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Figure 43-1. Normal Capnogram

(I) Anatomic dead space.

(II) Transition from anatomic dead space to alveolar plateau.

(III) Alveolar plateau (typically slight upward slope).

Reproduced from Longnecker DE, Brown DL, Newman MF, Zapol WM. Anesthesiology. Figure 31-7. Available at: Copyright © The McGraw-Hill Companies, Inc. All rights reserved.

  • Healthy lungs: 2–3 mm Hg awake, 5–8 mm Hg anesthetized
  • COPD: up to 10 mm Hg awake, 15–20 mm Hg anesthetized
  • Further increased by heat/moisture exchanger
  • Further increased by V/Q mismatch: PE, hypovolemia, lateral position

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Causes of Acute Changes in Capnogram
Exponential decreaseIncreased dead space (suspect PE or arrest)
Sudden drop to zeroLikely circuit disconnect
Gradual decrease but not to zeroLeakage or partial obstruction of airway
Air embolism, PE
Drop in CO: hypovolemia, IVC cross-clamping
Decrease in metabolic rate: hypothermia, deep anesthesia
Gradual increaseProlapse of expiratory valve or decreased minute ventilation
Sudden increaseRelease of tourniquet, aortic unclamping, MH
Figure 43-2. Changes in Capnogram in Different Disease States

(A) A normal capnograph demonstrating the three phases of expiration: phase I—dead space; phase II—mixture of dead space and alveolar gas; phase III—alveolar gas plateau. (B) Capnograph of a patient with severe chronic obstructive pulmonary disease. No plateau is reached before the next inspiration. The gradient between end-tidal CO2 and arterial CO2 is increased. (C) Depression during phase III indicates spontaneous respiratory effort. (D) Failure of the inspired CO2 to return to zero may represent an incompetent expiratory valve or exhausted CO2 absorbent. (E) The persistence of exhaled gas during part of the inspiratory cycle signals the presence of an incompetent inspiratory valve. Reproduced from Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 4th ed. Figure 6-25. Available at: Copyright © The McGraw-Hill Companies, Inc. All rights reserved.

1. Swedlow DB. Capnometry and capnography: the anesthesia disaster early warning system. Semin Anesth. 1986;3:194–205.
2. West JB. Respiratory Physiology: The Essentials. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2000.
3. Berengo A, Cutillo A. Single-breath analysis of carbon dioxide concentration records. J Appl Physiol. 1961;16:522–530.

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