This is about 2 ml/kg (150 ml in an adult). Early measurements of the volume were obtained by taking casts of the conducting airways but it may be measured non invasively by Fowler's method.
In Fowler's method the patient takes a single VC breath of 100% oxygen and exhales through a rapid nitrogen analyser. Expired N2 concentration is then plotted against expired volume (Figure RR.6). The initial gas from the dead space (Phase I) is free of nitrogen being pure oxygen, thereafter the nitrogen concentration increases with the introduction of alveolar gas (Phase II) until an alveolar' plateau is achieved (Phase III).
II Ml IV
II Ml IV
Figure RR.6 Fowler's method of dead space measurement
The dead space is found by dividing Phase II by a vertical line such that area A = area B and measure volume from zero.
Anatomical dead space will vary with changes in bronchial muscle tone and also with changes in position of the head and neck or the placing of an endotracheal tube. A functional decrease in anatomical dead space occurs at low VT's when gas flow in the airways is laminar.
Only fresh gas reaching the alveoli takes part in gas exchange. In each breath, only a portion of each VT, will reach the alveoli due to anatomical dead space (VD). This portion is the alveolar volume (VA), f p. for Vt = son nil and Vo - 150 ml Alveolar Ventilation
Alveolar ventilation is the volume of gas per min reaching the alveolar spaces. It can be calculated from the respiratory rate (n) and alveolar volume (VA), c.n- for Va = pi I and n ■ 15 breiths/miri
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