Physiology of Mechanical Ventilation

Part of: Mechanical Ventilation

Lung Mechanics

Definitions

$\dot{V}=V_{\text{T}}\times \text{Respiratory Rate}$

• Where:
  • $\dot{V}$ is the minute ventilation
  • $V_{\text{T}}$ is the tidal volume

${\dot{V}}_{\text{A}}=\left(V_{\text{T}}-V_{\text{D}}\right)\times \text{Respiratory Rate}$

• Where:
  • ${\dot{V}}_{\text{A}}$ is the alveolar ventilation
  • $V_{\text{D}}$ is the physiologic dead space

$Q=\frac{V}{T}$

$P=Q\times R$

$\text{Compliance}=\frac{\Delta \text{Volume}}{\Delta \text{Pressure}}$

$Q=\frac{\Delta P}{R}$

• Where
  • $Q$ is airflow
  • $\Delta P$ is pressure gradient
  • $R$ is airway resistance

$\text{Ventilation Pressure}=\text{Resistive Pressure}+\text{Elastic Pressure}$

• Where
  • Resistive pressure is the pressure required to push airflow through the airways
  • Elastic pressure is the pressure required to inflate lungs and chest wall

Phases of Mechanical Ventilation

• Four distinct phases, each of which has a governing variable which determines how that phase proceeds:
  • Trigger phase: initiate phase controlled by the trigger variable
  • Inspiratory phase: controlled by the limit variable
  • Cycling phase: controlled by the cycle variable
  • Expiratory phase: governed by the PEEP variable; the patient exhales passively

Monitoring

$P_{\text{Peak}}\propto \frac{\text{Airway Resistance}}{\text{Compliance}}$ $P_{\text{Plateau}}\propto \frac{1}{\text{Compliance}}$ $R_{\text{insp}}=\frac{P_{\text{peak}}-P_{\text{plateau}}}{Q_{\text{end inspiratory}}}$

• An increasing $P_{\text{peak}}$ in the absence of an increasing $P_{\text{plateau}}$ suggests airway resistance is increasing (e.g. bronchospasm, excessive secretions, mucous plug, foreign body aspiration, extrinsic airway compression)

$\text{Compliance}=\frac{V_{\text{T}}}{P_{\text{plateau}}-\text{PEEP}}$

• An increasing $P_{\text{plateau}}$ suggests compliance is decreasing (e.g. pulmonary oedema, pleural effusion, pneumothorax, right mainstem bronchus intubation, ascites or other abdominal distension)

$P_{\text{peak}}$	$P_{\text{plateau}}$	Likely problem
Increased	Normal	Increased airway resistance
Increased	Increased	Decreased lung compliance

---

Gas Exchange

Normal Gas Exchange

• Alveolar ventilation equation:

$$P_{\text{a}}\mathrm{CO}{}_2=\frac{\dot{V}\mathrm{CO}{}_2\times P_{\text{I}}}{{\dot{V}}_{\text{A}}}$$

• Where
  • $P_{\text{a}}\mathrm{CO}{}_2$ is the partial pressure of $\mathrm{CO}{}_2$ in arterial blood
  • $\dot{V}\mathrm{CO}{}_2$ is the rate of systemic $\mathrm{CO}{}_2$ production
  • $P_{\text{I}}$ is the pressure of inspired air
  • ${\dot{V}}_{\text{A}}$ is the alveolar ventilation
• Importantly $P_{\text{a}}\mathrm{CO}{}_2\propto \frac{1}{{\dot{V}}_{\text{A}}}$

Mechanism	Examples
VQ mismatch	Pneumonia, PE, pulmonary oedema, COPD
Shunt	Congenital heart disease, pulmonary AVM
Thickening of the alveolar-capillary membrane	Interstitial lung disease, pulmonary oedema
Destruction of the alveolar capillary membrane	Emphysema

Monitoring

• ABG Interpretation
• Pulse oximetry
• Capnography
  • Note that $P_{\text{a}}\mathrm{CO}{}_2\approx P_{\text{ET}}\mathrm{CO}{}_2+3.5\pm 1.5$
    • However, the gap can be:
      • Increased to >5 mmHg in low cardiac output, COPD, PE, advanced age
      • Decreased to <2 mmHg in high cardiac output states (e.g. septic shock)

PEEP

Preload

• Increased intrathoracic pressure, thus
  • Decreased venous return,
  • Thus reduced left ventricular stroke volume
  • Thus reduced left ventricular contractility
  • Thus reduced left ventricular oxygen demand
  • If the left ventricle is decompensating because it is overfilled and overstretched ( “congestive” heart failure) the decreased preload will push it back into the more efficient area of the Frank-Starling curve.
• If the PEEP is causing hemodynamic instability, the patient needs more fluid.

RV Afterload

• ↑ Intrathoracic pressure ⇒ ↑ pulmonary artery pressure
  • ↑ RV afterload
  • Increased right ventricular work and oxygen demand

LV Afterload