Download - Mechanical Ventilation and Blood Gases
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Mechanical Ventilation and Blood Gases
Resident Lecture Series
Soo Hyun Kwon, MD
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Goals 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
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Objectives 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
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Definition Assisted ventilation: movement of
gas into and out of lungs by external source connected directly to patient
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Factors to Consider
Pulmonary mechanics Gas exchange mechanisms Control of breathing Lung injury
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Normal Respiration
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Pulmonary Mechanics Compliance
Elasticity or distensibility of the respiratory structures (eg, alveoli, chest wall, and pulmonary parenchyma)
C=∆V/∆P
Resistance Inherent capacity of the air conducting
system (eg, airways, endotracheal tube) and tissues to oppose airflow
R= ∆P/∆F
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Pulmonary Mechanics in Newborns Shape of chest
More cylindrical and ribs more horizontal Less elevation of ribs therefore less volume
Compliance of chest wall Little resistance to expansion Little opposition to collapse
Surface tension Largest contributor to recoil on exhalation High surface tension will lead to atelectasis Surfactant reduces surface tension
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Normal Gas Exchange
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Gas Exchange in Newborns
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
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Ventilation and Hypercapnea
Ventilation (CO2 removal) Function of minute
ventilation Alveolar Minute
Ventilation = Tidal Volume x Rate
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Oxygenation and Hypoxemia
Oxygenation Function of FiO2
and MAP MAP =
[RRxItime/60] x (PIP-PEEP) + PEEP
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Time Constant
Time Constant: time required to allow pressure and volume to equilibriate Time constant (0.12s)= Compliance x
Resistance
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Indications for Assisted Ventilation Absolute Indications
Failure to initiate or sustain spontaneous breathing
Persistent bradycardia despite BMV
Major airway or pulmonary malformations
Sudden respiratory of cardiac collapse with apnea/bradycardia
Relative Indications 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
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Goals of Mechanical Ventilation
Improve gas exchange Decrease work of breathing Ventilation for patients with apnea
or respiratory depression Maintain airway patency
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Changing MAP and TV A: Flow B: PIP C: Insp time D: PEEP E: Exp time
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Ventilator Modes and Modalities
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Ventilator Settings (Pressure-targeted ventilation) Rate PIP
visible chest rise adequate breath sounds
PEEP 4-6 cm H2O
Tidal volumes (measured, not set) preterm: 4-7 ml/kg term: 5-8 ml/kg
Itime +/- PS FiO2
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Ventilator Induced Lung Injury
Barotrauma Volutrauma Atelectrauma Biotrauma
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Suggested Strategies For Conventional Ventilation in RDS
Conservative indications for conventional ventilation
Permissive hypercapnia Accept higher PCO2 values
Low tidal volume ventilation Lowest PIP (tidal volume) that inflates the
lungs Moderate PEEP (4 - 6 cm H2O) Aggressive weaning from conventional
ventilation
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Weaning from Assisted Ventilation Physiologic requisites
Adequate spontaneous drive Overcome respiratory system load
Elements of weaning Maintenance of alveolar ventilation Assumption of work of breathing Nutritional aspects
Impediments to weaning Infection Neurologic/neuromuscular dysfunction Electrolyte imbalance Metabolic alkalosis Congestive heart failure Anemia Sedatives/analgesics Nutrition
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Complications of Assisted Ventilation Airway
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
Lungs VA-PNA Air leaks Ventilator induced
lung injury CLD/BPD
Misc Imposed WOB PDA
Neurologic IVH PVL ROP
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Other Modes of Invasive Mechanical Ventilation
High Frequency Ventilation Jet ventilation Oscillatory ventilation
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Other Modes of Positive Pressure
Nasal Intermittent Positive Pressure Ventilation (NIPPV)
Continuous Positive Airway Pressure (CPAP)
High Flow Nasal Cannula
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Blood Gases
Objective evaluation of a patient’s oxygenation, ventilation and acid-base balance
Balance between lungs and kidneys
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Buffer Systems
Lungs 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
Excrete or retain bicarbonate HCO3 to maintain normal pH
As pH increases, kidneys excrete HCO3 through the urine
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Components of Blood Gas pH/PCO2/PO2/O2 sat/HCO3/Base excess
or deficit Measured
pH PCO2 PO2
Calculated O2 sat HCO3 Base excess or deficit
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Normal Values
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Steps to Interpreting Blood Gases Determine acidosis or alkalosis based on
pH Determine acidosis or alkalosis based on
PCO2 Determine if metabolic or respiratory Determine 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!
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Approach for Analysis of Simple Acid–Base Disorders
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Before Making Vent Changes Do you believe the blood gas result? Look at the baby
Is the chest moving? Is there good air-entry like? Is there increased WOB? Is the baby very tachypneic or is the baby apneic?
Look at the ventilator What tidal volume is the baby getting? Is there a significant leak?
Other things to consider How stable has the baby been over the past few hours or
days? Are there lots of secretions?
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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
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Common Causes of Acid-Base Status in Neonates
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Question 1 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
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Question 2
7.22/61/70/-1
What is the abnormality based on this gas?
Uncompensated respiratory acidosis
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Question 3 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.
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Question 4
CBG: 7.37/29/-3 What is the abnormality based on
this gas? Metabolic acidosis with Respiratory
compensation
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References
Fanaroff A, Martin R, Walsh M. Fanaroff and Martin's Neonatal-Perinatal Medicine. 2008.
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Thank You