Page 585 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 47: Upper Airway Obstruction 405

greater than 4000 predict turbulent flow. The driving pressure for laminar
flow is proportional to the flow rate and inversely related to the fourth
power of the radius, among other factors. Thus a relatively minor decrease
in the radius of a tube like the upper airway causes a large increase in the
driving pressure necessary to achieve the same flow rate. Turbulent flow
as occurs in a rough tube like the larynx and trachea requires greater
driving pressure than laminar flow, being proportional to the density of
the gas and the square of the flow rate. Inhalation of a low-density gas
such as helium therefore decreases the driving pressure required for air-
Hyoid flow through two mechanisms: reducing the Reynolds number, thereby
increasing the proportion of flow that is laminar; and decreasing the
Epiglottis density-dependent driving pressure where turbulent flow exists. 4
The clinical implications of these principles are as follows: (1) Upper
Aryepiglottic airway obstructing lesions, once symptomatic, can progress rapidly to a
fold crisis, with relatively small increases in size leading to proportionately
greater increases in the work of breathing; (2) the inhalation of a low-
Vestibule sure required for a given airflow. Helium-oxygen mixtures therefore may
Arytenoid density gas mixture such as helium-oxygen decreases the driving pres-
act to stabilize the airway in marginally compensated patients, bridging
Vestibular the patient toward definitive treatment of the upper airway obstruction.
fold Upper airway obstructing lesions may limit inspiratory flow, expi-
Ventricle ratory flow, or both, depending on the site and nature of the lesion.
Flow-volume loops obtained by spirometry are useful for understanding
Vocal cord
the physiology of different sites of upper airway obstruction. 5
Infraglottic ■ VARIABLE EXTRATHORACIC OBSTRUCTION
Thyrid cavity During normal inspiration the intrathoracic airways dilate, while the
cartillage
extrathoracic airways tend to collapse as the increase in gas velocity
causes a fall in intraluminal pressure (Bernoulli effect). As gas veloc-
ity increases past an obstructing lesion, this effect is increased, causing
dynamic collapse of the nonrigid airway and flow limitation, with a
Cricoid reduction in peak inspiratory flow and a flattening of the inspiratory
limb of the flow volume loop. In contrast, during forced expiration the
intraluminal pressure is positive relative to the atmosphere, preserving
expiratory flow. These lesions occur above the thoracic inlet and include
bilateral vocal cord paralysis, paradoxical movement of the vocal cords,
and tracheomalacia of the extrathoracic airway.
■ VARIABLE INTRATHORACIC OBSTRUCTION
FIGURE 47-1. Laryngeal anatomy. (Reproduced with permission from http://www. During forced expiration the intrathoracic airways have a tendency to nar-
wesnorman.com/lesson11.htm. Sagittal Section Through Larynx. Author Wesley Norman, PhD, DSc.)
row as a result of airway compression and the Bernoulli effect. This leads to
a reduction in peak expiratory flow and a flattening of the expiratory limb
PATHOPHYSIOLOGY OF UPPER AIRWAY OBSTRUCTION of the flow volume loop. Inspiratory flow, in contrast, is preserved as lung
expansion increases the radius of the airway at the site of the obstructing
While obstruction may occur at any point in the upper airway, laryngeal lesion. Common causes of variable intrathoracic obstruction include low
obstruction is most problematic because the airway is narrowest at this tracheal tumors and tracheomalacia of the intrathoracic airway. 6
point. The glottis is the narrowest region in adults, while the subglottic
region is the narrowest in infants. The basis for which seemingly minor ■ FIXED UPPER AIRWAY OBSTRUCTION
reductions in the cross-sectional area of the upper airway have impor- Fixed upper airway obstruction occurs when airflow at the site of obstruc-
tant effects on airflow is presented below. tion is insensitive to the effects of the respiratory cycle because the lesion
Alveolar ventilation is accomplished through the bulk flow of fresh gas imparts rigidity to the walls of the affected area. Affected patients have
down to the terminal bronchioles, at which point the cross-sectional area reductions in peak inspiratory and expiratory flow, while the flow volume
of the airways becomes so large that the forward velocity of gas molecules loop depicts flattening of the inspiratory and expiratory limbs. Examples
becomes negligible, and diffusive flow occurs. Prior to this point airflow of this disorder include subglottic stenosis and some tumors.
3
may be laminar, transitional, or turbulent. Laminar flow consists of orderly
streams of gas arranged in lines parallel to the airway. At higher flow rates
and at branch points flow may become transitional, with gas eddies that CLINICAL PRESENTATION AND INITIAL EVALUATION
break away from the parallel streams. Turbulent flow is the most disorga- Patients presenting with upper airway obstruction may complain of a vari-
nized pattern, and occurs at high flow rates. The Reynolds number (Re) is ety of symptoms including hoarseness, stridor, hemoptysis, dysphagia, ody-
a dimensionless number that derives from the ratio of inertial to viscous nophagia, drooling, and swelling of the neck or face. Dyspnea is typically
forces and allows prediction of whether flow will be laminar or not: exacerbated by exercise, and in the case of certain diseases—for example,
anterior mediastinal tumors—by the supine position. In many cases prior
Re = 2rvd/μ
evaluations have yielded a diagnosis of asthma or chronic obstructive
where r is the radius of the tube, v is the average gas velocity, d is the gas pulmonary disease. While certain disorders such as epiglottitis have very
density, and μ is the gas viscosity. Values less than 2000 predict laminar acute presentations, at times symptoms have developed so insidiously that
flow, values between 2000 and 4000 predict transitional flow, and values the patient has habituated to the condition, and has few or no complaints.

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