Pediatric Resident Curriculum for the PICU
UTHSCSA
PRINCIPLES OF MECHANICAL
VENTILATION and
BLOOD GAS INTERPRETATION
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
Types of Ventilation
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•
•
•
Volume Control
Pressure Control
Pressure Support-CPAP
Pressure-Regulated Volume Control
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
Triggering the Ventilator
Pediatric Resident Curriculum for the PICU
UTHSCSA
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-tobreath 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
Pediatric Resident Curriculum for the PICU
UTHSCSA
Volume vs. Pressure
Pediatric Resident Curriculum for the PICU
UTHSCSA
Changes in ARDS
Volume Control
Control
Pressure
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
SIMV + PS
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
Adjusting The Ventilator
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•
•
•
•
pCO2 too high
pCO2 too low
pO2 too high
pO2 too low
PIP too high
Pediatric Resident Curriculum for the PICU
UTHSCSA
pCO2 Too High
•
•
•
•
•
Patient’s 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 >7.20.
Pediatric Resident Curriculum for the PICU
UTHSCSA
pCO2 Too Low
•
•
•
•
Minute ventilation is too high.
Lower either the rate or TV.
Don’t need to lower the TV if the PIP is <20.
PIP <24 is fine unless delivered TV is still
>15ml/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 >7ml/kg.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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 812 hours.
• While patient is on ventilator, don’t wean FiO2
to <25% to give the patient a margin of safety
in case the ventilator quits.
Pediatric Resident Curriculum for the PICU
UTHSCSA
pO2 Too Low
• Increase either the FiO2 or the mean airway
pressure (MAP).
• Try to avoid FiO2 >70%.
• 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 doesn’t add
much at rates <30bpm.
• May need to increase the PEEP to over 10, but
try to stay <15 if possible.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
Weaning Priorities
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•
•
•
•
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Wean PIP to <35cm H2O
Wean FiO2 to <60%
Wean I-time to <50%
Wean PEEP to <8cm H2O
Wean FiO2 to <40%
Wean PEEP, PIP, I-time, and rate towards
extubation settings.
• Can consider changing to volume control
ventilation when PIP <35cm H2O.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
Acute Deterioration
• DIFFERENTIAL DIAGNOSES
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–
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–
–
Pneumothorax
Right mainstem intubation
Pneumonia
Pulmonary edema
Loss of airway
Airway occlusion
Ventilator malfunction
Mucus plugging
Air leak
Pediatric Resident Curriculum for the PICU
UTHSCSA
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
Pediatric Resident Curriculum for the PICU
UTHSCSA
Pressure Patterns
• Elevated peak and
plateau pressures
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–
–
–
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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
Pediatric Resident Curriculum for the PICU
UTHSCSA
Extubation Criteria
• Neurologic
• Cardiovascular
• Pulmonary
Pediatric Resident Curriculum for the PICU
UTHSCSA
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 doesn’t 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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
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 workof-breathing on the following settings:
– FiO2 <40%
– 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
Pediatric Resident Curriculum for the PICU
UTHSCSA
Blood Gas Interpretation
NORMAL VALUES
Arterial
Capillary
pH
7.4 (7.38-7.42)
pO2 80-100 mm Hg
pCO2 35-45 mm Hg
Sat
>95% on RA
HCO3 22-26 mEq/L
26mEq/L
BE
-2 to +2
Venous
7.36 (7.31-7.41)
35-40 mm Hg
41-52 mm Hg
60-80% on RA
22-26mEq/L
7.35-7.40
45-60 mm Hg
40-45 mm Hg
>70%
22-
-2 to +2
-2 to +2
Pediatric Resident Curriculum for the PICU
UTHSCSA
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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
Acid-Base Diagram
From Goldberg, M.,
Green, S.B., Moss, M.L.,
et al.: JAMA 223:269275, 1973
Pediatric Resident Curriculum for the PICU
UTHSCSA
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 <7.4.
• 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 >7.4.
Pediatric Resident Curriculum for the PICU
UTHSCSA
Metabolic Disturbances
• Acute metabolic acidosis gets compensated by CO2
reduction within 12-24 hours. The pH is still
usually <7.4.
• 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.
Pediatric Resident Curriculum for the PICU
UTHSCSA
Hypoxemia
There are five reasons for hypoxemia:
• FiO2 too low (high altitude)
• Global alveolar hypoventilation
• Right-to-left shunts
• V/Q mismatch
• Incomplete diffusion