2:1 to 4:1
• Adverse effects of IRV include increases in mean airway pressure and/or auto-PEEP, pulmonary edema, and complications of prolonged sedation
• Pressure control can be added to IRV to control mean airway pressure and auto-PEEP

Automatic Tube Compensation (ATC)

• ATC is the same as tubing compensation
• Additional pressure from the ventilator offsets or compensates for airflow resistance imposed by an artificial airway or an increased flow demand
• Pressure applied is active during inspiration and expiration

Neurally Adjusted Ventilatory Assist (NAVA)

• NAVA uses the patient's electrical activity of the diaphragm (Eadi or Edi) to guide optimal ventilator functions
• NAVA reduces incidence of disuse atrophy of diaphragm
• NAVA is available for adults, children, and neonates
• NAVA applications include treatment of spinal cord injury, head injury, COPD, and ventilator dependency

High-Frequency Oscillatory Ventilation (HFOV)

• HFOV utilizes extremely small volume and high frequency
• Primary settings used include Airway pressure amplitude (AP or power), frequency, mean airway pressure, percent inspiration, and inspiratory bias flow
• Ventilation is increased by decreasing frequency
• Ventilation can also be increased by Increasing amplitude, I-time, or bias flow (with leak)
• Control of oxygenation can be achieved by Increasing mean airway pressure