The body has its cycles. A great deal of attention to this point has spent on the cardiac cycle, the beating of the heart. But there is one other cycle near this frequency range that will concern our investigations here. That is the respiratory cycle, the breathing of the lungs. Much as the purpose of the cardiovascular system is to insure that blood volumes get to where they are needed throughout the body, the purpose of the respiratory system is to insure that gases get to where they are needed throughout the body and removed from where they are not.
The most readily apparent actors in this drama are the lungs, which are able to fulfill many of their roles with the help of the pressure changes caused by a thick muscular structure known as the thoracic diagram. Under normal circumstance, inhalation starts with the diaphragm contracting, flattening its curved shaped, increasing the volume of the thoracic cavity, inducing a negative pressure gradient and causing air to be drawn into the lungs. During exhalation, the diaphragm relaxes, returning to its dome shape, decreasing the volume of the lungs, increasing intrapulmonary pressure, and forcing air out of the lungs. This entire process takes about 2-5 seconds for average healthy adults (for an average rate of 12-30 breaths per minute).
Respiratory induced pressure changes are not merely confined to the lungs. The movement of the diaphragm and its and associated pressure/volume changes affect many other aspects of the body. Relevant to the interests of the work presented here are the effects such changes have on the cardiovascular system and more specifically on the measurement of hemodynamic parameters necessary to guide fluid treatment. These respiratory induced variations have the added benefit of acting as a kind of perturbation to the system, often in the form of cyclic loading. By dynamically exciting the system of interest, respiratory changes reveal more than a single static measurement can offer.
Dynamic respiratory responses are especially relevant in cases of mechanical ventilation, where a device assists or replaces a patients breathing by inducing a pressure gradient within the intrathoracic cavity. Ventilation can either be through positive pressure where air (or any specified gas mixture) is pushed through the trachea into the lungs or through negative pressure most famously embodied by the iron lung, where a large vacuumed en- vironment is formed around the outside of a patients chest, to force air to be pulled into the lungs. If we remember that much of the effort of clinicians in emergent and intensive care settings is to facilitate oxygen delivery throughout the body, then the prominent role of mechanical ventilation (to get oxygen to and remove carbon dioxide from the lungs of patients) in these settings becomes apparent. Mechanical ventilation can thus serve the dual role of treating the patient and enabling respiratory specific measures of circulating volume.
Variations in general
Perturbing a system and gauging its response is one of the most established means by which to characterize said system. By inputting a known signal, one can learn how the system behaves by measuring how the signal is modulated throughout. In the dynamic measures of volume status, some portion of the cardiovascular system is subjected to a perturbation (an increased pressure, an increased volume) and an end point is measured (vital sign change, stroke volume increase). The extent to which the input independent variable (say preload) causes a change in the dependent output variable (say cardiac output) can be viewed from a volume responsiveness perspective (in this case, how one moves along the Frank-Starling cardiac output curve). Variations in signals which are already present (such as stroke volume or the diameter of the inferior vena cava) in response to a stimulus thus offer a more nuanced view of volume status than any single static measure .
Many researchers have uncovered a trove of good techniques for assessing volume status and responsiveness using dynamic techniques based on respiratory and selective pressure loading including stroke volume and pulse pressure variation, esophageal Doppler monitoring, echocardiographic assessment of inferior vena cava collapse, and electrical impedance based volume measurements. The effectiveness and limitations of each of these techniques are discussed below.