2 - Basic Concepts of Mechanical Ventilation

external respiration

gas exchange occurring between alveoli and pulmonary capillaries

internal respiration

gas exchange occurring between systemic capillaries and tissues

accessory muscles for inspiration

• scalenes

• sternocleidomastoids

• pectorals

• trapezius

accessory muscles for expiration

• rectus abdominus

• external obliques

• internal obliques

• transverse abdominals

• serratus

• latissimus dorsi

mechanisms of inspiration

1. respiratory muscles contract

2. decrease in intrapleural and alveolar pressures

3. alveolar pressure < airway pressure

4. gas enters lungs

mechanisms of expiration

1. respiratory muscles relax

2. decrease in thoracic volume

3. increase in alveolar pressure

4. gas exits lungs

pressure gradients during ventilation

• airway opening pressure (P_awo)

pressure at the airway opening

• usually 0 or atmospheric pressure (P_atm)

pressure gradients during ventilation

• body surface pressure (P_bs)

pressure at the surface of the lungs

pressure gradients during ventilation

• intrapleural pressure (P_pl)

pressure within the pleural space

• end-expiration: −5 cmH₂O

• end-inspiration: −10 cmH₂O

• hard to measure; measured via esophagus (P_ES)

pressure gradients during ventilation

• alveolar pressure (P_A, P_alv)

pressure in the alveoli

• inhalation: −1 cmH₂O

• exhalation: +1 cmH₂O

pressure gradients during ventilation

• transairway pressure (P_TA)

pressure gradient between airway opening and alveoli

formula:

__ = P_awo − P_alv

pressure gradients during ventilation

• transthoracic pressure (P_W, P_TT)

pressure gradient between alveoli and body surface

formula:

__ = P_alv − P_bs

pressure gradients during ventilation

• transpulmonary pressure (P_L, P_TP)

pressure gradient between alveoli and pleural space

formula:

__ = P_alv − P_pl

pressure gradients during ventilation

• transrespiratory pressure (P_TR)

pressure gradient between airway opening and body surface

formula:

__ = P_awo − P_bs

V/Q mismatching

• shunt

perfusion without ventilation

formula:

Q_s / Q_t = (C_cO₂ − C_aO₂) ÷ (C_cO₂ − C_vO₂)

V/Q mismatching

• ranges for shunt

normal	<10%
mild	10-20%
significant	20-30%
critical	>30%

V/Q mismatching

• dead space

ventilation without perfusion

types:

• anatomical (V_Danat)

• alveolar (V_DA)

• physiological (V_D)

• mechanical

formula:

V_D / V_T = (P_aCO₂ − P_ECO₂) ÷ P_aCO₂

normal range for lung compliance (C_L)

• non-intubated

0.05-0.17 L/cmH₂O (50-170 mL/cmH₂O)

normal range for lung compliance (C_L)

• intubated

• males: 40-50 mL/cmH₂O, up to 100 mL/cmH₂O

• females: 35-45 mL/cmH₂O, up to 100 mL/cmH₂O

dynamic compliance (C_dyn)

lung compliance during airflow (R_aw)

formula:

__ = V_T ÷ (PIP − PEEP)

static compliance (C_stat)

lung compliance without airflow (R_aw)

formula:

__ = V_T ÷ (P_plat − PEEP)

normal range for airway resistance (R_aw)

• non-intubated

0.6-2.4 cmH₂O/L/sec

normal range for airway resistance (R_aw)

• intubated

≥ 6 cmH₂O/L/sec

normal range for airway resistance (R_aw)

• non-intubated emphysema/asthma

13-18 cmH₂O/L/sec

formula for R_aw

__ = P_TA ÷ °V

or

__ = (PIP − P_plat) ÷ °V

time constant (T_c)

amount of time (sec) needed to fill and empty ~63% of lungs

• ≥ 3 units needed to allow adequate exhalation

formula:

__ = R_aw × C_stat

RR for high-frequency positive pressure ventilation (HFPPV)

60-100 breaths/min

RR for high-frequency jet ventilation (HFJV)

100-400 breaths/min

RR for high-frequency oscillatory ventilation (HFOV)

up to 4,000 breaths/min

baseline pressure = 0

no extra pressure is added at airway opening during expiration and before inspiration

baseline pressure > 0

PEEP present

• extrinsic: set by RT

• intrinsic: auto-PEEP (air trapping)