Relative refractory time

The precise time from which on a new perturbation can excite the system depends on the stimulus. One therefore usually talks about the relative refractory time. 

However, there is a period of time following the absolute refractory period during which a premature electrical stimulus can be conducted, and is often conducted abnormally. This period of time is called the relative refractory period (Fig. 6-2).2 The relative refractory period corresponds roughly to the latter half of phase 3 repolarization on the action potential and to the latter half of the T wave on the ECG. If a new... 

O Ventricular tachycardia is usually initiated by a precisely timed VPD, occurring during the relative refractory period, which provokes reentry within ventricular tissue. 

FIGURE 23-1 T The cardiac action potential recorded from a Purkinje cell. The effective refractory period is the time during which the cell cannot be depolarized, and the relative refractory period is the time in which a supranormal stimulus is required to depolarize the cell. Action potential phases [0-4] and the ionic basis for each phase are discussed in the text. From Keefe DLD, Kates RE, Harrison DC. New antiarrhythmic drugs their place in therapy. Drugs. 1981 22 363 with permission.]... 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

Fig. 1 shows a graphical representation of these operational regions. WAO and SCWO processes are often referred to as hydrothermal oxidation technologies (HTOs). The major difference between the processes is that, in SCWO, organics are completely oxidized in a relatively short time (seconds to minutes), whereas in WAO, the reaction may require a longer time (minutes to hours). Furthermore, in WAO, some refractory organics are not completely oxidized because of the lower temperature of operation... 

The flash filament experiment as first described by Becker and Hartman (14) has since been used extensively in studies of the adsorption of gases onto refractory metals, particularly in association with other techniques. The basic method is to allow gas introduced at a known input rate to adsorb for a measured time onto a previously cleaned wire or ribbon. The gas is then desorbed by heating the sample, and the resulting pressure bursts monitored. The pressure versus time curve is referred to as a desorption spectrum, as illustrated in Fig. 4 and 5. Sticking probabilities can then be obtained from the relative adsorption times and desorption quantities. Methods of analysis of these desorption spectra (15, 16) and of the variation in thermal resolution by different heating schedules such as linear or reciprocal increase in temperature with time, have been discussed extensively by a number of authors... 

ERP includes both the action potential duration and the time for membrane repolarization and stimulus to threshold. Mexiletine, by blocking fast sodium channels, blunts the amplitude of the cardiac action potential (phase 0 is decreased). The decrease in sodium influx and decrease in phase 0 amplitude results in a lower membrane potential as the action potential enters phase 2 and 3. This results in the membrane potential, at the end of phase 0. being closer to the membrane potential for potassium and calcium, so the influx of these ions is of shorter duration, resulting in a decrease in the length of phase 1 and 2 and a steeper slope of phase 3 repolarization. Phase 4, however, is lengthened, and the rate of rise decreased, with the decrease in the rate of sodium influx. Thus, the cardiac membrane remains relatively refractory. even after repolarization, and the ratio of the ERP to the action potential duration increases. 

Besides the existence of a threshold for stimulation, excitable systems are characterized by the existence of a refractory period during which the response to further stimulation is reduced or even totally absent (Fitzhugh, 1961). As shown in fig. 5.33, the cAMP signalling system of D. discoideum shares this property. Represented in this figure is the ratio Aic/Aiii of the maxima of two successive peaks of intracellular cAMP, as a function of the time interval separating the second stimulus from the maximum of the first response, which defines time zero. For 4 min, no cAMP synthesis can be elicited by the second stimulus. This phase defines an absolute refractory period. Thereafter, the response to the second stimulus increases gradually, until the second maximum reaches the value of the first, after some 15 min. This second phase defines a relative refractory period. The two types of refractory period are known in other excitable systems, for example in nerve cells (Fitzhugh, 1961). 

The depolarization and repolarization phases of action events can occur quickly over intervals of tens of microseconds, although the actual durations depend very much on die cell type. During the time when the cell is depolarized, it cannot be restimulated to another action event. This interval is known as the cell s absolute refractory period. The cell s relative refractory period is the interval... 

The refractory carbides are hard and wear resistant, have high melting points, and are chemically inert. In a relatively short time, they have become major industrial materials with numerous applications such as cutting and grinding tools, bearings, textile-machinery components, oxidation-resistant gas burners, and many others. 

In order to avoid laborious and time-consuming procedures for converting the phosphoms to measurable phosphate ions some improved methods have been reported that may be effective for online monitoring, e.g. thermal induced digestion, ultraviolet photooxidation [110], and microwave-assisted digestion [81,97,105]. Whatever the choice, it is important to ensure that quantitative oxidation of relatively refractory organic phosphoms compounds, i.e. condensed phosphates, and further hydrolysis and release of P from polyphosphates occurs prior to the detection step. 

The fractionation of these refractory elements is beheved to be the result of relative efficiencies of incorporation of condensed sohds rich in early high temperature phases into the meteorite parent bodies at different times and locations in the solar nebula. The data are taken from Reference 3. 

Thermal oxidizers must be built to provide the residence time and temperatures to achieve the desired destruction efficiency (DE). As such, thermal oxidizers are comparatively larger than catalytic oxidizers since their residence time is two to four times greater. Historical designs of thermal oxidizers were comprised of carbon steel for the outer shell and castable refractory or brick as the thermal liner (a refractory is like a cement, which is put on the inside of the rector shell to act as a thermal insulation barrier). Modern units are designed and built using ceramic fiber insulation on the inside, which is a lightweight material, and has a relatively long life. Old refractory would tend to fail over a period of years by attrition of expansion and contraction. 

If a stroke patient receives intravenous (IV) thrombolysis, care often continues in the ED until the patient arrives in the ICU. Close monitoring must continue during this time, with special attention to the blood pressure. The blood pressure is most commonly checked via an arm cuff, since the placement of invasive lines (e.g., arterial catheterization) is relatively contraindicated once the patient has received intravenous thrombolysis (unless the situation is emergent and mandates such treatment). The systolic pressure must not exceed 185 mm Hg, and the diastolic pressure limit should be 110 mm Hg. Should the blood pressure exceed these limits, IV antihypertensive agents should be administered. IV pushes of labetolol (10-20 mg over 1-2 minutes) may be effective, but if patients are refractory to these initial measures then a continuous infusion of labetolol (0.5-2.0 mg/minute), nicardipine (5-15 mg/hour), or nitro-prusside (0.25-10 mg/kg/minute) may be necessary to keep the patient s blood pressure within the range. There will be a more detailed discussion of these antihypertensive agents, including their side effect profiles, later in this chapter. 

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