Advance directives: barriers to completion.

The concept of advance directives, although sound in theory, appears to be failing to meet the needs of many patients. Advance directives have been endorsed by both the healthcare profession and the general public, yet few individuals have documented their wishes for treatment preferences in end-of-life decisions. A review of 15 research articles revealed three main barriers to the completion and effective use of advance directives. These barriers are: (a) lack of education or knowledge regarding advance directives, (b) difficulties with paperwork completion, and (c) discordance between patient, family members or proxy, and provider. Future research is suggested to generate information that will facilitate meeting the needs of patients and their families in this most difficult aspect of healthcare delivery.

Color management of cosmetic restorations.

Ideally, a cosmetic restoration should be indistinguishable from the surrounding unrestored dentition. The cosmetic dentist faces the challenging task of creating the restoration using materials that do not possess the light-transmitting properties of natural tooth structures. A thorough knowledge of the physical and physiologic properties of light, color production and perception, and skill in the art of color matching can lead to rewarding success.

The effect of heart rate on in utero left ventricular output in the fetal sheep.

The effect of heart rate on left ventricular output was examined in seven fetal lambs at ages of 128 to 140 gestational days. The fetuses had been surgically instrumented at least 4 days previously with an ascending aortic flow probe, left ventricular dimension transducers, and left and right atrial pacing electrodes. Natural variations in heart rate of the lambs taken as a group correlated positively with left ventricular output, and negatively with ventricular end-diastolic dimension and stroke volume (P less than 0.0001). Rate did not affect output with right atrial pacing. With left atrial pacing, it correlated negatively with output (P less than 0.0001). At both pacing sites, rate correlated negatively with end-diastolic dimension and stroke volume (P less than 0.0001). The introduction of a longer interval during each pacing rate circumvented the rate-related changes in dimension and allowed the ventricle to fill to the same end-diastolic dimension. The systole at the end of the longer interval had a greater stroke volume than the preceding systoles. The faster the preceding paced rate, the greater was the stroke volume (P less than 0.0001). This study demonstrates that experimentally induced variations in heart rate produce changes in end-diastolic volume and contractility which prominently affect stroke volume. Over a broad range of rates, however, the effect of rate on left ventricular output is either negative or absent. With naturally occurring rate changes, there are additional changes in contractility and venous return which affect stroke volume. These combine to produce a positive relation between heart rate and left ventricular output. These effects of heart rate on output are qualitatively similar to those described for the adult animal.

Developmental changes in cardiac contractility in fetal and postnatal sheep: in vitro and in vivo.

Developmental changes in contractility were sought in the fetal and postnatal sheep heart by using postextrasystolic potentiation and force, pressure, and wall-motion measures. Two different preparations were used, isolated myocardium and the chronically instrumented lamb. In the isolated muscle, the following increased significantly with age: force of contraction, the maximum rate of rise of force, and postextrasystolic potentiation. In the intact heart prior to birth [period of study, 20 +/- 4 (SD) days] heart rate (HR) fell significantly, and the following increased significantly: postextrasystolic potentiation [measured with the maximum rate of rise of left ventricular (LV) pressure (Pmax)], LV peak systolic pressure (LVP), end-diastolic dimension (EDD), end-systolic dimension (ESD), and aortic diastolic pressure. After birth, LVP, Pmax, HR, LVEDP, EDD, and ESD increased and postextrasystolic potentiation fell. The latter fall was not found in vitro and probably demonstrates a transient change in contractility, related to hormonal or neural stimulation. Over the subsequent postnatal days (6-122 days), HR fell while potentiation, EDD, and ESD increased significantly. Both in vitro and in vivo, the overall increase in postextrasystolic potentiation demonstrates a similar long-term change in contractility. The similarity of this change to that induced by mild hypertrophy suggests that development and mild hypertrophy alter myocardial contractility through a common mechanism.

Biophysics of the developing heart. III. A comparison of the left ventricular dynamics of the fetal and neonatal lamb heart.

Changes in heart rate, left ventricular dimensions, and inotropic state of chronically instrumented lambs from in utero to neonatal life are described and analyzed. Six lambs were instrumented from 5 to 23 days prior to parturition and studied prior to and after birth. Heart rate, aortic systolic and diastolic pressure, left ventricular end-diastolic and peak systolic pressure, and left ventricular minor axis end-diastolic (EDD) and end-systolic dimension (ESD) were monitored. The maximum rate of rise of left ventricular pressure (Pmax), percentage fractional shortening [(EDD - ESD divided by EDD) x 100%; %FS], and the Pmax -interval ratio (this ratio of Pmax of the postextrasystolic potentiated systole to Pmax of the previous regular systole is independent of volume) described contractility. Subsequent to birth: heart rate, aortic systolic and diastolic pressure and left ventricular dimensions increased; during spontaneous rhythm and at equal fetal and neonatal heart rates and ventricular dimensions, %FS and P max increased significantly; and P max -interval ratio changed significantly. An increase in myocardial inotropic state occurs with birth. This enhancement is in addition to the effects of the increase in heart rate and end-diastolic volume that occur with the adaptation to birth.