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Respiratory Pathophysiology

From a broad view, breath is the set of "physiological processes that allow the use of O2 at the cellular level. In this perspective, breathing involves not only the respiratory system itself, the respiratory muscles, the central and peripheral nervous systems that control the ventilation, the cardiovascular system and blood, which carry O2 from the lung cells, and, also to the enzyme systems that allow the use of O2 to-cellular level.

EVALUATION OF RESPIRATORY FUNCTION

To evaluate a system as complex as the one described, there are multiple functional tests. From a clinical standpoint, this evaluation can be divided into three main parts:

a) Evaluation of mechanical properties of the lung and airways
b) Evaluation of respiratory muscles
c) Evaluation of gas exchange.

The equipment needed to perform these precise measurements are relatively complex, so they generally do pulmonary function laboratory. However, there are simple devices that can be used in hospital wards or in clinics. With these parameters are obtained not as precise as the previous ones, but useful for making clinical decisions. In this chapter we will emphasize the use of these simple tests.

Evaluation of mechanical properties of lung ventilation

From the point of view of ventilatory function, lung involves alveolar space, where gas exchange takes place, and the airways, allowing air to enter and exit the alveolar gas. The lung contains a specific permanent alveolar gas volume is only partially renewed with each breath. The volume of gas that exists in the lung at the end of a quiet breathing is called functional residual capacity (FRC) (Figure 261). Despite its importance, this volume is not measured in routine clinical practice because they require complex equipment.

The volume of air in and out with each breath is called tidal volume (VT), which is about 500 ml.

ventilatory lung function can be assessed by simple equipment, the ventilómetros, with measured ventilation or expiratory volume in one minute (VE), which equals: VE = VC



xf where f is the respiratory rate. Not everything

inhale fresh air reaches the alveoli, as a part of VC (approximately 150 ml) is in the airways. This volume of air inhaled, which does not participate in gas exchange is called dead space volume (EEV). Consequently, the fresh air reaches the lung with each breath is only 350 ml.

According to the above, the index actually related to gas exchange is the alveolar ventilation (VA), which is calculated as:

VA = (VC - VEM) xf

In practice, the VA estimates only from VE, since the measurement of VEM is too complex for current clinical use.

lung size

A useful way to evaluate the functional status of lung volume measurement, since there are diseases that increase or decrease. For this measure the maximum volume that can be exhaled after a maximal inspiration made. This volume, called vital capacity, varies according to size, age (decreases progressively from 20 to 25 years) - and sex (higher in males). This index can be measured with a ventilómetro or more complex equipment, spirometers.


Airways

Airways conduct air into the alveoli and oppose a resistance to airflow, which increases when they are tightened in various obstructive diseases. In the clinical setting, the resistance of airways assessed indirectly by measuring the output speed of the air during a forced expiration. For this patient is asked to perform a deep breath and then breathes out with maximum effort. In diseases with airway obstruction exit velocity of the air decreases. This could be assessed through forced expiratory volume in one second (FEV1), measured with a spirometer (Figure 261) or, more simply, with expiratory flow (PEF, Peak expiratory Flow English) with a flowmeter. Elasticity

pulmonary

In patients who are on mechanical ventilation, ventilators, it is useful to assess lung elasticity, as a index of the evolution of the underlying disease .- In example, in patients with pulmonary edema, this body is more rigid, a property that tends to return to normal as the swelling improves. The elasticity is evaluated through the distensibility or compliance of the respiratory system, which is the ratio between tidal volume and pressure necessary to inflate the lung:

distensibility = delta V / deltaP

The value of delta V to get a spirometer and the delta P of a manometer, both included in respirators.

Evaluation of respiratory muscles

can be considered that the respiratory system is made up of two components: first the lung, in charge of gas exchange, and the other a pump, formed by the respiratory muscles and rib cage in charge of moving air to produce gas exchange. This pump is critical because it must run continuously for a lifetime. Its failure leads to decreased ventilation alveolar hypoxemia and hypercapnia (respiratory failure).

inspiratory muscles are the ones who must bear all the burden, since the expiration is usually passive. Even when there are multiple functional tests that allow to know in detail the role of different respiratory muscles, the only simple enough for common clinical use is the measurement of maximal inspiratory pressure (MIP). "This index, which can be measured with an aneroid manometer or water, evaluating the strength of all respiratory muscles is diminished in cases of inspiratory muscle fatigue or neuromuscular disease.

Evaluation of gas exchange

is performed clinically by examining arterial blood gas, which measures the pressures of O2 and CO2 dissolved in the blood (PaO2 and PaCO2, respectively). In addition, the determination of arterial pH enables assessment of acid-base balance is closely related to PaCO2.


oxygen. Under normal conditions, the pressure of O2 in the alveolar gas is approximately 100 mm Hg. Because the thickness of the alveolar-capillary membrane is minimal, almost spread-alveolar O2 unimpeded into the pulmonary capillary blood, which is why the Normal PaO2 is only a few mmHg lower than the pressure of oxygen in the alveolar gas. This slight reduction works also the existence of venous blood that is oxygenated in the lungs, but passes directly to the arterial side (physiologic shunt). This blood comes from the bronchial veins and heart.
The normal value of PaO2 breathing air at sea level is about 90 mm Hg. The decrease in PaO2 is called hypoxemia. Although PaO2 measurement is extremely important, it is always useful to keep in mind that the index actually related to O2 transport to tissues is the content of O2 in arterial blood, representing the number O2 molecules contained in the blood. "It's important to note that there may be hypoxaemia with normal O2 content, which is of great importance to understand the compensation that is seen in diseases with chronic hypoxemia, and compensation for people living in areas of g-ran high. Carbon dioxide



cells continually produce CO2 as a result of the metabolism of carbohydrates and fats. This gas diffuses into the interstitium and then into the blood capillary, where it is transported to the lung. In the venous blood PCO2 approximately 45 mmHg, while in the alveolar gas is 40 mm Hg. The pressures in this balanced level very quickly, so that the PaCO2 in practice is equal to alveolar pressure of CO2 (PaCO2).

PaCO2 varies widely according to changes in alveolar ventilation. If there is a higher VA, more CO2 is removed and therefore falls under the normal limit PaCO2 35 mm Hg, which is called hypocapnia. The reduction of ventilation on the other hand, produces the opposite effect, increasing PaCO2 above 45 mm Hg, a condition called hypercapnia.

These changes in PaCO2 alterations in the concentration of hydrogen ions, which are discussed in the chapter on acid-base balance.