1.2 Respiratory Physiology, Lung Volumes & Chronic Pulmonary Pathophysiology

1.2 Respiratory Physiology, Lung Volumes & Chronic Pulmonary Pathophysiology

Intraoperative pulmonary management requires an intricate understanding of respiratory mechanics, gas exchange kinetics, ventilation-perfusion ($V/Q$) relationships, and the pathophysiologic alterations imposed by general anesthesia, muscle paralysis, and mechanical ventilation.

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1. Lung Volumes, Capacities & Anesthesia Effects

Respiratory function is categorized into four primary static volumes and four capacities (which consist of two or more combined volumes).

+-------------------------------------------------------------------------+
|                         LUNG VOLUMES & CAPACITIES                       |
+-------------------------------------------------------------------------+
| Total Lung Capacity (TLC ≈ 5800-6000 mL)                                |
| +-----------------------------------------------+---------------------+ |
| | Vital Capacity (VC ≈ 4500-4800 mL)            | Residual Volume     | |
| | +-----------------------+-------------------+ | (RV ≈ 1200 mL)      | |
| | | Inspiratory Capacity  | Expiratory        | |                     | |
| | | (IC ≈ 3500 mL)        | Reserve Volume    | |                     | |
| | | +---------+---------+ | (ERV ≈ 1100 mL)   | |                     | |
| | | | Tidal   | IRV     | |                   | |                     | |
| | | | Volume  | (3000   | |                   | |                     | |
| | | | (TV     |  mL)    | |                   | |                     | |
| | | |  500 mL)|         | |                   | |                     | |
| +-+---+-------+---------+-+-------------------+-----------------------+ |
|                           | Functional Residual Capacity (FRC ≈ 2300) | |
|                           +-------------------------------------------+ |
+-------------------------------------------------------------------------+

Primary Volumes & Capacities

• Tidal Volume ($V_T$): Volume inhaled or exhaled with each normal breath ($6 - 8 \text{ mL/kg}$ predicted body weight, $\approx 500 \text{ mL}$ in a 70-kg adult).
• Inspiratory Reserve Volume ($IRV$): Maximal volume inhaled above normal end-inspiration ($\approx 3000 \text{ mL}$).
• Expiratory Reserve Volume ($ERV$): Maximal volume exhaled below normal end-expiration ($\approx 1100 \text{ mL}$).
• Residual Volume ($RV$): Volume remaining in lungs after maximal forced exhalation ($\approx 1200 \text{ mL}$). Cannot be measured by simple spirometry (requires helium dilution, nitrogen washout, or plethysmography).
• Functional Residual Capacity ($FRC = ERV + RV$): Resting lung volume at end-expiration ($\approx 2300 - 2500 \text{ mL}$). Represents the equilibrium point where the outward recoil of the chest wall matches the inward elastic recoil of the lungs.

Factors Impacting FRC During Anesthesia

FRC functions as the body's primary oxygen reservoir during periods of apnea or hypoventilation.

• General Anesthesia: Reduces FRC by $15 - 20%$ (approximately $400 - 500 \text{ mL}$) within minutes of induction due to loss of phrenic nerve and intercostal muscle tone, cephalad displacement of the diaphragm by abdominal contents, and immediate development of dependent atelectasis.
• Positioning: Moving from upright to supine decreases FRC by $\approx 800 - 1000 \text{ mL}$. Prone, Trendelenburg, and lithotomy positions decrease FRC further.
• Surgical & Patient Factors: Obesity (exponential reduction in ERV), pregnancy (gravid uterus displacing diaphragm), laparoscopy (pneumoperitoneum pressure), and thoracic/upper abdominal incisions.

Closing Volume (CV) & Closing Capacity (CC)

• Closing Capacity ($CC = CV + RV$): The lung volume at which small, non-cartilaginous conducting airways ($<1 \text{ mm}$ diameter) in dependent lung regions begin to close during expiration.
• Relationship to FRC:
  • In healthy young standing adults: $FRC > CC$ (airways remain patent throughout the respiratory cycle).
  • Supine at Age $\approx 44$ Years: $CC$ exceeds $FRC$ in the supine position.
  • Upright at Age $\approx 66$ Years: $CC$ exceeds $FRC$ in the upright position.
  • When $CC > FRC$, airway collapse occurs during normal tidal ventilation, leading to absorption atelectasis, intrapulmonary shunting, and worsening arterial hypoxemia.

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2. Respiratory Mechanics: Compliance & Airway Resistance

+-------------------------------------------------------------------------+
|                        PULMONARY COMPLIANCE EQUATIONS                   |
+-------------------------------------------------------------------------+
|                                                                         |
|   Static Compliance (C_stat):     Dynamic Compliance (C_dyn):           |
|                                                                         |
|              V_T                             V_T                        |
|   C_stat = -----------------      C_dyn = -----------------             |
|            P_plat - PEEP                  P_peak - PEEP                 |
|                                                                         |
|   Normal: 60 - 100 mL/cmH₂O       Normal: 50 - 80 mL/cmH₂O              |
|   Reflects: LUNG/CHEST ELASTICITY Reflects: ELASTICITY + AIRWAY RESIST. |
+-------------------------------------------------------------------------+

Differentiating Elevated Airway Pressures

• Peak Inspiratory Pressure ($P_{peak}$): Pressure required to overcome dynamic airway resistance plus the elastic recoil of the respiratory system.
• Plateau Pressure ($P_{plat}$): Static pressure measured during an end-inspiratory pause (zero gas flow), reflecting true alveolar distending pressure and total respiratory compliance.

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3. West Zones of the Lung & Pulmonary Blood Flow

Pulmonary circulation is a low-pressure, high-compliance system where distribution of blood flow is governed by the vertical hydrostatic gradient between alveolar pressure ($P_A$), pulmonary arterial pressure ($P_a$), and pulmonary venous pressure ($P_v$).

                    [TOP OF LUNG (Apex)]
                          |
    Zone 1:  PA > Pa > Pv  --> Alveolar Dead Space (V/Q = Infinity)
                          |
    Zone 2:  Pa > PA > Pv  --> Waterfall Effect / Starling Resistor
                          |
    Zone 3:  Pa > Pv > PA  --> Continuous Flow / PA Catheter Zone
                          |
    Zone 4:  Pa > P_ISF > Pv > PA --> Pathologic Interstitial Edema
                          v
                   [BASE OF LUNG]

Characteristics of West Zones:

1. Zone 1 ($P_A > P_a > P_v$): Alveolar pressure compresses and collapses pulmonary capillaries. Ventilation occurs without perfusion (Alveolar Dead Space, $V/Q \to \infty$). Does not normally exist in healthy spontaneous breathing; induced by positive-pressure ventilation, high PEEP, hypovolemia, or severe hypotension.
2. Zone 2 ($P_a > P_A > P_v$): Flow is determined by the difference between arterial and alveolar pressure ($P_a - P_A$), acting as a waterfall or Starling resistor.
3. Zone 3 ($P_a > P_v > P_A$): Arterial and venous pressures exceed alveolar pressure, producing continuous, non-interrupted flow determined by $P_a - P_v$.
   • PA Catheter Tip Rule: Pulmonary artery catheter tips must be positioned in West Zone 3 to maintain a continuous, uninterrupted column of blood between the catheter transducer and the left atrium, ensuring that PAOP accurately reflects left atrial and left ventricular end-diastolic pressure.
4. Zone 4 ($P_a > P_{ISF} > P_v > P_A$): Occurs at the extreme lung bases in the setting of pulmonary interstitial fluid accumulation or dependent atelectasis.

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4. Ventilation-Perfusion ($V/Q$) Relationships, Shunt & Dead Space

• Global $V/Q$ Ratio: Total alveolar ventilation ($\approx 4 \text{ L/min}$) divided by cardiac output ($\approx 5 \text{ L/min}$) yields a resting normal $V/Q \approx 0.8$.
• Dead Space ($V/Q = \infty$): Alveoli are ventilated but not perfused.
  • Calculated via the Bohr Equation (modified by Enghoff): $\frac{V_D}{V_T} = \frac{PaCO_2 - P_{\bar{E}}CO_2}{PaCO_2}$
  • Normal $V_D/V_T$ is $0.20 - 0.35$ (or $\approx 2 \text{ mL/kg}$ anatomical dead space).
  • Increased by positive-pressure ventilation, pulmonary embolism, hypovolemia, erect positioning, and anticholinergic bronchodilation.
  • Results in hypercapnia and widening of the arterial-to-end-tidal $CO_2$ gradient ($PaCO_2 - P_{ET}CO_2$).
• Shunt ($V/Q = 0$): Alveoli are perfused but not ventilated.
  • Anatomic shunt ($\approx 2 - 5%$) via thebesian, bronchial, and pleural veins.
  • Physiologic shunt caused by atelectasis, endobronchial intubation, pneumonia, ARDS, pulmonary edema.
  • NCE Rule on Shunt: Hypoxemia caused by true absolute right-to-left shunt ($V/Q = 0$) cannot be corrected by administering 100% $FiO_2$ alone because shunted blood never contacts alveolar gas; it requires alveolar recruitment maneuvers and PEEP.

Hypoxic Pulmonary Vasoconstriction (HPV)

• Mechanism: A protective physiologic reflex whereby small pulmonary precapillary arterioles constrict in response to local alveolar hypoxia ($P_A O_2 < 60 \text{ mmHg}$). This diverts desaturated blood flow away from hypoxic, poorly ventilated alveoli toward well-oxygenated lung regions, minimizing intrapulmonary shunt.
• Inhibitors of HPV:
  • Volatile anesthetics in a dose-dependent manner (clinically significant at $>1.0 - 1.5 \text{ MAC}$).
  • Direct vasodilators: Sodium nitroprusside, nitroglycerin, hydralazine, calcium channel blockers, phosphodiesterase inhibitors (milrinone, sildenafil).
  • Hypocapnia/alkalosis, severe hypothermia, elevated left atrial pressure/volume overload.

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5. Oxyhemoglobin Dissociation Curve & The CADET Rule

The oxyhemoglobin dissociation curve describes the relationship between oxygen tension ($PaO_2$) and hemoglobin oxygen saturation ($SaO_2$).

• Normal $P_{50}$: $26.8 \text{ mmHg}$ (the $PaO_2$ at which $50%$ of hemoglobin is saturated with oxygen).

   Left Shift (Increased Affinity)         Right Shift (Decreased Affinity)
   Holds O₂ (Lower P₅₀)                    Releases O₂ (Higher P₅₀)
   -------------------------------         -------------------------------
   ↓ CO₂                                   ↑ CO₂ (Hypercapnia)
   ↓ H⁺ / ↑ pH (Alkalosis)                 ↑ Acidosis (H⁺ / low pH)
   ↓ 2,3-DPG (Banked Blood)                ↑ 2,3-DPG
   ↓ Temperature (Hypothermia)             ↑ Exercise
   Carboxyhemoglobin, Methemoglobin        ↑ Temperature (Fever)
   Fetal Hemoglobin (HbF)                  [Mnemonic: CADET Face Right]

$\text{Right Shift (CADET): } \mathbf{C}O_2 \uparrow, \; \mathbf{A}\text{cidosis} \uparrow, \; \mathbf{D}PG \uparrow, \; \mathbf{E}\text{xercise} \uparrow, \; \mathbf{T}\text{emperature} \uparrow$

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6. Obstructive vs. Restrictive Pulmonary Pathophysiology