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