Table of Contents

A well-matched V/Q ratio is 1.0, i.e. the lung unit receives as much ventilation as blood flow, and this should establish ideal conditions for good gas exchange. Wherever the V/Q ratio is low, there is an excess of blood flow as compared to ventilation, and therefore the effluent blood will be relatively hypoxic. Wherever there is an excess of ventilation, the global CO2 clearance will be poor in spite of vigorous airflow because the amount of blood delivered to these units is insufficient. These conditions are relatively absent in the healthy organism, but they can arise in disease states such as COPD, asthma, pulmonary oedema, and under the effects of positive pressure ventiltion.

Global lung perfusion (Q)is equal to the cardiac output, and global lung ventilation (V) is equivalent to the minute volume. Under ideal circumstances, these flow rates would be equal, but in reality there is usually some inequality in the distribution of blood flow between different lung regions. The main reason for this inequality is the gravity-related vertical distribution of blood flow and pleural pressure, but there are multiple other regional factors which independently affect regional ventilation and blood flow.

West's Zones of the lung are physiologically distinct but anatomically blurry regions which represent different relationships between alveolar pressure, arterial pressure and venous pressure. Zone 1 is characterised by minimal perfusion (it is essentially dead space), Zone 2 is perfused intermittently, and Zone 3 has good perfusion and receives most of the blood flow. An additional fourth zone represents the region of lung which is collapsed and in which interstitial fluid pressure is the dominant Starling resistor.

The expired CO₂ waveform can identify a variety of pulmonary and airway pathology. It all but eliminates the need to auscultate the lung, for the lazy intensivist who never lays his hands on the patient. Do you really need to hear a wheeze? The end-tidal trace, sloping up, not only alerts you to the bronchospastic airways disease, but also to the fact that it is improving with your nebs.

Pulmonary vascular resistance is affected by numerous factors. It is lowest at FRC. Vasoconstricting stimuli include high and low lung volumes, hypoxia, hypercapnia, acidosis, low pulmonary blood flow and drugs like noradrenaline. Vasodilating stimuli include alkalosis, sepsis, high pulmonary blood flow rates and drugs like nitric oxide, milrinone, levosimendan, and sildenafil.

Context-sensitive half time  is the concept which relates the drug distribution into and out of tissue compartments to the change in plasma concentration after sustained infusion. If the infusion has not achieved a steady-state of concentration, the context-sensitive half time will more closely resemble the α half-life (distribution half-life).  If the infusion is at steady-state and the tissue compartment is well-saturated with drug molecules, the context-sensitive half time will resemble the β half-life (elimination half-life). The context-sensitive half time is not expressed as a number like a half-life would normally be, but is rather a constant.

This inhaled pulmonary vasodilator is becoming more of a footnote in the history of ARDS pharmacotherapy. Otherwise known as nitrogen monoxide, it is a free radical with the formula NO. At room temperature, it is colourless and non-flammable.

This chapter focuses on the physiological effects of CO₂, taking care to view it as a drug, including pharmacokinetics and pharmacodynamics. In summary, CO₂ is a potent inotrope and vasopressor which exerts its effects mainly by being an indirect sympathomimetic. It is also an anaesthetic, a bronchodilator, a cerebral vasodilator, and it causes QT prolongation.