Slides 68 - 85 Ventilation
Expansion of the lower chest is approximately 50% greater than that of the upper chest. This is because:
-Configuration of the thoracic bony structures and the action of the respiratory muscles -Action of the diaphragm preferentially inflates the lower lobes of the lung
two factors that affect regional distribution of gas in the healthy lung:
-Relative differences in thoracic expansion -Regional transpulmonary pressure gradients.
There are several methods to measure mechanical and metabolic work of breathing
-clinical exam -metabolic cart to measure work of breathing -transdiaphragmatic pressure -esophageal balloon placement -calculations by ventilator -diaphragmatic electromyogram -ultrasonography of diaphragm
The oxygen cost of breathing in healthy individuals averages from:
0.5 to 1.0 ml of O2 per liter of increased ventilation
At FRC the apices have more lung volume than the bases, but the bases receive
4 times the ventilation because they expand more
Expansion of the lower chest is approximately
50% greater than that of the upper chest
Time Constants =
Compliance X Resistance
frequency dependence of compliance
Compliance of the lung appears to decrease as breathing frequency increases
what are two factors account for this unevenness?
Regional and local
Patients with decreased compliance, increased elastic work of breathing will assume:
a rapid, shallow breathing pattern
Pulmonary disease patients ability to perform
any exercise is limited partially due to this problem
This is less than 5% of the
bodies total oxygen consumption
Each unit relies on its
compliance and resistance to fill
In pressure control modes, inspiratory time must be at least three time constant long to
deliver 95% of the volume that is possible with the given pressure settings and lung mechanics
Ventilation is still greatest in the
dependent lung zones which are the lowest regions
Individual respiratory units and their associated airways may
differ from each other and contribute to uneven ventilation in healthy lungs
Large tidal volumes increase the:
elastic component of work
The transpulmonary pressure gradient is not
equal throughout the thorax
At in increased breathing rates, units with long time constants
fill less and empty more slowly than normal units
high breathing rates increase:
frictional work
When more inspired volume goes to a smaller number of lung units, higher transpulmonary pressures must be
generated to maintain alveolar ventilation
This does not create a large difference since the distance between top to bottom is not as
great as the upright lung
Airway size influences
how much driving pressure reaches the distal lung units
Mismatching of ventilation and perfusion can result in
hypoxemia, severely limiting an individual's ability to perform daily activities
In disease, the distribution of ventilation can worsen and this can cause:
impairment of oxygen and carbon dioxide exchange
In the presence of pulmonary disease (either obstructive or restrictive), the oxygen cost of breathing may:
increase dramatically with increasing ventilation
Alveoli at the top of the lung are:
larger but distend less easily
Airway obstruction can cause high resistance to gas flow, the pressure drop across the obstruction may be large, so
less driving pressure to inflate the alveoli and therefore less volume
Lung units with high compliance have
less elastic recoil than normal
Lung units with short time constants fill and empty rapidly than
long units with normal compliance and resistance
Lung units with low compliance have high elastic recoil compared to
normal and they fill and empty faster but have a smaller volume (e.g., fibrosis).
Ventilation is
not distributed evenly in healthy lungs
In healthy individuals, the mechanical work of breathing depends:
on the pattern of ventilation
In the work of breathing the respiratory muscles consume:
oxygen
Changes in the transpulmonary pressure gradient are greatest in
peripheral alveoli so they expand more than central ones
The weight of the lung and gravity effect the
pleural pressure apex -10 cmH2O and base -2.5 cmH2O
Gravity also causes most blood flow through:
pulmonary capillaries to go to the bases
Gravity-dependent areas are also seen in individuals that are
recumbent, whether supine, prone, or right and left lateral
A lung unit has a long time constant if what is high?
resistance or compliance
Lung units have a short time constant when
resistance or compliance is low
The time constant is expressed in
seconds
In the upright lung, the weight of the lung tissues causes alveoli at the basis to be:
smaller but more easily distended
If dynamic compliance decreases as the respiratory rate increases,
some lung units must have abnormal time constants
in healthy lungs the pressure drop between
the airway opening and alveolus is minimal
In upright individuals, these factors direct more ventilation to the bases and periphery of the lungs than to
the apices and central zones
Patients with airway obstruction assume a ventilatory pattern that reduces:
the frictional work, slow deep breathing
Transdiaphragmatic pressure (Pdi) is
the inspiratory pressure generated by the diaphragm (pressure ∆ reflects muscle VO2)
The lung bases expand to a greater extend than
the lung apices
By placing the patient in a position in which functional lung units are dependent and diseased units are elevated,
the matching of ventilation and perfusion can be optimized and gas exchange often improves
Alveolar inflating pressure is directly related to
the pleural pressure
It varies within the lung, as well as from
the top to the bottom of the lung
The rate of oxygen consumption (VO2) by the respiratory muscles reflects:
their energy requirements (indirect measurement obtained from this information)
Lung units with high compliance have less elastic recoil than normal; therefore
they fill and empty more slowly (e.g., emphysema).
Lung units with high compliance have high time constant;
they fill and empty slower, lower volume in given time frame.
For the mode, expiratory time must be set to at least
three time constants for the lung to empty passively to 95% (i.e., 5% of inspired volume still remains).
Units with long time constants take longer to fill and to empty than
units with normal compliance and resistance.
Dynamic compliance is
used to assess pressure-volume relationships during breathing (it includes airway resistance)
The pressure-volume characteristics of the upright lung direct most ventilation to these dependent portions, thus matching
ventilation and blood flow to promote gas exchange
This phenomenon can be useful clinically when localized lesions (e.g. lobar pneumonia) cause
ventilation-perfusion abnormalities
Understanding of the time constant is essential
when setting mechanical ventilators
Gravity to a large extent, determines:
where ventilation goes in the lungs