Mechanical Ventilation and Blood Gases

Resident Lecture Series

Soo Hyun Kwon, MD

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

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

Definition Assisted ventilation: movement of

gas into and out of lungs by external source connected directly to patient

Factors to Consider

Pulmonary mechanics Gas exchange mechanisms Control of breathing Lung injury

Normal Respiration

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

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

Normal Gas Exchange

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

Ventilation and Hypercapnea

Ventilation (CO2 removal) Function of minute

ventilation Alveolar Minute

Ventilation = Tidal Volume x Rate

Oxygenation and Hypoxemia

Oxygenation Function of FiO2

and MAP MAP =

[RRxItime/60] x (PIP-PEEP) + PEEP

Time Constant

Time Constant: time required to allow pressure and volume to equilibriate Time constant (0.12s)= Compliance x

Resistance

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

Goals of Mechanical Ventilation

Improve gas exchange Decrease work of breathing Ventilation for patients with apnea

or respiratory depression Maintain airway patency

Changing MAP and TV A: Flow B: PIP C: Insp time D: PEEP E: Exp time

Ventilator Modes and Modalities

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

Ventilator Induced Lung Injury

Barotrauma Volutrauma Atelectrauma Biotrauma

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

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

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

Other Modes of Invasive Mechanical Ventilation

High Frequency Ventilation Jet ventilation Oscillatory ventilation

Other Modes of Positive Pressure

Nasal Intermittent Positive Pressure Ventilation (NIPPV)

Continuous Positive Airway Pressure (CPAP)

High Flow Nasal Cannula

Blood Gases

Objective evaluation of a patient’s oxygenation, ventilation and acid-base balance

Balance between lungs and kidneys

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

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

Normal Values

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!

Approach for Analysis of Simple Acid–Base Disorders

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?

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 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

7.22/61/70/-1

What is the abnormality based on this gas?

Uncompensated respiratory acidosis

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.

Question 4

CBG: 7.37/29/-3 What is the abnormality based on

this gas? Metabolic acidosis with Respiratory

compensation

References

Fanaroff A, Martin R, Walsh M. Fanaroff and Martin's Neonatal-Perinatal Medicine. 2008.

Thank You