PRINCIPLES OF MECHANICAL VENTILATION and

1

• PRINCIPLES OF MECHANICAL VENTILATION and
• BLOOD GAS INTERPRETATION

2
Definitions

• Tidal Volume (TV) volume of each breath.
• Rate breaths per minute.
• Minute Ventilation (MV) total ventilation per
  minute. MV TV x Rate.
• Flow volume of gas per time.
• Compliance the distensibility of a system. The
  higher the compliance, the easier it is to
  inflate the lungs.
• Resistance impediment to airflow.
• SIMV patient breathes spontaneously between
  ventilator breaths. Allows patient-ventilator
  synchrony, making for a more comfortable
  experience.

3
Definitions

• PIP maximum pressure measured by the ventilator
  during inspiration.
• PEEP pressure present in the airways at the end
  of expiration.
• CPAP amount of pressure applied to the airway
  during all phases of the respiratory cycle.
• PS amount of pressure applied to the airway
  during spontaneous inspiration by the patient.
• I-time amount of time delegated to inspiration.

4
Types of Ventilation

• Volume Control
• Pressure Control
• Pressure Support-CPAP
• Pressure-Regulated Volume Control

5
Volume Control

• The patient is given a specific volume of air
  during inspiration.
• The ventilator uses a set flow for a set period
  of time to deliver the volume TV (cc) Flow
  (cc/sec) x i-time (sec)
• The PIP observed is a product of the lung
  compliance, airway resistance and flow rate. The
  ventilator does not react to the PIP unless the
  alarm limits are violated.
• The PIP tends to be higher than during pressure
  control ventilation to deliver the same volume of
  air.
• With SIMV, the patient can breath spontaneously
  between vent breaths. This mode is often
  combined with PS.

6
Triggering the Ventilator
7
Pressure Control

• Patient receives a breath at a fixed airway
  pressure.
• The ventilator adjusts the flow to maintain the
  pressure.
• Flow decreases throughout the inspiratory cycle.
• The pressure is constant throughout inspiration.
• Volume delivered depends upon the inspiratory
  pressure, I-time, pulmonary compliance and airway
  resistance.
• The delivered volume can vary from
  breath-to-breath depending upon the above
  factors. MV not assured.
• Good mode to use if patient has large air leak,
  because the ventilator will increase the flow to
  compensate it.

8
Volume vs. Pressure
9
Changes in ARDS

• Volume Control
  Pressure Control

10
CPAP-Pressure Support

• No mandatory breaths
• Patient sets the rate, I-time, and respiratory
  effort.
• CPAP performs the same function as PEEP, except
  that it is constant throughout the inspiratory
  and expiratory cycle.
• Pressure Support (PS) helps to overcome airway
  resistance and inadequate pulmonary effort and is
  added on top of the CPAP during inspiration.
• The ventilator increases the flow during
  inspiration to reach the target pressure and make
  it easier for the patient to take a breath.

11
SIMV PS
12
Pressure-Regulated Volume Control

• In this mode, a target minute ventilation is set.
  
• The ventilator will adjust the flow to deliver
  the volume without exceeding a target inspiratory
  pressure.
• Decelerating flow pattern.
• No change in minute ventilation if pulmonary
  conditions change.
• Can ventilate at a lower PIP than in regular
  volume control.
• Hard to use on a spontaneously breathing patient
  or one with a large air leak.
• Not a weaning mode.

13
Initial Ventilator Settings

• Rate 20-24 for infants and preschoolers
  16-20 for grade school kids 12-16 for
  adolescents
• TV 10-15ml/kg
• PEEP 3-5cm H2O
• FiO2 100
• I-time 0.7 sec for higher rates, 1sec for lower
  rates
• PIP (for pressure control) about 24cm H2O.
• Pressure Support 5-10cm H2O.

14
Adjusting The Ventilator

• pCO2 too high
• pCO2 too low
• pO2 too high
• pO2 too low
• PIP too high

15
pCO2 Too High

• Patients minute ventilation is too low.
• Increase rate or TV or both.
• If using PC ventilation, increase PIP.
• If PIP too high, increase the rate instead.
• If air-trapping is occurring, decrease the rate
  and the I-time and increase the TV to allow
  complete exhalation.
• Sometimes, you have to live with the high pCO2,
  so use THAM or bicarbonate to increase the pH to
  gt7.20.

16
pCO2 Too Low

• Minute ventilation is too high.
• Lower either the rate or TV.
• Dont need to lower the TV if the PIP is lt20.
• PIP lt24 is fine unless delivered TV is still
  gt15ml/kg.
• TV needs to be 8ml/kg or higher to prevent
  progressive atelectasis
• If patient is spontaneously breathing, consider
  lowering the pressure support if spontaneous TV
  gt7ml/kg.

17
pO2 Too High

• Decrease the FiO2.
• When FiO2 is less than 40, decrease the PEEP to
  3-5 cm H2O.
• Wean the PEEP no faster than about 1 every 8-12
  hours.
• While patient is on ventilator, dont wean FiO2
  to lt25 to give the patient a margin of safety in
  case the ventilator quits.

18
pO2 Too Low

• Increase either the FiO2 or the mean airway
  pressure (MAP).
• Try to avoid FiO2 gt70.
• Increasing the PEEP is the most efficient way of
  increasing the MAP in the PICU.
• Can also increase the I-time to increase the MAP
  (PC).
• Can increase the PIP in Pressure Control to
  increase the MAP, but this generally doesnt add
  much at rates lt30bpm.
• May need to increase the PEEP to over 10, but try
  to stay lt15 if possible.

19
PIP Too High

• Decrease the PIP (PC) or the TV (VC).
• Increase the I-time (VC).
• Change to another mode of ventilation.
  Generally, pressure control achieves the same TV
  at a lower PIP than volume control.
• If the high PIP is due to high airway resistance,
  generally the lung is protected from barotrauma
  unless air-trapping occurs.

20
Weaning Priorities

• Wean PIP to lt35cm H2O
• Wean FiO2 to lt60
• Wean I-time to lt50
• Wean PEEP to lt8cm H2O
• Wean FiO2 to lt40
• Wean PEEP, PIP, I-time, and rate towards
  extubation settings.
• Can consider changing to volume control
  ventilation when PIP lt35cm H2O.

21
Complications

• Pulmonary
• Barotrauma
• Ventilator-induced lung injury
• Nosocomial pneumonia
• Tracheal stenosis
• Tracheomalacia
• Pneumothorax
• Cardiac
• Myocardial ischemia
• Reduced cardiac output

• Gastrointestinal
• Ileus
• Hemorrhage
• Pneumoperiteneum
• Renal
• Fluid retention
• Nutritional
• Malnutrition
• Overfeeding

22
Acute Deterioration

• DIFFERENTIAL DIAGNOSES
• Pneumothorax
• Right mainstem intubation
• Pneumonia
• Pulmonary edema
• Loss of airway
• Airway occlusion
• Ventilator malfunction
• Mucus plugging
• Air leak

23
Physical Exam

• Tracheal shift
• Pneumothorax
• Wheezing
• Bronchospasm
• Mucus plugging
• Pulmonary edema
• Pulmonary thromboembolism

• Asymmetric breath sounds
• Pneumothorax
• Mainstem intubation
• Mucus plugging with atelectasis
• Decreased breath sounds bilaterally
• Tube occlusion
• Ventilator malfunction
• Loss of airway

24
Pressure Patterns

• Elevated peak and plateau pressures
• Pneumonia
• Pulmonary edema
• Pneumothorax
• Atelectasis
• Right mainstem intubation

• Elevated peak pressure, normal plateau pressure
• Airflow obstruction
• Mucus plugging
• Partial tube occlusion
• Reduced peak and plateau pressure
• Cuff leak
• Ventilator malfunction
• Large bronchopleural fistula

25
Extubation Criteria

• Neurologic
• Cardiovascular
• Pulmonary

26
Neurologic

• Patient must be able to protect his airway, e.g,
  have cough, gag, and swallow reflexes.
• Level of sedation should be low enough that the
  patient doesnt become apneic once the ETT is
  removed.
• No apnea on the ventilator.
• Must be strong enough to generate a spontaneous
  TV of 5-7ml/kg on 5-10 cm H2O PS or have a
  negative inspiratory force (NIF) of 25cm H2O or
  higher.
• Being able to follow commands is preferred.

27
Cardiovascular

• Patient must be able to increase cardiac output
  to meet demands of work of breathing.
• Patient should have evidence of adequate cardiac
  output without being on significant inotropic
  support.
• Patient must be hemodynamically stable.

28
Pulmonary

• Patient should have a patent airway.
• If no air leak, consider decadron and racemic
  epinephrine.
• Pulmonary compliance and resistance should be
  near normal.
• Patient should have normal blood gas and
  work-of-breathing on the following settings
• FiO2 lt40
• PEEP 3-5cm H2O
• Rate 6bpm for infants, 2bpm for toddlers,
  CPAP/PS for 1hr for older children and
  adolescents
• PS 5-8cm H2O
• Spontaneous TV of 5-7ml/kg

29
Blood Gas Interpretation

• NORMAL VALUES
• Arterial Venous
  Capillary
• pH 7.4 (7.38-7.42) 7.36 (7.31-7.41) 7.35-7.40
• pO2 80-100 mm Hg 35-40 mm Hg 45-60 mm Hg
• pCO2 35-45 mm Hg 41-52 mm Hg 40-45 mm Hg
• Sat gt95 on RA 60-80 on RA gt70
• HCO3 22-26 mEq/L 22-26mEq/L 22-26mEq/L
• BE -2 to 2 -2 to 2 -2 to 2

30
Rules Of Interpretation

• ? in pCO2 of 10mm Hg should ? pH by 0.08.
• pH ? of 0.15 is equal to ? in HCO3 of 10mEq/L.
• Normal pCO2 in the face of respiratory distress
  is a sign of impending respiratory failure.

31
Acid-Base Diagram
From Goldberg, M., Green, S.B., Moss, M.L., et
al. JAMA 223269-275, 1973
32
Respiratory Disturbances

• Acute respiratory acidosis occurs when CO2 is
  retained acutely.
• Chronic respiratory acidosis occurs when the
  retained CO2 gets buffered by renal retention of
  HCO3. The pH is higher than in acute respiratory
  acidosis, but it is still lt7.4.
• Acute respiratory alkalosis occurs when CO2 is
  blown off acutely.
• Chronic respiratory alkalosis occurs when the
  reduction of CO2 is compensated for by the renal
  excretion of HCO3. The pH is lower than in acute
  respiratory alkalosis, but it is still gt7.4.

33
Metabolic Disturbances

• Acute metabolic acidosis gets compensated by CO2
  reduction within 12-24 hours. The pH is still
  usually lt7.4.
• Metabolic alkalosis is rare. Usual causes are
  pyloric stenosis, chronic diuretic use, and
  bicarbonate infusions.
• Otherwise healthy people do not usually retain
  CO2 to compensate for metabolic alkalosis.
• Patients who are severely dehydrated or have lung
  disease will retain CO2 to compensate for
  metabolic alkalosis.

34
Hypoxemia

• There are five reasons for hypoxemia
• FiO2 too low (high altitude)
• Global alveolar hypoventilation
• Right-to-left shunts
• V/Q mismatch
• Incomplete diffusion