Measurement of Respiratory Dead Space

BACKGROUND

The Respiratory Dead Space is the volume of breath that does not participate in the gas exchange and is the ventilation of the lungs without perfusion. Physiologic or total dead space is the sum total of alveolar dead-space and anatomic dead-space.

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The alveolar dead-space is the volume of gas within unperfused alveoli and is very negligible amount. The anatomic dead-space is the volume of gas within the conducting zone and amounts to about 2 ml/kg in the upright position.¹

The anatomic dead-space is measured by Fowler’s method. This amounts to around 450-500 ml.

REQUIREMENTS

Subjects:        Human

Apparatus:     Ruben Valve, Douglas Bag

PROCEDURE

Physiological dead space

The examination is carried out by subject lying on the bed supported by pillow. A nose clip is applied and the subject is asked to breathe in and out through Ruben non-return valve. The latter is firmly supported in the mouth by the standard mouth piece.

The expirate is passed through the dry gas meter and into the atmosphere via two-way tap. The rate of respiration and minute volume are recorded. Slowly the expirate is switched to Douglas bag once the respiration became steady.²

Physiological dead space is calculated as follows:

Alveolar dead–space is calculated as follows:

Anatomic dead-space:

The subject breathes out and deeply inhales from 0% nitrogen gas mixture and exhales into the equipment that measures nitrogen and gas volume. The exhalation has three phases.

The first phase has no nitrogen, the second phase nitrogen concentration increases and third phase is plateau phase. The anatomic dead-space is the sum of volume exhaled during the first phase and half of volume exhaled during the second phase.³

CONCLUSION

The dead-space is increased by following factors: general anesthesia, artificial airway, emphysema, age, neck extension, positive pressure ventilation, pulmonary embolus, pulmonary artery thrombus, hemorrhage, hypotension, emphysema.

REFERENCES

1. https://www.openanesthesia.org/aba_respiratory_function_-_dead_space. Accessed on 22 July 2018.
2. https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2044.1960.tb13355.x. Accessed on 22 July 2018.
3. https://en.wikipedia.org/wiki/Dead_space_(physiology). Accessed on 22 July 2018.

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