Table of Contents

The term "nitrate vasodilators" usually refers to drugs which act as nitric oxide donors. They are united by their mechanism of action, which leads to a NO-mediated activation of guanylate cyclase and smooth muscle relaxation. These drugs include inorganic substances such as sodium nitroprusside, organic nitrites such as amyl nitrite, and nitrates such as GTN. The chemical and pharmacological properties of this class of drugs is rather varied, and it includes rapidly active IV members (nitroprusside), volatile inhaled agents (nitric oxide) and long-acting oral alternatives (isorobide mononitrate)

The historical eponymous gas laws as identified below have subsequently been combined into the unifying Ideal Gas Law, which is discussed last. A brief summary of the important features is available elsewhere, for the purposes of revision. This chapter is more of a reflective digression on the history of gas research, lingering delicately on the important roles played in it by lamb bladders and mobs of angry French villagers.

In summary, crystalloid is currently “in”, and colloid is “out”. Recent guidelines (eg. Scandinavian guidelines) recommend crystalloid over colloid, and this reflects the broad trend in critical care over the last 20 years. All available evidence (eg. meta-analysis with n=16,000) points to there being no difference in mortality. Theoretical benefits in the use of colloid (eg. defence of the glycocalyx with albumin) or even known practical benefits (eg. a more rapid achievement of haemodynamic goals) have thus far not translated into measurable hard outcome advantages. Some colloids have a propensity to cause renal failure (eg. hydroxyethyl starch) or coagulopathy (eg. gelatins); all are more expensive then crystalloid (particularly human albumin).

This chapter is basically a set of long, enormously elaborate footnotes to the Wiggers Diagram, which describes the timing of pressure and volume changes in the chambers of the heart. In short, the cardiac cycle can be split into seven fairly predictable phases, each with their clearly defined boundaries, containing well described events which are a favourite of the CICM examiners.

This is a classification of antihypertensive agents by their mechanism of action, with a brief outline of each mechanism, and an example of a drug in each class.

Apart from "classical" varieties of antihypertensive drugs (ACE-inhibitors and ARBs, beta-blockers, calcium channel blockers, diuretics and nitrate vasodilators), there are multiple classes which are either rarely resorted to or have become forgotten because newer drugs have made them obsolete. This group includes alpha-1 antagonists, alpha-2 agonists, potassium channel activators, ganglionic blockers, phosphodiesterase inhibitors, inhibitors of monoamine transport and synthesis, and other drugs with mechanisms of effect so obscure that most pharmacology textbooks do not usually mention them.

The coronary circulation consists of three main arteries (LAD, LCx and RCA) and one main vein (the coronary sinus). The most exciting feature of this regional circulation is its ability to autoregulate its blood flow in a way which permits the heart to massively increase its metabolic demand. As myocardial oxygen extraction is already quite high (60-70%), coronary blood flow needs to increase to match demand. Mediators implicated in this autoregulation include variou metabolic substrates and bytproducts, such as ATP, adenosine, potassium, oxygen, carbon dioxide, lactate and hydrogen ions.

Diffusion and perfusion are the two main processes which govern the rate of alveolar-capillary gas transfer. Diffusion-limited gas exchange is where the rate of gas uptake in the capillary is determined by the rate of diffusion across the blood-gas barrier (eg. where the rate of diffusion from alveolus to blood is very slow). Perfusion-limited gas exchange is where the rate of gas uptake in the capillary is determined by capillary blood flow, eg. where the rate of gas diffusion into the capillary is very rapid. Carbon monoxide is a diffusion-limited gas, whereas oxygen is perfusion-limited under most normal circumstances.

Pressure-volume loops can inform us about  changes in the patient's lung compliance, air leaks, patient-ventilator dyssynchrony, and increased work of breathing.  For instance, they may reveal alveolar overdistension, or help determine the optimal level of PEEP (the so-called "critical opening pressure") for a patient with ARDS.

The trigger phase variable is said to determine whether a mode of ventilation can be described as "mandatory" or "spontaneous", but there is more to the “spontaneousness” of a mode. Generally, the patient is also given some control over how the breath is terminated (i.e the cycling variable is anything other than time-cycled). There may be even more input from the patient (for example, NAVA proportions the level of ventilator support according to diaphragmatic contraction). In contrast, mandatory modes of ventilation take this control away, in return for a totally effortless respiratory experience where you just lie back and let the blower do the work.

Transport across the cell membrane can occur by diffusion or by active transport. Diffusion can take the form of simple passive diffusion or "facilitated" diffusion which occurs via pores or protein channels. Active transport can be primary (where ATP is used as the energy source) or secondary (where concentration gradients are used as the source of energy). Endocytosis and exocytosis are mechanisms of delivering larger molecules into and out of the cell, and involve the transport of such molecules inside membrane-bound vesciles.

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.

This is a condensed revision of Ray Raper's chapter on post-op management of cardiothoracic surgical patients in the ICU. It has been expanded upon by details from the 2004 update to the 1999 AHA guidelines relating to the management of patients recovering from coronary artery bypass graft surgery.