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 ProblemPossible Solutions Low oxygenationIncrease FiO2, MAP High oxygenationDecrease FiO2, MAP Over-ventilationDecrease TV, Rate Under-ventilationIncrease 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 /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

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