VENTILATION MECHANICAL Phunsup Wongsurakiat, MD, FCCP Division of Respiratory Disease and TB Department of Medicine, Siriraj Hospital

Mechanical Ventilation Invasive (intubation) Non-invasive (other interface such as face mask): - negative pressure - positive pressure

DISTENDING PRESSURES Plat P - Base P (PEEP) = pressure to distend resp system (lung + chest wall) Plat P = peak alveolar pressure Transpulmonary pressure = Plat P – Pleural pressure

Influence of chest wall stiffness 35 cm H2O 35 cm H2O 5 cm H2O 10 cm H2O 30 cm H2O 25 cm H2O

FLOW Resistance (R) Peak P - PlatP = pressure for flow R = Flow/(PeakP - Plat P)

Physiology of Respiration O2 consumption (VO2) ≈ 250 mL/ min CO2 production (VCO2) ≈ 200 mL/ min Cardiac output (CO) ≈ 5 L/min Minute ventilation = RR X VT ≈ 5 - 8 L/min VT ≈ 5 - 8 mL / kg RR ≈ 12 - 20 bpm PaCO2 = 35-45 mmHg pH = 7.35-7.45

Physiology of Respiration PAO2 = PIO2 – PaCO2 / R [PIO2 = FIO2 x (barometric pressure – 47)] PaCO2 = k x VCO2 / VA

Mechanical Ventilation Variables Mode Objectives Clinical settings Complications

Mechanical Ventilation Variables Tidal volume Rate Total time (inversely related to rate): - inspiratory time (adjustable) - expiratory time (total time - inspiratory time) Flow (tidal volume / inspiratory time) Minute ventilation

Mechanical Ventilation Variables Trigger sensitivity FiO2 Pressure: - peak pressure - plateau pressure Compliance

Mechanical Ventilation Modes Limit variables rise no higher than some preset value and increase to preset value before inspiration ends = limit variable Cycle Variable that terminate inspiration = cycle variable

Breath characteristics Gas Delivery

Mechanical Ventilation Bird Limit: pressure, flow Cycle: pressure Bennett 7200 (volume assist-control) Limit: volume, flow Cycle: volume Pressure support Limit: pressure Cycle: flow

Mechanical Ventilation Modes Volume-targeted: Pre-set tidal volume Pressure-targeted: Pre-set inspiratory pressure Mandatory breaths: Breaths that the ventilator delivers to the patient at a set frequency, volume/pressure, flow/time Spontaneous breaths: Patient initiated breath

Mechanical Ventilation Modes Volume-targeted Controlled mechanical ventilation (CMV) Assist/control (A/C) mechanical ventilation Synchronized intermittent mandatory ventilation (SIMV)

Modes of Ventilation (CMV) Background: Full preset tidal volume at a fixed preset rate Rate and minute ventilation cannot  by patient effort Trigger sensitivity is locked out, need sedated or paralyzed

Modes of Ventilation (CMV) Advantages: Near complete resting of ventilatory muscles Respiratory muscle rest, secured minute ventilation Disadvantages: “Stack" breaths (air trapping) and develop barotrauma “AutoPEEP" with barotrauma or hypotension Need sedated or paralyzed Respiratory muscle atrophy Uses: apnea, little breathing effort, unstable

Modes of Ventilation (Assist/Control) Background: Full preset tidal volume at a minimum preset rate Additional full tidal volumes given if the patient initiates extra breaths

Modes of Ventilation (Assist/Control) Advantages: Near complete resting of ventilatory muscles Comfortable, respiratory muscle rest, secured minute ventilation Effectively used in awake, sedated, or paralyzed patients Disadvantages: Hyperventilate and become alkalotic “Stack" breaths (air trapping) and develop barotrauma “AutoPEEP" with barotrauma or hypotension Uses: apnea, little breathing effort, unstable

(SIMV + Pressure support) Modes of Ventilation (SIMV + Pressure support) Background: Preset tidal volume at a fixed preset rate Ventilator waits a predetermined trigger period Patient can take additional breaths but tidal volume of these extra breaths is dependent on the patient's inspiratory effort

(SIMV + Pressure support) Modes of Ventilation (SIMV + Pressure support) Advantages: Improved venous return: intermittent negative pressure (spontaneous) breaths More comfortable: more control over their ventilatory pattern and minute ventilation Disadvantages: Can result in chronic respiratory fatigue if set rate is too low; Uses: Weaning, bronchopleural fistula

Mechanical Ventilation Modes Pressure-targeted Pressure support ventilation (PSV) Pressure-control ventilation (PCV) Pressure-targeted assist-control (A/C-PC) Pressure-targeted SIMV (SIMV-PC)

Pressure support Ventilation Background: Patient triggers, a preset pressure support is delivered Terminate inspiration by flow rate Tidal volume and minute ventilation are dependent on the preset pressure and patient’s lung-thorax compliance

Pressure support Ventilation Advantages: Avoids patient-ventilator asynchrony More comfortable: full control over ventilatory pattern and minute ventilation Avoids breath stacking and autoPEEP (especially in patients with COPD) Disadvantages: Required patient’s triggering, cannot be used in heavily sedated, paralyzed, or comatose patients Respiratory muscle fatigue if pressure support is set too low

Pressure Control Ventilation Background: Ventilator control predetermined pressure in a fixed predetermined time and rate

Pressure Control Ventilation Advantages: Pressure and time are controlled High flow rate Disadvantages: Tidal volume variable Produced autoPEEP if inspiratory time too long Uncomfortable mode for most patients

Pressure-targeted assist-control (A/C-PC) All breaths machine-delivered at a preset inflation pressure Patients can  rate by triggering additional machine breaths if desired Same as assist/control volume targeted mode

Pressure-targeted SIMV (SIMV-PC) Fixed rate of machine-delivered breaths at a preset inflation pressure Patients can breathe spontaneously between machine-delivered breaths if desired Same as SIMV volume targeted mode

Mechanical Ventilation Objectives: Physiology Comfortable Least complications

Mechanical Ventilation Usual settings Minute Ventilation = RR X VT ( 5 - 8 L/min) VT = 5 - 8 mL / kg RR = 12 - 20 bpm PaCO2 = 35-45 mmHg pH = 7.35-7.45

Mechanical Ventilation Triggering: sensitivity of ventilator to patient’s respiratory effort. Flow or pressure setting that allows ventilator to detect patient’s inspiratory effort Allows ventilator synchronize with patient’s spontaneous respiratory efforts Improving patient’s comfort during mechanical ventilation. Setting: lowest but not self cycling Less work in flow triggering

Mechanical Ventilation Flow rate: adjust to patient’s comfort Usually > 60 L/min Pressure: Plateau pressure < 30 cmH2O

Positive End Expiratory Pressure (PEEP)  Functional residual capacity Move fluid from alveoli into interstitial space Improve oxygenation

PEEP In COPD : To offset auto-PEEP To improve oxygenation in acute lung injury To improve oxygenation, preload, afterload in cardiogenic pulmonary edema

PEEP Titration Goal : -  PaO2 -  FiO2 (< 0.5)   PEEP Titration Goal : -  PaO2 -  FiO2 (< 0.5) - No adverse effect: cardiac output lung compliance from overdistension ( plateau pressure) Obtained baseline respiratory & hemodynamic Change (PEEP) only , keep other parameter constant  PEEP 5 cmH2O q 15 – 20 min Repeat respiratory and hemodynamic data at each new PEEP level  

PEEP to previous level  O2 delivery O2 delivery SaO2 BP or co SaO2 No adverse effect SaO2 Compliance SaO2 Volume + vasopressor Adequate Tidal volume Indequate PEEP to previous level   Use current PEEP  FiO2 PEEP more

Contraindication Absolute: none Relative: unilateral lung disease, bronchopleural fistulae, intracranial pressure, high plateau pressure, pulmonary embolism

Ventilator Alarms Setting High minute ventilation 10-15% > set or target minute volume Low minute ventilation 10-15% < set or target minute volume High Vt 10-15% > set or target Vt Low Vt 10-15% < set or target Vt High-system pressure 10 cmH20 > average peak airway pressure Low-system pressure 5-10 cmH20 < average peak airway pressure Loss of PEEP 3-5 cmH2O < PEEP O2 analyzer 0.05 </> Fio2

Alarms & Troubleshooting High Peak Inspiratory Pressure: Secretions Patient biting ETT Patient coughing Changing patient’s clinical status Low Pressure Alarm or low PEEP alarm: Disconnect (check all connections) Apnea

Mechanical Ventilation Complications Disconnection Malfunction Patient-ventilator asynchrony hemodynamic effects: Barotrauma Ventilator-induced lung injury Oxygen toxicity Infection - ventilator-associated pneumonia - sinusitis

Mechanical Ventilation Complications Hemodynamic effects: Impaired venous return, increased pulmonary vascular resistance   cardiac output AutoPEEP Mean lung volume or mean alveolar pressure correlate best with hemodynamic effects

AutoPEEP Exhalation is not complete by the time the next breath is given: - expiratory airflow obstruction - high minute ventilation (> 15-20 l/min) Alveolar pressure & volume remain increased at end-expiration

AutoPEEP Adverse Effect Same effect as externally applied PEEP: - Hyperinflation -  cardiac output Difficult to trigger ventilator → ↑ work of breathing

AUTOPEEP Auscultation: persistent exhalation (specific but not sensitive) Untriggering Flow-time graphs End-expiratory hold

Steps for Reducing Dynamic Hyperinflation & AutoPEEP Eliminate unnecessary ventilation: Weaning Minute ventilation Permissive hypercapnia Expiratory airflow obstruction: Rx bronchospasm Keep airways free of secretions Maximize expiratory time: peak inspiratory flow rate to 70 – 100 l/min

Mechanical Ventilation Complications Oxygen toxicity High FiO2 is potentially injurious Tissue injury depends on FiO2 and duration of exposure and some diseases/conditions No evidence that sustained exposure to FiO2 < 0.5 causes tissue injury Lowest FiO2 with adequate tissue oxygenation Measurements to keep FiO2 < 0.5

Hemoglobin and 02 Transport 280 million hemoglobin/RBC. Each hemoglobin has 4 polypeptide chains and 4 hemes. In the center of each heme group is 1 atom of iron that can combine with 1 molecule 02. Insert fig. 16.32

Oxyhemoglobin Dissociation Curve Insert fig.16.34

Mechanical Ventilation Respiratory Distress Patient Ventilator (malfunction) Patient-ventilator asynchrony - flow rate - trigger - autoPEEP

Ventilator-Induced Lung Injury (VILI) Barotrauma : extraalveolar air, pneumothorax Diffuse alveolar damage, pulmonary edema

Overdistention

Overdistention may be regional Even a “normal” VT can create regional overdistention

Ventilator Management of ARDS INITIAL VENTILATOR TIDAL VOLUME AND RATE ADJUSTMENTS Calculate predicted body weight (PBW) Male= 50 + 2.3 [height (inches) - 60] Female= 45.5 + 2.3 [height (inches) - 60] Mode: Volume Assist-Control