A free online resource for Intensive Care Medicine.
An unofficial Fellowship Exam (CICM Part 2) preparation resource.
Deranged Physiologyis a slowly growing archive of discussions and study notes relevant (or if not relevant, then at least interesting) to the practice of intensive care medicine. The content provides an introduction to the fundamental themes in intensive care: mechanical ventilation, vasopressors, electrolyte management, hemodynamic monitoring, dialysis, and so forth. Attention is directed at equipment in intensive care, and there are attempts to revisit interesting pharmacology and physiology. The aim of this resource is to supplement the bedside teaching of senior staff, and to consolidate resources for intensive care trainees in the initial stages of their training.
Amperometry, or galvanometry, or polarography (turns out these are not interchangeable terms) is used around this site seemingly interchangeably to describe a sensory process where changes in current are measured ( though it turns out these are not interchangeable terms). Unlike the potentiometric electrodes, which measure the change in potential difference which is generated when ions migrate across a membrane, the amperometric chain subjects the migrating ions to a potential difference, and then measures the current which is generated.
The inspiratory hold manoeuvre abolishes the pressure contribution from the airway resistance and reveals the pressure in the alveoli. This is available on virtually every specialist-grade ventilator, and consists of a manual override of the expiratory valve, forcing it to close and essentially producing a super-syringe-style test of lung compliance, where the entire respiratory system (including the ventilator circuit) is challenged with a static volume..
Diffusion describes solute transport across a semi-permeable membrane generated by a concentration gradient. The major determinant of diffusion rate in dialysis is the concentration gradient; however several other factors influence the rate of diffusion. These factors include the characteristics of the membrane, the temperature of the solution, the available surface area and the diffusivity coefficient of the molecule (which is a complex composite measure of how fast a substance diffuses across a solvent volume, expressed in length squared per second).
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.
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 changing relationship of pressure and volume over the course of a breath can provide us with information about the compliance of the respiratory circuit. Inspiration creates a negative pressure, which gradually trends to zero as the lungs fill to the full capacity of the tidal volume. At expiration, the elastic recoil of the chest wall and lung tissue creates a positive pressure, which decreases towards zero as the volume is exhaled.
The flow waveform is the most interesting waveform. Much information can be derived from its shape. When flow is being used to generate a controlled level of pressure, the shape of the inspiratory flow waveform is informative regarding the necessary inspiratory time (if flow reaches zero, then the inspiratory time could be shorter without compromising volume). The expiratory flow pattern is also informative, as a slow return to baseline is an indication of the resistance to airflow.
The pressure waveform can give one information about the compliance of the different parts of the respiratory system. The waveform which is of greatest interest is the one generated when you put the patient on a mode of ventilation which features a constant inspiratory flow, such as a volume controlled mode of ventilation. In the presence of constant flow, the waveform represents the change in circuit pressure over time.
Though the terms are often used interchangeably, dynamic hyperinflation and intrinsic PEEP are distinct entities. Dynamic hyperinflation is the increase in end-expiratory volume caused by incomplete end-expiratory emptying of the lungs, and intrinsic PEEP is the raised alveolar pressure during expiration which is caused by this progressive hyperinflation. This is usually caused by Increased airway resistance causing airflow limitation, but it can also happen due to an increased respiratory rate with insufficient time for alveolar emptying.
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.
This device was asked about in Question 27.3 from the second paper of 2011. A more detailed exploration is carried out in a chapter dedicated to the Passy-Muir Valve from the Mechanical Ventilation section. In brief, it is a one-way valve which allows a uni-directional movement of air across a tracheostomy with an inflated cuff. Exhaled air comes out through the upper airway, allowing speech, expectoration of secretions, and a more effective cough.
This device is described in great detail in another chapter. This brief summary is constructed purely to improve the candidate's recall of its most important features, so as to answer questions such as Question 27.2 from the second paper of 2011. Thus far, several aspects of the DLT have been interrogated. Question 27.2 asked about the indications for its use.
The item discussed here is the Mallinckrodt size 8.5 endotracheal tube with an above-the-cuff suction port. There being millions of different types, I thought it would be better to just pick a representative style, and to discuss it. The suction port may be a bit of a gimmick, and many places don't use this style of tube. Of course, the "representative style" available to me was the one which was already unwrapped, in the ICU nurse educator's office. There are several structural features of note, which each deserve some brief mention. These features are common to the vast majority of ETTs.