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Lung Mechanics and the Effects of Anesthesia

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CA-1 Didactic Lecture

Columbia University Irving Medical Center

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Aaron Mittel, MD

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Outline/ ABA ITE Keywords

  • Lung volumes and capacities: definition
  • PFTs
  • Flow volume loop
  • Lung resection outcome: PFTs
  • FRC: ventilatory setting effects
  • PEEP: Lung volume effect
  • PEEP to treat hypoxia
  • Obesity: Lung volumes
  • Morbid obesity: PFTs
  • Pregnancy: Lung volumes

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Basic Spirometry

  • Forced vital capacity = FVC = maximal volume of air exhaled with maximal force from a position of full inspiration
    • Low in airflow restriction (i.e. blockage), poor effort/weakness, restrictive lung disease
  • Forced expiratory volume in one second = FEV1 = maximal volume of air exhaled during the first second of FVC maneuver
    • Low in the presence of significant obstruction (e.g. bronchospasm)
    • Decreases with age
    • Predicted to be roughly 4 L for men, 3 L for women (though is IBW adjusted)
  • FEV1/FVC
    • Normal 0.75
    • < 0.70 associated with obstructive disease
    • > 0.75 associated with restrictive disease

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Other Spirometry Values (not exhaustive)

  • FEF25-75 = forced expiratory flow between 25 to 75% of the FVC = maximal mid-expiratory flow
    • Poorly reproducible
    • Thought to be unrelated to patient effort
    • “Small airway disease”, but not used in adults
  • Maximal volume ventilation = total volume of air exhaled during 12 seconds of rapid, deep breathing
  • DLCO = diffusing capacity for carbon monoxide = measure of the conductance of gas transfer from inspired gas to red blood cells
    • Affected by factors that change the membrane properties, hemoglobin, capillary blood volume

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Lung Volumes

  • Vital capacity = maximum volume exhaled after maximum inspiration
  • Functional residual capacity = FRC = volume of air remaining in chest at the end of a normal tidal volume breath
  • Residual volume = volume of air remaining after maximum exhalation
  • Expiratory reserve volume = ERV = volume of air exhaled from end-tidal volume to point of maximum exhalation
  • Inspiratory capacity = maximum inspiration from end-tidal volume to total lung capacity
  • Total lung capacity = volume of air in lungs at end of maximal inspiration
  • Tidal volume = normal breathing

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Lung Volumes, Quantified

  • Normal tidal volume = 6-8 cc/kg
  • Normal VC = 60 cc/kg +/- 20%
    • 10x tidal volume
  • FRC can not easily be measured (residual volume is difficult to quantify)
    • ~ 2.5 L
    • FRC is the gas tank for apneic gas exchange

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Practice Question #1

Following induction of general anesthesia in a 70 kg patient, assuming you have pre-oxygenated, how long can you completely ignore the patient before SpO2 falls?

    • 2 min
    • 4 min
    • 6 min
    • 8 min
    • 10 min

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Practice Question #1, Solution

  • VO2 at ~ 1 MET (i.e. anesthetized, not moving) = 3.5 cc O2/kg/min

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  • 70 kg * 3.5 cc O2/kg/min = 245 cc O2/min
  • 2.5 L FRC with 100% O2 (completely de-nitrogenated) = 2500 cc O2
  • 2500 cc / (245 cc O2/min) = 10 min

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Closing Capacity

  • Closing capacity = volume at which small airways (respiratory bronchioles) begin to close
    • Closing volume + residual volume
    • NOTE: this is not graphed on the usual “lung volume” figure!
    • Usually smaller than the FRC
      • Therefore… there is normally enough air in the lungs to keep small airways open during tidal volume ventilation

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Closing Capacity

  • Increased closing capacity 🡪 increased atelectasis/air trapping
    • Think… old, weak, distended airways can’t stay open
      • Leads to gradual increase in alveolar-arterial oxygen gradient
    • Small airways collapse during passive ventilation
    • Aging, COPD, asthma, etc
      • Age in 40s: closing capacity = FRC at supine
      • Age in 60s: closing capacity = FRC when upright

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Flow Volume�Loops

  • Volume is on x-axis
  • Flow is on y-axis
  • Spontaneous ventilation: inspiration is on bottom
  • Mechanical positive pressure ventilation: inspiration is on top
  • Dynamic = not same in inspiration + expiration

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Lung Resection Outcome: PFTs

  • Preoperative pulmonary function can be used to predict postoperative pulmonary complications
    • Exercise tolerance
    • Dyspnea, oxygen requirements
    • Spirometry
    • Lung volumes
  • Historically used to evaluate candidacy for pneumonectomy
    • Pulmonary artery occlusion (clamping) frequently therefore used as most “aggressive” measure of anticipating outcomes post-resection
      • (not realistic)

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Lung Resection Outcome: PFTs

  • Bad outcomes with…
    • FEV1 < 2L
    • MVV > 50%
    • RV/TLC < 50%

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Poor Outcomes after Lung Resection if…

  • Oxygen use problem: VO2 max < 15 mL/kg/min
  • Oxygen removal problem: ppoFEV1 < 30%
    • i.e. removing useful lung
  • Oxygen delivery problem: DLCO < 40%

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  • ppoFEV1 is the most predictive of pulmonary complications.
    • ppoFEV1% = preoperative FEV1% x (1 – %functional lung tissue removed/100)
    • ppoFEV1% > 40% is low-risk
    • ppoFEV1% < 30% is high risk

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FRC: Ventilatory Setting Effects

  • At FRC, inward elastic recoil of lung = outward elastic recoil of the chest (including resting diaphragmatic tone)
  • Positive airway pressure increases FRC by increasing trans-pulmonary pressure
    • Compliance = (change in volume) / (change in pressure)
    • Trans-pulmonary pressure = alveolar pressure – pleural pressure
      • PEEP = increased alveolar pressure (pressurizes the alveoli throughout the respiratory cycle)
      • By adding PEEP, you increase the FRC above the closing capacity!
  • The major effect of PEEP on the lungs is to increase FRC

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PEEP 201

  • PEEP may improve compliance
  • (Compliance) * (change in pressure) = change in volume
  • By increasing FRC, PEEP may improve region of lung that is “recruited”
    • Opens lung and allows for better V/Q matching
  • Alternatively, PEEP may over-distend areas that are already recruited
    • Over-distends = over-pressurizes areas
    • Worsens V/Q matching
  • PEEP must be effectively titrated to optimize compliance!

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PEEP

  • PEEP increases FRC
    • Compared to no PEEP
  • Adding PEEP increases FRC
    • Improves lung compliance
    • Improves V/Q matching
  • May lead to barotrauma by causing regional over-distention

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Positioning

  • FRC falls by 10% when lying supine
  • Increased abdominal pressure and “malpositioning” may occur during steep Trendelenburg

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Anesthetics and Lung Mechanics

  • Inhaled anesthestics decrease tidal volume, increase respiratory rate 🡪 increased dead space ventilation
  • Inhaled anesthetics 🡪 cephalad diaphragm, inward displacement of rib cage 🡪 reduction in FRC
    • Additional 20% reduction in FRC beyond what occurs in the supine position

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Obesity

  • Restrictive spirometry pattern
    • Decreased FEV1 and decreased FVC but preserved FEV1/FVC
    • Decreased compliance of the respiratory system (chest wall >> lung) due to fat layers
      • Lung compliance is unchanged
  • Dead space is unchanged
  • Tidal volume is unchanged
  • Closing capacity is unchanged
    • Reduced FRC can result in lung volumes below the closing capacity
    • V/Q mismatch, R -> L shunting
  • Reduced expiratory reserve volume leads to reduced FRC
    • Occasionally reduced residual volume
  • TLC is usually unchanged, but may be reduced in morbid obesity
  • Under anesthesia, FRC of the obese patient decreases by 50%

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Pickwickian Syndrome

  • Obesity hypoventilation syndrome due to decreased ventilatory response to CO2 and O2, resulting in sleep apnea, hypoxemia, hypercarbia, pulmonary HTN, polycythemia, and (eventually) biventricular failure (pHTN and RV dysfunction differentiates this from obesity hypoventilation syndrome)
  • Compared to the morbidly obese, have increased upper airway obstruction -> more CO2 retention -> less response to chronic hypercarbia
  • Both morbidly obese patients and Pickwickian patients will have increased O2 demand and increased metabolic rate (simply due to tissue mass)
  • Lung volumes are comparable to that of morbidly obese patients

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Pregnancy

  • Decreased FRC, due to compression of the diaphragm by the uterus
  • 5% reduction in TLC
  • Tidal volume increases by 30-40%
  • Minute ventilation increases by 30-40%

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Ascites

  • Restrictive type physiology due to abdominal swelling
  • Significantly impeded diaphragm movement when supin
  • All lung volumes are reduced
    • FEV1/FVC ratio preserved
  • Rapid, shallow breathing due to arterial hypoxemia

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Age-related Changes

  • Diminished responses to hypoxia, hypercapnia, stress with aging
  • Increased lung compliance
  • FRC increases
    • Higher residual volume
  • Closing capacity increases
  • Increased anatomic dead space
  • Decreased DLCO
  • Total lung capacity is unchanged

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Practice Question #2

Which of the following flow-volume loops potentially corresponds to a patient with the problem seen here?

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Practice Question #3

As we age, which of the following is true?

  1. FRC:TLC ratio increases
  2. Closing capacity:TLC ratio decreses
  3. Tidal volume decreases
  4. Ventilatory response to CO2 increases

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Practice Question #4

Which of the following is not true in obese patients?

  1. Lung compliance is unchanged
  2. FRC is reduced due to a reduction in residual volume
  3. Reduced FRC can create V/Q mismatch and arterial hypoxemia
  4. FEV1/FVC is reduced

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