Table of Contents

Positive pressure ventilation affects preload, afterload and ventricular compliance. The net effect in most situations is a decrease in cardiac output. However, the effect may be beneficial in the context of decompensated heart failure, where the decreased preload and afterload result in a return to a more productive part of the Starling curve. In this rests the chief benefit of CPAP in the management of acute pulmonary oedema.

The pulmonary circulation is a low pressure, highly elastic system, with vessel walls which are much thinner and less muscular than the systemic circuit. At any given time, contains ~10% of the circulating blood volume, or about 500ml, and it varies by around 50ml over the course of a single cardiac cycle. The mean pressure in the pulmonary arteries is usually only 9-16 mmHg. The resistance of pulmonary arteries is quite low (100-200 dynes/sec/cm-5).

The lung has multiple non-respiratory functions. It acts as a filter for blood and a trap for dust particles. It has multiple immune and mechanical barrier functions. The lungs modulate acid-base balance, the clotting cascade, heat and water loss through respiration, and it metabolises important molecules like angiotensin-1.

The pulmonary circulation is a low pressure, highly elastic system, with vessel walls which are much thinner and less muscular than the systemic circuit. The pulmonary trunk divides into pulmonary arteries which can be divided into elastic (large), muscular (small) and nonmuscular (the smallest), though further subdivisions are histologically apparent. Pulmonary arteries and veins travel with bronchi, nerves and lymphatics in bronchovascular bundles, which are extensions of the visceral pleura. Pulmonary veins are thinner and more collagen-rich than pulmonary arteries. The bronchi are supplied by the systemic circulation which arises from the intercostal arteries on the right and from the aorta on the left.

The pulmonary circulation is a low pressure, low resistance system, and it contains much less blood than the systemic circulation (500ml vs. 4500ml). Where the systemic arterioles would vasodilate (eg. hypoxia, hypercapnia), the pulmonary arteries will do the opposite and vasodilate. The blood flow in the systemic circulation is distributed regionally according to organ system demands, using resistance arterioles to redistribute blood flow, whereas in the lung there nothing of the sort, and it can barely redistribute blood flow away from hypoxic regions. In short, the pulmonary and systemic circulatory systems are vastly different.

This is a hyponatremia where there is either an actual low volume state (and water and sodium are being appropriately retained by the kidneys), or a volume overloaded state (where the renal mechanisms have been fooled into retaining sodium and water by some other pathological process). This category encompasses true hypovolemia and SIADH, as well as the organ failures (cardiac, renal, liver), thiazide diuretic effects, hypoadrenalism and hypothyroidism.

There are two possible approaches to the diagnosis of hyponatremia: the classical method and the "lazy man's method". Both are offered here. In their most basic form, the approaches require essentially the same tests (serum osmolality, urine osmolality and urinary sodium). In the classical method, one is expected to examine the patient and come to a conclusion regarding their fluid status. The lazy man will of course do no such thing, as it requires for him to get up. Instead, the examination of the patient is deferred until the absolute last moment, and consists of deciding whether the patient has SIADH or cerebral salt wasting on the basis of their volume status and urine output.

Methylene blue, VA ECMO, high dose insulin, angiotensin-II, polyclonal IVIG, high cut-off haemofiltration membranes and dichloroacetate. All the weird exotic therapies for profoundly shocked sepsis are collected in this chapter, more for amusement than for education. Certainly, the value of these issues for the exam candidate approaches zero in terms of last minute revision.

The FRC is the volume of gas present in the lung at end-expiration during tidal breathing. It is composed of ERV and RV. This is usually 30-35 ml/kg, or 2100-2400ml in a normal-sized person. It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure.  The measurement of FRC is an important starting point for the measurement of other lung volumes, and its decrease has consequences for gas exchange and lung mechanics

Respiratory compliance is defined as the change in lung volume per unit change in transmural pressure gradient. It is usually about 100ml/cm H₂O. Static compliance is defined as the change in lung volume per unit change in pressure in the absence of flow. Dynamic compliance is defined as the change in lung volume per unit change in pressure in the presence of flow. Specific compliance is lung compliance which is normalised to a lung volume or capacity, which permits comparison between lungs of different size.

In the past papers, pulmonary hypertension crops up now and then as a sideshow to some other major problems, or as a comorbidity to discuss. For instance, in Question 12 from the second paper of 2017 the patient has among other things pulmonary hypertension, and the candisates were asked to discuss their periop Question 2 from the second paper of 2015 asks the candidates to identify the causes of pulmonary hypetension. The update article by Siomonneau (2013) is an excellent resource for this answer, and is used to generate the table offered below. For a more thorough review of this disease process, one may look at the massive 2015 ESC/ERS Guidelines. 

Question 1 from the second paper of 2009 and Question 24 from the second paper of 2006 asked for a detailed discussion about the utility of using CPP as a therapeutic target. Unfortunately, the college model answer was very brief; it probably reflects some sort of minimal level of competence. An ideal synopsis of this issue is offered by LITFL. No resource improves on the brevity or clarity of their short note on this concept. If brevity and clarity are not your thing, then you should probably read this 2013 review article from the BJA, by Kirman and Smith, and the canonical "Guidelines for the management of severe traumatic brain injury" from the Brain Trauma Foundation.

There are four volumes and four capacities recognised in respiratory physiology, where lung volumes are measurable gas-filled spaces in the lung, whereas capacities are combinations of two or more volumes (where the definition of capacity is the measure of the lungs' ability to hold a gas).