12 - Introduction to Mechanical Ventilation (2/2)

normal pleural pressure

-5 cmH₂O

What can disruption of pressure cause?

pneumothorax or effusion

lung compliance (C_L)

ability of the lungs to stretch and accept air

static lung compliance (C_stat)

lung compliance without airflow

dynamic lung compliance (C_dyn)

lung compliance with airflow (R_aw)

airway resistance (R_aw)

pressure needed to move gas through airways

how to prevent barotrauma

keep plateau pressure (P_plat) ≤ 30 cmH₂O

C_dyn formula

C__ = V_T ÷ (PIP − PEEP)

PIP = peak inspiratory pressure; aka: peak pressure (P_peak)

C_stat formula

C__ = V_T ÷ (P_plat − PEEP)

P_plat = plateau pressure

R_aw formula

__ = (PIP − P_plat) ÷ flow

negative pressure ventilation

ventilation that mimics natural breathing

ex: iron lung

positive pressure ventilation

ventilation that forces air into lungs; standard method of mechanical ventilation

dead space ventilation

ventilation without perfusion

ex: pulmonary embolism

shunt

perfusion without ventilation

ex: atelectasis, ARDS

volume control (VCV)

MV mode that has a preset volume; pressure will vary

pressure control (PCV)

MV mode that has a preset pressure; volume will vary

pressure support (PS) and synchronized intermittent mandatory ventilation (SIMV)

MV modes that assist spontaneous breathing

parameters for breathing in MV

• trigger

  • time, patient effort

• limit

  • maximum volume allowed (like pressure)

• cycle

  • what ends breath (time, flow, volume)

volume control

• pressure graph

volume control

• volume graph

volume control

• flow graph

pressure control

• pressure graph

pressure control

• volume graph

pressure control

• flow graph

triggering for VCV

• time-triggered (mandatory) or patient-triggered

• pressure or flow trigger starts breath if patient effort detected

• trigger sensitivity must balance between responsiveness and auto-trigger

triggering for PCV

• time-triggered for mandatory breaths

• patient-triggering breaths use flow/pressure sensors

• preset pressure delivered for fixed time

cycling for PCV

time cycle: inspiration ends after preset i-time

• constant pressure given until time ends

• V_T varies based on C_L/R_aw

cycling for VCV

volume cycle: inspiration ends after set V_T delivered

• flow constant, pressure will vary

• may cause high pressure with poor C_L

cycling for PS

flow cycle: breath ends when inspiratory flow drops to set percentage of peak flow (usually 25%)

• patient-triggered and patient-cycled

• emulates natural breathing

MV settings and interpretation

• PEEP

__ improves oxygenation by preventing alveolar collapse

MV settings and interpretation

• F_iO₂

__ is titrated to maintain S_pO₂ ≥ 90%

MV settings and interpretation

• critical waveform indications

• air trapping (auto-PEEP)

• secretions

• patient-ventilator dyssynchrony