Principles of Mechanical Ventilation Mazen Kherallah, MD, FCCP

Oxygenation Parameters Alveolar P O2 Arterial P O2 Tension-based indices –P (A-a)o2 –P aO2 /P AO2 –P aO2 /F iO2 Respiratory index Pulmonary Shunt

Distribution of Normal Ventilation-Perfusion Ratios

Oxygenation Status Monitoring Alveolar - arterial Oxygen Tension Difference P (A-a)o2 PAo2 = Fio2 (PB-P H2O ) - Pa co2 /R = (Fio2  713) - (Pa co2 /0.8) at sea level = (Pa co2 /0.8) at sea level on room air A-a Gradient = PAo2 - PaO2 Normal Value: 5-25 mmHg

Oxygenation Status Monitoring A-a Gradient Increased: –Decreased Fio2 –V/Q mismatch –Shunting process –Diffusion abnormalities Decreased –Hyperventilation –Increased Fio2

Pa O2 /PA O2 Remains stable when FiO2 changes Can be used to determined FiO2 needed for desired PO2 –FiO2 needed=[(desired PaO2)/(PaO2/PAO2)+Paco2]/(PB-47) Value of less than 0.75 indicates pulmonary dysfunction due to V/Q abnormality, shunt or diffusion abnormality

Pa O2 /FI O2 Oxygenation index Value of less than 200 is associated with severe shunt in patients with acute respiratory failure Easy to calculate

Respiratory Index P(A-a) O2 /PaO2 Normal value 0.1 Values higher than 0.1 indicate respiratory abnormality Better indicator of oxygenation dysfuntion

Pulmonary Shunt Q S /Q T = (CcO2-CaO2)/(CcO2-CvO2) Q S /Q T = (CcO2-CaO2)/(3.5+ CcO2-CaO2) when pulmonary catheter is not in place

Oxygenation Status Monitoring Oxygen Delivery Do2 = CI  Ca O2 CaO2 = SaO2  1.36  Hgb + (  PaO2) CI = CO/ BSA Normal Value: mL/min

Oxygenation Dissociation Curve

Oxygenation Status Monitoring Oxygen Consumption Vo2=CI (CaO2-CvO2) CaO2 = SaO2  1.36  Hgb + (  PaO2) CvO2 = SvO2  1.36  Hgb + (  PvO2) Normal Value: mL/min

Oxygenation Status Monitoring Oxygen Extraction O2 ext = Vo2 / Do2 Normal value: 27%

Oxygenation Status Monitoring Relationship between Vo2 and Do2

Oxygenation Status Monitoring Oxygen Transport Variables

Anatomic and Capillary Shunts

Dead Space

Ventilation-Perfusion Inequality Acute Exacerbation of COPD

Ventilation-Perfusion Inequality Asthma

Ventilation-Perfusion Inequality Pulmonary Embolism

Shunting Process ARDS

The Effect of Increasing Ventilation- Perfusion Inequality on Arterial Po2 and Pco2

The effect of changing the inspired oxygen concentration on arterial Po2 for lung’s shunts of 10 to 50%

Assessment of Hypoxia

Ventilation Status Monitoring Tidal Volume: Vt Minute ventilation: Vm Respiration Rate: RR CO2 production: Vco2 Dead Space: V DS /V T

Dead Space Ventilation V D /V T =(Pa CO2 -PE CO2 )/Pa CO2 Normal is PEco2 is measured by collecting condensate from the water trap on the expiratory limb of the ventilator circuit and the measure PCO2 using blood gas analyzer

Causes of Increased Dead Space Ventilation Pulmonary embolism pulmonary hypoperfusion positive pressure ventilation High rate-low tidal volume ventilation

Arterial CO2 Pa CO2 = V CO /V E.(1-Vd/Vt)

High Minute Ventilation Increased CO2 production –Sepsis –Fever –Thyrotoxicosis –High carbohydrate feeding Increased ventilation: –Agitation –Pain –Central hyperventilation –Increased dead space

Low Minute Ventilation Decreased CO2 production –Hypothermia –Hypothyroidism –Severe sedation –Low carbohydrate feeding –Paralysis Decreased ventilation: –Sedation –Central hypoventilation –Decreased dead space

Airway Pressure Waveform

Pulmonary Mechanics Peak pressure Plateau pressure I E Airway Resistance

Mean Airway Pressure Paw= (PIP-PEEP).(T I /T T )+PEEP.(T E /T T )

Methods to Increase Mean Airway Pressure Increase in tidal volume Increase in respiratory frequency Reduction in T E Decrease in respiratory flow rate: increase in T I Addition of end-inspiratory pause Addition of PEEP

Equation of Motion

Work of Breathing Mechanical work is performed when a force moves its point of application through a distance In the case of three dimensional fluid system, work is done when a pressure (P) changes the volume (V) of the system W = P.V: {PIP-(0.5). (Pplat)/100}.v T 0.5 J/L

Static Pressure-volume curve in ARDS

Ventilatory System

Control Variables during Inspiration

Phase Variables

Modes of Ventilation

Breath Type during Mechanical Ventilation

Pressure Waveforms

Flow, Pressure, and Volume Waveforms with Constant Flow, Volume Ventilation

Flow, Pressure, and Volume Waveforms with Decelerating Ramp Flow, Volume Ventilation

Waveforms for Decelerating and Accelerating Ramp Flows

Full and Partial Decelerating Ramp Flow with Volume Ventilation

Flow, Pressure, and Volume Waveforms with Pressure Ventilation

Full and Modified Sine-flow Waveforms during Volume Ventilation

Flow, Pressure, and Volume Waveforms with Pressure Support Ventilation

Active Inspiration during Positive Pressure Ventilation

Airway Flow Waveform during Mechanical Ventilation

Airway Volume Waveform during Mechanical Ventilation

Flow-Volume and Pressure-Volume loops with COPD

Changes in Flow-Volume and Pressure- Volume loops with Bronchodilators

Pressure-Volume Loop Work Performed to Trigger the Ventilator

Pressure-Volume Loop Lung/Chest Wall Compliance

Dynamic Pressure-Volume LOOP Restrictive Work

Inspiratory Work of Breathing

Pressure-Volume Loop Deflection Points

Modes of Mechanical Ventilation Volume-Cycled Control Mode Ventilation

Modes of Mechanical Ventilation Assist-Control Ventilation

Indications: – for patients who are awake, moderately sedated or paralyzed and able to initiates ventilation –increase metabolic demands: infection, burns, multisystem organ failure –Respiratory muscle strengthening and weaning Limitations: –patient-ventilator dysynchrony –ventilator assisted hyperventilation in agitated patients with increased inspiratory drive –auto-PEEP in COPD patients

Modes of Mechanical Ventilation Intermittent Mandatory Ventilation

Modes of Mechanical Ventilation Synchronized Intermittent Mandatory Ventilation

Pressure Waveform for SIMV

Synchronized Intermittent Mandatory Ventilation Indications: –patients with minimal spontaneous respiratory efforts –respiratory muscle conditioning –ventilator weaning Limitations: –patient-ventilator dysynchrony especially in agitated patients –nonphysiologic way of respiratory muscle conditioning

Modes of Mechanical Ventilation Pressure Support Ventilation

Indications: –weaning –more physiologic conditioning of respiratory muscles: low pressure-high volume load –improved patient- ventilator dysynchrony Limitations:

Modes of Mechanical Ventilation Inverse Ratio Ventilation

Auto-PEEP

Normal Lung Mechanics and Gas Exchange

Severe Airflow Obstruction

Acute on Chronic Respiratory Failure

Acute Hypoxemic Respiratory Failure