What is Physiological Dead Space



What is Physiological Dead Space?

S.M.Galvagno, D.O., Ph.D.

1. Definition: portion of each tidal volume that does not participate in gas exchange with capillary blood

a. Alveoli ventilated, but not perfused

b. Equal to sum of alveolar and anatomical dead space

i. Anatomical dead space: gas in tracheal and conducting airways

1. No gas exchange

a. Trachea & bronchi

b. Y-piece on ventilator tubing, etc.

2. About 150 mL normally

3. Measured by nitrogen washout method (Fowler’s)

ii. Alveolar dead space: volume contained in lung units without gas exchange

1. Normally only 5-10 mL in healthy patients

2. The Bohr Equation

a. VD= dead space

b. VT= tidal volume

c. PECO2= end tidal CO2

d. PaCO2= arterial CO2

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e. Normal value: 0.2-0.4

1. May increase to > 0.5 (50%) in lung disease

2. Elevated values are associated with ↑ death in ARDS

3. Causes of Increased Alveolar (and physiological) Dead Space:

a. Any process that disturbs normal V/Q matching

b. Any process that increases West zone I

Pulmonary Embolism

Air, Amniotic, or Fat embolism

Anticholinergics (atropine, scopolamine, glycopyrrolate)

ARDS

COPD

Asthma

Inhibitors of Hypoxic Pulmonary Vasoconstriction (HPV)

Excessive PEEP

Excessive Tidal Volumes

Decreased Cardiac Output

Posture (standing >sitting >supine)

Advanced Age

4. References / Further Reading

a. Wagner PD. Causes of a high physiological dead space in critically ill patients. Crit Care 2008; 12(3): 148.

b. Tang, Y, Turner MJ, Baker AB. Effects of alveolar dead space, shunt, and V/Q distribution on respiratory dead space measurements. Br J Anaesth 2005; 95(4): 538-548.

c. Nuckton TJ, Alonso JA, Kallet RH, et al. Pulmonary dead-space fraction as a risk factor for death in ARDS. N Eng J Med 2005; 346: 1281-1286.

d. West JB. Respiratory Physiology: The Essentials. Philadelphia: Lippincott Williams & Wilkins, 2008.

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