Topics in Critical Care Medicine and Applied Physiology

An overview of the modalities of renal replacement therapy

Modalities of RRT can be classified into groups on the basis of the manner of access, the duration of therapy, and the dominant mechanism of solute clearance. RRT can be continuous, or intermittent - in which case it could be very brisk (IHD runs over 4 hours, using 120L of dialysate) or relatively slow (SLED runs over 8-12 hours). Access can be veno-venous (conventionally), or arterio-venous (historically, as well as in chronic maintenance dialysis). Depending on whether diffusion or convection (or both) are used to purify the blood, the modality is called haemodialysis, haemofiltration or haemodiafiltration.

Physical and chemical characteristics of dialysis membranes

Dialysis filters contain about 10,000 hollow fibres with a total surface area around 1-2m2. Desirable membrane properties are low cost, minimal thickness, maximal strength and biological compatibility. Most membranes these days are synthetic, highly biocompatible and approximately 100-200 μm thick  with an internal fibre diameter of 200-500 100-200 μm. Because the CICM examiners have never asked the trainees anything about this in either the fellowship or the primary exams, the minimum level of understanding for this topic is therefore actually less than this short summary.

Causes of acidosis in hyperlactataemia

Though the term "lactic acidosis" is imbibed together with mother's milk in one's medical training, the very concept of lactate causing acidosis is mired in sordid controversy. The question of whether or not lactate actually causes acidosis is itself debated; highly respected members of the intensive care community disagree on this rather important point. Less respected members are even more conflicted. Capture a senior intensivist, and corner them with the question: can infusion of Hartmanns cause lactic acidosis in an anhepatic patient? Why or why not? How can there be severe hyperlactataemia with a normal base excess? One may be disappointed with the results of this experiment.

Interpreting the shape of the flow-volume loop

Typically, when one thinks of flow-volume loops, one refers to the classical loops of forced expiratory spirometry. Most of what is written about flow-volume loops refers to these. Indeed, both the LITFL entry on flow-volume loops and the AnaesthesiaUK revision article use the formal pulmonary function test standard of flow-volume loop interpretation. The shape of the curves is quite similar, but the fundamental difference is in the fact that in the ventilator loop is by convention upside-down.

Multi-organ system failure: the "Final Common Pathway"

Definition

Multi-organ-system-failure is a constellation of several horrible problems, each of which would be life-threatening all on its own. This entity is a luxury of our enlightened age. In the olden days people never survived long enough to develop this syndrome.

As per Oh's Manual, The Definition is: "in an acutely ill patient, the function in two or more organ systems altered such that homeostasis cannot be maintained without intervention"

Tricuspid stenosis

Keep the heart rate slow, give them milrinone if you absolutely have to, and avoid AF like the plague.

Physiological consequences of tricuspid stenosis

The limitation of right heart preload by a stenotic mitral valve manifests as right heart failure. Hepatomegaly and portal venous congestion with ascites tend to complicate the picture.

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