MECHANICAL VENTILATION

MODULE IIIJeffery C Rooks (Craig) MMHC, RRTArna Banerjee MD, MMHC, FCCM

Learning Objectives• Select appropriate initial ventilator settings

(ventilator prescription).• Modify the ventilator prescription in response

to pressure changes and arterial blood gas analysis.

• Describe the differences between mandatory and spontaneous modes of mechanical ventilation.

Ventilator Circuit

• Oxygen/air source• High pressure lines to ventilator• Inspiratory circuit to patient• Expiratory circuit• Exhalation

Other Features

• Humidifier/heater• Bacterial filters• Attached suction devices

Control panel• Airway pressure gauge/digital display• Buttons for entry of settings• General alarm displays• Others– Flow rate– Pressure volume curves– Inspiratory to expiratory ratio (I:E)

Mr. Z, a 43-year-old, 60-kg patient, is admitted with a multidrug overdose. After observation in the intermediate care unit for 2 hours, the nurses have observed a worsening of the patient’s mental status with a decline in his GCS score to 6. The house physician has intubated the patient. After transfer to the ICU, the staff calls you to confirm the mechanical ventilator parameters ordered by the house physician.

CASE I

MODES OF VENTILATION

Use the mode that you are most familiar with

Settings

• Mode AC or SIMV • Rate 12-15 breaths/min• Tidal volume (VT) 8 mL/kg ideal body weight • FIO2 100% • PEEP 5 cm H2O• PSV 5-10 cm H2O, if mode is SIMV (+PSV)

Assist Control Ventilation (AC)

• Every breath delivered to patient is a mechanical breath (breath may be triggered by a timing mechanism – respiratory rate or patient effort)

• Tidal volume is preset• Breaths are delivered at a preset

frequency/rate• Pressure is variable throughout the delivered

breath

Assist Control

Volume ControlledYou set:

Tidal volumePeak flow (or I:E)

Rate

Patient efforts recognized by ventilator

Assist Control (A/C)

Pressure SupportCPAP

PS/CPAP

Positive pressure maintained in the ventilator circuit during the

inspiratory phase

Inspiratory limb

Expiratory limb

To patient

Pressure Support

Set PS level

CPAP level

Time (sec)

Flow(L/m)

Volume(mL)

Flow Cycling

SIMVSynchronized Intermittent Mandatory Ventilation

• Volume Control + Pressure Support• Mandatory breaths are Volume Control

breaths (controlled)• Spontaneous breaths are pressure support

(supported)• Ventilator provides mandatory breaths which

are synchronized with patient’s spontaneous efforts at a preset rate

Spontaneous Breaths

Flow(L/m)

Pressure(cm H2O)

Volume(mL)

Modifying Ventilator Settings

30 minutes after initiating mechanical ventilation, you obtain the following ABG:

• What abnormalities does this show?– Acute (uncompensated) RESPIRATORY ACIDOSIS– Supranormal OXYGENATION

• What adjustments should you make?

• To treat the RESPIRATORY ACIDOSIS:– Increase the respiratory rate to improve the

minute ventilation

• To treat the SUPRANORMAL OXYGENATION:– Decrease the FiO2

• Repeat the ABG in 20-30 minutes

Tips

• PaC02 > 45 (or ETC02 > 50)– Increase Respiratory

Rate– Increase Tidal Volume

• PaC02 < 35 (or ETC02 < 30)– Decrease Rate– Decrease Tidal Volume

• Pa02 < 60 (Sp02 < 90%)– Increase Fi02– Increase PEEP

• Sp02 > 95% (or appropriate oxygenation for patient)– Reduce Fi02– Reduce PEEP to 5

Pressure Pressure

Pressure

Dynamic pressure OR

Peak pressure(Ppeak)

Static pressure OR

Plateau pressure

(Pplat)

What pressures are we interested in?

Increased Resistance

• Normally Pplat = Ppeak – 5to10 cm H2O• Why are we more interested clinically in Pplat?

Ppeak Pplat

Puts a pause in the Inspiratory Cycle – no flow – measures pressure

• Estimates alveolar pressure at end-inspiration• Indirect indicator of alveolar distension• Goal is Ppeak <40, Pplat <30

High Pressure

AlarmDifferential

Diagnosis

• Endotracheal tube compromise

• Acute bronchospasm• Pneumothorax• Gastric distension• Main stem intubation• Pulmonary

edema/change in lung compliance

• Patient-ventilator dyssynchrony

If hypoxemic or decompensating, disconnect from ventilator and initiate manual ventilation

• Auscultate lung fields and order Chest Xray• Confirmation of tube position and patency• Suction the Endotracheal Tube• Assess for tension pneumothorax• Bronchospasm • Patient agitated• Abdomen: rigid or distended

• Peak airway pressure – plateau pressure – if >10 cm H2O = airway resistance– if <5 cm H2O = decreased compliance

In the patient with significant distress while on the ventilator, disconnection from the ventilator and reassessment can be diagnostic and therapeutic. (NOT APPLICABLE FOR COVID +VE OR SUSPECTED)

If hypoxemic or decompensating, disconnect from ventilator and initiate manual ventilation

Decreased lung/chest compliance?

• Chest wall rigidity• Alveolar dysfunction/collapse

Increased airway resistance

• Airway obstruction• Bronchospasm• Circuit kink• Water in endotracheal tube and / or circuit• Check Filter if being used

• A 62-year-old, 70-kg admitted for a small bowel obstruction – aspiration pneumonitis -intubated

• SIMV rate 12, tidal volume 650 mL, PS 10 cm H2O, PEEP 5 cm H2O, FIO2 100%

• CXR - diffuse alveolar infiltrates• Peak Pressure 45 cm H2O • Plateau pressure is 40 cm H2O. • ABG: pH 7.35, PaCO2 45 mm Hg, PaO2 80 mm

Hg, HCO3 24 mmol/L.

Case II

Change to the Current Ventilator Settings

• Reduce the VT to 6 mL/kg– VT calculation should be based on

predicted body weight – not measured weight.

– Further reduce VT to 5 mL/kg if plateau pressure remains above 30 cm H2O.

– Reduction in VT may adversely affect CO2 clearance

The patient continues to demonstrate worsening oxygenation with a SpO2 of 86% on a FIO2 100%

What would likely improve oxygenation?

• Increase PEEP in 2- to 3-cm increments every 30-min intervals

• The effect is not immediate and as such it is generally recommended to allow 30 min after an adjustment before assessing efficacy

• Prolong the inspiratory time, thus the amount of time spent at the inspiratory driving pressure.

Improve Oxygenation

• 18-year-old man severe asthma exacerbation -bronchodilator and IV corticosteroids

• ABG severe respiratory acidosis PaCO2 >100• Intubated• Vital signs: HR 160, BP 70/55, R 30, SpO2 88%. • Auscultation: minimal air movement. • Ventilator alarms: high respiratory rate, low

tidal volume, and high peak airway pressure.

CASE III

What is “Auto-Peep”?

FLOWINSP

EXP

Expiratory flow ends before next

breath

Next breath begins before

exhalation ends

Obstructive lung disease (pursed lip)Rapid breathing (breath stacking)

Forced exhalation (anxiety)

Decrease Auto-PEEP• Ventilator maneuvers include actions to

enhance expiratory time. • Decreasing the set respiratory rate• Reducing the Tidal Volume• Increasing peak inspiratory flow rates.

Acknowledgement

• Raeanna Adams, MD, MBA• Nathan Ashby, MD• Rob Hood, MD• Meredith Pugh, MD, MSCI• Kimberly Rengel, MD• Kyla Terhune, MD, MBA

Questions ?