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Mechanical Ventilation


Chronic airway obstruction - emphysema - chronic asthma - chronic bronchitis ... asthma - chronic bronchitis - bronchiolitis - atelectasis - thromboemboli ... – PowerPoint PPT presentation

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Title: Mechanical Ventilation

Mechanical Ventilation
  • Defined as the support of a patients ventilatory
    needs by artificial means
  • Is an interim life support measure that gives the
    physician an opportunity to medically correct or
    stabilize a patients cardiopulmonary problem
  • Does not cure the patient, even though it may
    prolong life

Medical Indication for M. V.
  • Is generally indicated to prevent patient from
    going into respiratory failure or to provide life
    support and stabilize those who are already in
    respiratory failure
  • Resp. failure the inability of the lungs to
    maintain either the normal delivery of O2 to the
    tissues or the normal removal of CO2 from the
  • Table 37.2, p. 824 (Egans)

Respiratory Failure
  • Two categories - Type I (hypoxemic R.
    F.) - Type II (hypercapnic R. F.)
  • Type I is caused by - V/Q mismatch -
    Intrapulmon. Shunt - alveolar hypoventilation
    - decreased FIO2 - diffusion impair. -
    diffusion/perfusion impairment

Resp. Failure (contd)
  • Type II causes - Decreased ventilatory
    drive d/t drugs, brainstem lesions, sleep apnea,
    hypothyroidism - Respiratory muscle
    fatigue, e.g., CNS problems, muscular dystrophy
    - Increased work of breathing d/t
    COPD, pneumothorax, rib fx., pleural

Respiratory Failure (contd)
  • Clinical manifestations of acute R. F.
  • - restlessness - confusion
  • - tachycardia - diaphoresis
  • - headache - hypotension - central
    cyanosis - tremors - depressed ventilation -
  • ID R.F. by a deteriorating clinical picture
    combined with ABG values (PaO2 lt 50 PaCO2 gt 50
    on room air

Causes of Impaired Ventilation
  • Chronic airway obstruction - emphysema -
    chronic asthma - chronic bronchitis
  • Restrictive defects - interstitial
    fibrosis - pneumothorax - pleural effusion -
    chest surgery - flail chest - kyphoscoliosis -
    severe obesity - abd. surgery - peritonitis

Impaired Vent. (contd)
  • Neuromuscular defects - myasthenia gravis -
    Guillain Barre - multiple sclerosis -
    tetanus - spinal injuries - poliomyelitis -
    drugs an poisons
  • CNS damage or depression d/t - anesthetics -
    narcotics - barbiturates - tranquilizers -
    head trauma

Impaired a-c Gas Exchange
  • Due to diseases/conditions such as - fibrosing
    alveolitis - pneumoconiosis - sarcoidosis -
    tumors - pulmonary edema - thromboemboli
  • - pneumonectomy - collagen disease

V/Q abnormalities
  • Caused by anatomic and physiologic shunts
  • Anatomic shunt - blood flows from R. side of
    heart to L. side without passing through pulmon.
  • Physiologic shunt d/t - emphysema -
    asthma - chronic bronchitis - bronchiolitis -
    atelectasis - thromboemboli
  • - resp. distress syndrome - pneumonia

Goals of Mechanical Vent.
  • Provide adequate ventilation - best indicator
    of alveolar vent. is PaCO2
  • Adequate Oxygenation
  • Reduced work of breathing - Normally WOB
    requires 2-3 of total O2 consumption, but
    decreased compliance and resistance may cause
    200-300 increase in WOB

Complications and Hazards of Mechanical
  • Effects on systemic circulation by increased
    intrathoracic pressure
  • Barotrauma - modes such as SIMV and high
    frequency ventilation can help prevent -
    physical findings of pneumothorax
  • Hyperventilation - PaCO2 should not be
    allowed below 35 unless to lower ICP

Complications (contd)
  • Effects on other organ systems, i.e., hepatic,
    renal, GI
  • See decreased liver function d/t diaphragm
    pressing down on liver interfering with portal
    blood flow, may see blood clotting problems and
    decr. detoxification of drugs
  • Renal dysfunction and decreased urine output
    caused by incr. prod. of ADH and decr. renal
    blood flow (interstitial edema, decr. PaO2)

Complications (contd)
  • GI problems, i.e., bleeding, d/t pressure
    interfering with circulation to spleen and GI
    tract leading to mucosal ischemia and incr.
    effects of acid in tract. Prevention with

Mech. Vent. Control Circuits
  • Mechanical, i.e., levers, pulleys, and cams
  • Pneumatic, which uses gas pressure to operate
    diaphragms, jet entrainment devices, and pistons
  • Fluidic , which are similar to electronic logic
    circuits and use minute gas flows to operate
    timing systems and pressure switches
  • Electric, which use only simple switches

Control Circuits (contd)
  • Electronic, which use devices such as resistors,
    capacitors, diodes, and transistors in an
    integrated circuit. Can range in complexity from
    simple logic gates to microprocessors.

Ventilator Drive Mechanisms
  • Lung inflation takes place d/t a pressure
    gradient created between the mouth and alveoli.
    To generate this pressure various drive
    mechanisms are used
  • Seven common types
  • - weighted bellows - blowers
  • - injectors - press. reducing valves -
    pistons (linear nonlinear)
  • - spring-loaded bellows
  • -microprocessor controlled pneumatics

Weighted Bellows
  • Pressure generated within the bellows is a
    function of wt. acting on the cross-sectional
    area of the bellows
  • Pressure generated is constant

Pressure Reducing Valve
  • Reduces a high input press. to a lower constant
    output press.
  • May be adjustable or preset

  • Is an electric motor connected to a fan and
    rotates at a high constant speed

  • Mechanisms powered by press. reducing valves or
    blowers and main function is to increase the
    overall flowrated capability of the ventilator

Linear Driven Pistons
  • Pressure is generated with the use of an electric
    motor and a piston
  • Linear motion is transferred to the piston and
    positive pressure is generated during the
    pistons forward stroke

Non-linear driven pistons
  • Electric motor rotates a large wheel to which a
    connecting rod and piston are attached and
    positive pressure is generated during the forward
    stroke of the piston

Spring-loaded Bellows
  • Tension of a spring applies a continuous downward
    force to top of bellows
  • Pressure in the bellows does not remain constant

Microprocessor controlled Pneumatic Drive Systems
  • Use proportional solenoid valves and
    microprocessor controls
  • Uses programmed algorithms to open and close the
    solenoid valves to mimic virtually any flow or
    pressure wave pattern

Chatburns Mechanical Ventilator Classification
  • To understand ventilators in general, we need to
    understand basic functions of - Power
    input - Drive mechanisms - Control
    scheme - Output ( pressure, volume, and flow
    waveforms) - Alarm systems

Power Input
  • Electric - use 110-115 volts AC (60 Hz),
    can then be converted to DC to power
    electronic control circuits - DC(from lead
    acid batteries), which supplies about 2.5
    amp-hours of energy. Battery will usually power
    a vent. for up to 1 hour (requires 8-12 hours to
  • Pneumatic

Drive Mechanisms
  • Compressor - External -
    Internal 1. Piston and cylinder 2.
    Diaphragm 3. Bellows 4. Rotating vane

Drive Mechanisms (contd)
  • Motor and linkage - electric motor/rotating
    crank and piston rod - electric
    motor/rack and pinion - electric
    motor/direct (turns compressor - compressed gas
    regulator/direct (gas is used as the motor) i.e.
  • Output control valve - used to regulate the
    flow of gas to the patient

Drive Mechanisms (contd)Output Control Valve
  • Electromagnetic Poppet Valve - uses magnetic
    force caused by an electric current to control
    a pneumatic pressure in an on/off fashion
  • Pneumatic Poppet Valve - similar to a
    solenoid valve except it uses a small pneumatic
    press. to control a larger pneumatic press.

Output Control Valve (contd)
  • Proportional Valve - Is a mass flow control
    valve - similar to a solenoid, but uses a
    stepper motor where it is not just on/off but
    changes the diameter of the outflow port to
    give a variety of waveforms - PB 7200, Hamilton
    Veolar, Servo 900C, Bear 5
  • Pneumatic Diaphragm - mushroom valve,
    exhalation valve

Control Scheme
  • A ventilator can directly control only one
    variable at a time pressure, volume, or flow
  • Consists of 1. Control circuits 2.
    Control variables 3. Phase variables 4.
    Modes of ventilation and conditional variables

Control Circuits
  • Is the subsystem responsible for controlling the
    drive mechanism and/or the output control valve.
    A vent. may have more than one control circuit,
    which may be of several types.
  • Mechanical, Pneumatic, Fluidic, Electric, and

Control Variables and Waveforms
  • A vent. may be classified as either a pressure,
    volume, or flow controller, and less often as a
    time controller, and may also be characterized by
    the type of waveforms it can generate.
  • Vents. can also combine control schemes to create
    complex modes, e.g., PB7200 can mix flow-
    controlled breaths with pressure-controlled
    breaths (SIMV PSV)

Control Variables and Waveforms (contd)
  • PressureController - the vent. can control
    either the airway pressure or the pressure on the
    body surface, hence the classification of
    positive pressure vs. negative pressure - a
    positive press. controller generates a
    rectangular pressure waveform and an iron lung
    would generate a sinusoidal waveform

Control Variables and Waveforms (contd)
  • Volume controller - to be classified as a
    volume controller a vent. must (1) maintain a
    consistent volume waveform in the presence of a
    varying load and (2) measure volume and use the
    signal to control the volume waveform

Control Variables and Waveforms (contd)
  • Flow Controller - if the volume change
    remains consistent when compliance and
    resistance are varied, and volume change is
    not measured and used for control -
    Servo 900C is a flow controller because it
    measures flow and adjusts the output control
    valve accordingly

Control Variables and Waveforms (contd)
  • Time Controller - if both pressure and
    volume are affected substantially by changes
    in lung mechanics, then the only variables being
    controlled are inspir. and expir. times - see
    this in some high-freq. vents

Phase Variables
  • Still use division into 4 phases 1. Change
    from E to I 2. Inspiration 3. Change
    from I to E 4. Expiration
  • In each phase a particular variable is measured
    and used to start, sustain, and end the phase
  • Uses trigger, limit, cycle, and baseline variables

Phase Variables (contd)Trigger Variable
  • Trigger variable refers to an initiation of a
    breath when one of the variables of pressure,
    volume, flow, or time reaches a preset value
  • Most common trigger variable is time
  • Other trigger variables include manual and on
    some infant vents. see chest wall movement as the
    trigger variable

Phase variables (contd)Limit Variable
  • During inspiration pressure, volume, and flow
    increase above their baseline values
  • A variable is limited if it increases to a
    pre-set value before inspiration ends. In other
    words, inspiration does not end when the variable
    reaches its preset value
  • Limit variable in other words, sustains

Phase Variable (contd)Cycle Variable
  • Inspiration ends because some variable (pressure,
    volume, flow, or time) has reached a pre-set
  • This ending variable must be measured by the
    ventilator and used as a feedback signal to end
    inspiratory flow delivery, which then allows
    exhalation to begin

Phase Variables (contd)Baseline Variable
  • Baseline variable is the variable that is
    controlled during the expiratory time
  • Pressure is the most practical value to control
    and is used by all commonly used ventilators

Conditional Variables
  • Conditional variables can include pressure, tidal
    volume, inspiratory flow, minute ventilation,
    time, etc.
  • If the value of a conditional value reaches some
    pre-set threshold, then some action occurs to
    change the ventilatory pattern, e.g., MA-1 vent.
    when giving a sigh breath (the conditional
    variable is time), or as in SIMV if vent.
    detects pt. effort and window is open then
    spont. breath

  • The study of ventilatory operation requires the
    examination of output waveforms
  • The waveforms we look at are pressure, volume,
    and flow
  • Waveforms are grouped into 4 basic
    categories - rectangular - exponential -
    ramp - sinusoidal

Rectangular(see with pressure, flow)
Sinusoidal(see with volume, flow)
Ascending Ramp(see with pressure, volume, and
Descending Ramp(see with flow)
Exponential (rise)(see with pressure)
Exponential (decay)(see with flow)
Characteristic Waveforms
Pressure/Volume Curve
Flow/Volume Loop