Mechanical Ventilation

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Mechanical Ventilation
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Introduction

• 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

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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
  tissues
• Table 37.2, p. 824 (Egans)

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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

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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
  effusions

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Respiratory Failure (contd)

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

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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

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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

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Impaired a-c Gas Exchange

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

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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.
  vasculature
• Physiologic shunt d/t - emphysema -
  asthma - chronic bronchitis - bronchiolitis -
  atelectasis - thromboemboli
• - resp. distress syndrome - pneumonia

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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

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Complications and Hazards of Mechanical
Ventilation

• 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

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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)

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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
  antacids

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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

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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.

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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

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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

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Pressure Reducing Valve

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

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Blowers

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

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Injectors

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

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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

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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

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Spring-loaded Bellows

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

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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

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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

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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
  recharge)
• Pneumatic

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Drive Mechanisms

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

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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.
  PB7200
• Output control valve - used to regulate the
  flow of gas to the patient

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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.
  (fluidic)

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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

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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

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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
  electronic

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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)

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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

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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

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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

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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

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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

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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

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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
  inspiration

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Phase Variable (contd)Cycle Variable

• Inspiration ends because some variable (pressure,
  volume, flow, or time) has reached a pre-set
  value
• 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

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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
• PEEP, CPAP

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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

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Output

• 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

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