Table of Contents

The mechanism underlying the nerve impulse is the action potential, a phenomenon whereby the neuronal membrane depolarises and creates a current which in turn causes the depolarisation of nearby membrane areas. This is mediated by voltage-gated sodium channels which respond to a change in transmembrane voltage by opening (provided the threshold voltage is achieved). Nerve impulse propagation is faster in fibres that are myelinated, and in those which have a larger diameter.

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.

It is a general rule among intensivists, that our job satisfaction frequently varies inversely in proportion to our distance from the nearest ECMO circuit. It is therefore all the more remarkable that ECMO has seen so little attention from the CICM examiners, by whom all the major ECMO centres in Australia are extensively colonised. The only question ever to involve any discussion of the evidence was Question 11 from the second paper of 2010. Later, Question 23 from the first paper of 2014 asked about the indications for ECMO and the relative merits of VV and VA ECMO, but only a very limited discussion of the literature was required.  

The kidneys are paired retroperitoneal organs. The parenchyma is divided into the cortex (where the glomeruli and convoluted tubules are) and the medulla (where the straight tubules and ducts reside). From there, the urine drains into the minor, and then the major calyces, collecting ultimately in the renal pelvis.

Glomerular filtration is influenced by Starling forces acting on the glomerulus (for water) and the size and charge barrier of the glomerular filtration membrane (for solutes). The net ultrafiltration rate of the glomerulus depends on the permeability of the filtration surface to water and to the net filtration pressure, which is a balance of hydrostatic pressures and oncotic pressures between the glomerular capillaries and Bowman's space.

Renal acid-base handling consists of reabsorbing filtered bicarbonate, eliminating non-volatile acids, and local synthesis of ammonium which is eliminated as a means of removing acid (H⁺ by the classical interpretation) or chloride (by the physicochemical interpretation). Of these mechanisms, regulation of bicarbonate reabsorption is the most important mechanism of compensation for respiratory acid-base disturbances, and ammonium elimination is the most important mechanism of compensating for metabolic acid-nase disturbances.

Serum creatinine is a low-molecular-weight byproduct of muscle metabolism which is cleared exclusively by the kidneys and is therefore a convenient endogenous marker of renal function. Its relationship to glomerular filtration is non-linear, and large changes in glomerular filtration can take place before the serum creatinine will change. Its serum concentration is also affected by dietary protein, muscle metabolism, patient body mass, age, ethnicity and sex. Creatinine clearance can be estimated from the serum creatinine measurements and empirically derived values which estimate creatinine production.

Beta-blockers can be divided into selective and nonselective, depending on their affinity for β1 receptors where most of their desirable effects are exerted. Their main positive effects are a decrease in heart rate, decrease in cardiac contractility, and an increase in diastolic filling time, which results in a decrease in myocardial oxygen demand and an increase in coronary blood supply.They have additional positive antiarrhythmic effects. Negative (β2 receptor-mediated) effects consist of peripheral vascular vasoconstriction and increased bronchial smooth muscle tone.

GFR can be measured by the urinary clearance of a marker solute (such as inulin or creatinine) or by the plasma clearance of another suitable solute. The concentration of the solute in the blood and in the urine is used to calculate the volume of blood cleared of the marker per unit time. Renal blood flow can be approximated from renal plasma flow, which can be measured by the use of para-aminohippuric acid (PAH).

Diuretics do not have a defined classification system in the same way as antiarrhmics may, but they do fall into discrete classes. Carbonic anhydrase inhibitors, loop diuretics, osmotic diuretics, aldosterone antagonists, thiazides and drugs acting on the vasopressin-aquaporin system have discrete and unique mechanisms of actions, and they generally do not overlap. This chapter tries to consolidate all of these substances into some sort of logical structure.