Section 1

Section 2

Section 3 

Section 4 

Section 5

Section 6

Section 7

Section 8

Section 9

Section 2b

MONITORING OF PULMONARY FUNCTION
(ADEQUACY OF OXYGEN EXCHANGE)

Percent Oxygen Saturation of Hemoglobin

The percent of oxygen saturation of hemoglobin in arterial and venous blood is a major factor in the oxygen delivery, oxygen extraction, and tissue oxygen utilization. The value of SaO₂ by itself does not reflect the adequacy of blood oxygen content if, e.g. anemia is present. In general, at the normal PaO₂ of 90 to 100 mmHg (at sea level, breathing room air) the SaO₂ is approximately 97%. As long as the PaO₂ is kept above 60mmHg, the SaO₂ will be at least 90% ( except co-toxicity is present). Increasing the PaO₂ above 100 mmHg will have little effect on the SaO₂ because the oxygen saturation cannot exceed 100%.

Arterial Oxygen Tension

This parameter measures the amount of oxygen dissolved in plasma and determines the percent saturation of hemoglobin, the major factor in blood oxygen content. Arterial oxygen tension (PaO₂ ) will be affected by pulmonary processes impairing oxygen exchange, i.e., impaired diffusion, increased shunt, V/Q mismatch. (Normal value is greater than 70mmHg on room air)

Alveolar-Arterial Oxygen Gradient

This parameter is a more sensitive measure of an impairment to oxygen exchange from lung to blood.

Although measurable at any given FiO₂ , it is commonly determined with the patient inhaling 100% oxygen for 20 minutes. The normal gradient is less than 50mmHg on 100% oxygen (less than 30 mmHg on room air).

ADEQUACY OF VENTILATION

Arterial Carbon Dioxide Tension

The normal value of PaCO₂ is 39 to 43 mmHg with the normal value in mixed venous blood being 42 to 50 mmHg. The alveolar (end-expiratory) PCO₂ should be numerically close to PaCO₂, assuming reasonable uniformity of ventilation. An increase in PaCO₂ reflects either an impairment of respiratory drive or impaired CO₂ clearance by the lung as a result of an increase in dead space ventilation. Both are characteristics of smoke inhalation.

Respiratory Rate

A respiratory rate below 8 or above 25 reflects ventilatory dysfunction. A low value reflects a decrease in CNS drive while an increased rate reflects impaired CO₂ removal. An increased rate markedly increases the work of breathing.

Work of Breathing

Work of breathing is an important parameter in ventilation. Normally, less than 5% of total oxygen consumption (VO₂ ) is used for the work of breathing. An increase in dead space, or shunt, or decrease in compliance can markedly increase the work load. If the increased oxygen demands cannot be met or muscle fatigue occurs, respiratory distress will occur. Chest wall retractions and increased diaphragmatic breathing are markers of excess work after the result of bronchospasm on airways edema.

Minute Ventilation

Minute ventilation (V) is defined as total air movement through the lungs per unit time. The value of V includes both alveolar and dead space ventilation:

V - tidal volume x rate A value exceeding 8 liters/min usually reflects lung injury and impaired CO₂ removal.

Dynamic Lung Compliance

This parameter is a measure of the ease of lung expansion during air flows.

C_DYN= tidal volume/peak inspiratory pressure - PEEP

It is a reflection of not only lung expansibility but resistance to flow of air. The resistance includes the ventilator, the airways, and chest wall. A value less than 50ml/cm H₂O reflects a stiff chest wall, lung or airways.

Static Lung Compliance

Static lung compliance (C_STAT) is a measure of the recoil properties of lung and chest wall or the ability of the system to stay inflated at end inspiration. C_STAT is measure at no flow, at the end of inspiration. The V_T is held in the lung at the end of inspiration and pressure measured. On a ventilator, this is obtained by using an end-inspiratory pause or plateau, the pressure now measured at no air flow is called the plateau pressure:

C_DYN = tidal volume/plateau pressure - PEEP

Decreased C_STAT can reflect a tendency toward alveolar collapse, such as surfactant denaturation or alveolar edema. A stiff chest wall will also decrease C_STAT. A value less than 60ml/cm H₂O indicates a stiff lung or chest wall.