Mechanical Ventilation and
Blood Gases
Resident Lecture Series
Soo Hyun Kwon, MD
Goals
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Understand the principles of respiratory physiology
Learn differences in respiratory physiology of neonate
Learn different modes of mechanical ventilation
Discuss some of complications of mechanical
ventilation and issues related to weaning the
ventilator
Review how to interpret blood gases and causes of
acid-base disturbances
Objectives
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List indications for mechanical ventilation
Describe the basics of respiratory mechanics
Describe the interaction between the ventilator and
the infant
Compare modes of conventional ventilation
Delineate the factors on which ventilator adjustments
should be based
Describe how mechanical ventilation may cause lung
injury
Interpret blood gases and changes to ventilator
settings based on a gas
Definition
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Assisted ventilation: movement of gas
into and out of lungs by external source
connected directly to patient
Factors to Consider
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Pulmonary mechanics
Gas exchange mechanisms
Control of breathing
Lung injury
Normal Respiration
Pulmonary Mechanics
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Compliance
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Elasticity or distensibility of the respiratory
structures (eg, alveoli, chest wall, and pulmonary
parenchyma)
C=∆V/∆P
Resistance
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Inherent capacity of the air conducting system
(eg, airways, endotracheal tube) and tissues to
oppose airflow
R= ∆P/∆F
Pulmonary Mechanics in
Newborns
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Shape of chest
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Compliance of chest wall
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More cylindrical and ribs more horizontal
Less elevation of ribs therefore less volume
Little resistance to expansion
Little opposition to collapse
Surface tension
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Largest contributor to recoil on exhalation
High surface tension will lead to atelectasis
Surfactant reduces surface tension
Normal Gas Exchange
Gas Exchange in Newborns
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High metabolic rate
Propensity for decreased functional
residual capacity (FRC)
Increased resistance
Potential for right-to-left shunts through
the ductus arteriosus, foramen ovale, or
both
Ventilation and Hypercapnea
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Ventilation (CO2
removal)
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Function of minute
ventilation
Alveolar Minute
Ventilation = Tidal
Volume x Rate
Oxygenation and Hypoxemia
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Oxygenation
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Function of FiO2 and
MAP
MAP = [RRxItime/60]
x (PIP-PEEP) + PEEP
Time Constant
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Time Constant: time required to allow
pressure and volume to equilibriate
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Time constant (0.12s)= Compliance x Resistance
Indications for Assisted
Ventilation
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Absolute Indications
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Failure to initiate or
sustain spontaneous
breathing
Persistent bradycardia
despite BMV
Major airway or
pulmonary malformations
Sudden respiratory of
cardiac collapse with
apnea/bradycardia
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Relative Indications
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High likelihood of
subsequent respiratory
failure
Surfactant administration
Impaired pulmonary gas
exchange
Worsening apnea
unresponsive to other
measures
Need to maintain airway
patency
Need to control CO2
elimination
Goals of Mechanical
Ventilation
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Improve gas exchange
Decrease work of breathing
Ventilation for patients with apnea or
respiratory depression
Maintain airway patency
Changing MAP and TV
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A: Flow
B: PIP
C: Insp time
D: PEEP
E: Exp time
Ventilator Modes and
Modalities
Ventilator Settings
(Pressure-targeted ventilation)
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Rate
PIP
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PEEP
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4-6 cm H2O
Tidal volumes (measured, not set)
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visible chest rise
adequate breath sounds
preterm: 4-7 ml/kg
term: 5-8 ml/kg
Itime
+/- PS
FiO2
Ventilator Induced Lung Injury
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Barotrauma
Volutrauma
Atelectrauma
Biotrauma
Suggested Strategies For
Conventional Ventilation in RDS
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Conservative indications for conventional
ventilation
Permissive hypercapnia
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Low tidal volume ventilation
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Accept higher PCO2 values
Lowest PIP (tidal volume) that inflates the lungs
Moderate PEEP (4 - 6 cm H2O)
Aggressive weaning from conventional
ventilation
Weaning from Assisted
Ventilation
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Physiologic requisites
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Elements of weaning
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Adequate spontaneous drive
Overcome respiratory system load
Maintenance of alveolar ventilation
Assumption of work of breathing
Nutritional aspects
Impediments to weaning
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Infection
Neurologic/neuromuscular dysfunction
Electrolyte imbalance
Metabolic alkalosis
Congestive heart failure
Anemia
Sedatives/analgesics
Nutrition
Complications of Assisted
Ventilation
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Airway
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Upper: trauma/injury,
abnl dentition,
esophageal perforation,
acquired palatal groove
Trachea: subglottic cysts,
tracheal enlargement,
tracheobronchomalacia,
tracheal perforation,
vocal cord
paralysis/paresis,
subglottic stenosis,
necrotizing
tacheobronchitis
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Lungs
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Misc
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VA-PNA
Air leaks
Ventilator induced lung
injury
CLD/BPD
Imposed WOB
PDA
Neurologic
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IVH
PVL
ROP
Other Modes of Invasive
Mechanical Ventilation
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High Frequency Ventilation
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Jet ventilation
Oscillatory ventilation
Other Modes of Positive
Pressure
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Nasal Intermittent Positive Pressure
Ventilation (NIPPV)
Continuous Positive Airway Pressure
(CPAP)
High Flow Nasal Cannula
Blood Gases
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Objective evaluation of a patient’s
oxygenation, ventilation and acid-base
balance
Balance between lungs and kidneys
Buffer Systems
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Lungs
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Cellular metabolism  CO2
CO2 in lungs + H20  carbonic acid (H2CO3).
Carbonic acid changes blood pH
Triggers lungs to either increase or decrease rate/depth of
ventilation
In an effort to maintain the pH of the blood within its normal
range, the kidneys excrete or
Kidneys
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Excrete or retain bicarbonate HCO3 to maintain normal pH
As pH increases, kidneys excrete HCO3 through the urine
Components of Blood Gas
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pH/PCO2/PO2/O2 sat/HCO3/Base excess or
deficit
Measured
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pH
PCO2
PO2
Calculated
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O2 sat
HCO3
Base excess or deficit
Normal Values
Steps to Interpreting Blood
Gases
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Determine
Determine
PCO2
Determine
Determine
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acidosis or alkalosis based on pH
acidosis or alkalosis based on
if metabolic or respiratory
compensation
For every 10 change in PCO2 above or below 40
 0.08 change in pH in opposite direction
Acidosis and alkalosis may be partially or fully
compensated by the opposite mechanism
Body NEVER OVERCOMPENSATES!
Approach for Analysis of
Simple Acid–Base Disorders
Before Making Vent Changes
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Do you believe the blood gas result?
Look at the baby
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the chest moving?
there good air-entry like?
there increased WOB?
the baby very tachypneic or is the baby apneic?
Look at the ventilator
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Is
Is
Is
Is
What tidal volume is the baby getting?
Is there a significant leak?
Other things to consider
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How stable has the baby been over the past few hours or days?
Are there lots of secretions?
Vent Changes
Problem
Possible Solutions
Low oxygenation
Increase FiO2, MAP
High oxygenation
Decrease FiO2, MAP
Over-ventilation
Decrease TV, Rate
Under-ventilation
Increase TV, Rate
Common Causes of Acid-Base
Status in Neonates
Question 1
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Baby Brown is a 24-week-gestation male infant who
is 4 days old. His birth weight was 600 grams and he
is on a conventional ventilator.
Vent settings: 30 19/5 PS6 40%
Na: 151 Glucose: 180
Weight today: 510 grams
ABG: 7.17/45/55/-10
What is the abnormality based on gas? What is the
most likely cause of this abnormality?
Metabolic acidosis
Question 2
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7.22/61/70/-1
What is the abnormality based on this
gas?
Uncompensated respiratory acidosis
Question 3
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33 weeker
SIMV 25 18/5 30%
CBG: 7.49/26/+2
What is the abnormality based on this
gas? How would you change the vent
settings?
Uncompensated respiratory alkalosis.
Decrease Rate, PIP.
Question 4
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CBG: 7.37/29/-3
What is the abnormality based on this
gas?
Metabolic acidosis with Respiratory
compensation
References
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Fanaroff A, Martin R, Walsh M. Fanaroff
and Martin's Neonatal-Perinatal
Medicine. 2008.
Thank You