Page 132 - Biomedical Engineering and Design Handbook Volume 2, Applications

CHAPTER 4

DESIGN OF RESPIRATORY

DEVICES

David M. Shade
Johns Hopkins Hospital, Baltimore, Maryland

Arthur T. Johnson
University of Maryland, College Park, Maryland

4.1 INTRODUCTION 111 4.5 COMMON RESPIRATORY
4.2 PULMONARY PHYSIOLOGY 111 MEASUREMENTS 126
4.3 IMPORTANT PRINCIPLES OF GAS 4.6 OTHER DEVICES 135
PHYSICS 114 4.7 DESIGN OF RESPIRATORY DEVICES 135
4.4 DEVICE COMPONENTS 118 REFERENCES 140

4.1 INTRODUCTION

Respiratory medical devices generally fall into categories designed to measure volume, flow, pres-
sure, or gas concentration. Many of these devices have been used in some form for many years, so
design is both sophisticated and incremental. A thorough knowledge of pulmonary physiology and
existing devices can be very helpful when proposing improvements. Most respiratory devices are
composed of one or more simpler components, often linked to a processing unit of some type, such
as a standalone personal computer (PC). Remarkably varied and sophisticated diagnostic instruments
can be constructed from these basic building blocks combined with values, tubing, pumps, and
mouthpieces.

4.2 PULMONARY PHYSIOLOGY

Before delving too deeply into the instrumentation, it will be helpful to review briefly the function
of the respiratory system and the parameters most often measured in pulmonary medicine.
The human respiratory system is composed of two lungs contained within the thorax, or chest-
cavity. The primary function of the lungs is gas exchange with the blood, providing a continuous
source of oxygen for transport to body tissues, and eliminating carbon dioxide produced as a waste
product of cellular metabolism. Gas exchange occurs in the alveoli, tiny thin-walled air-filled sacs
numbering approximately 300 million in normal adult lungs. The alveoli are connected to the out-
side environment through a system of conducting airways that ends with the oral and nasal cavities.
Alveoli are surrounded by and in close proximity to pulmonary capillaries, the tiny vessels containing

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