Blood volume and hematocrit in regional circulations during isovolemic hemodilution in dogs

Comparison of blood flow models and acquisitions for quantitative myocardial perfusion estimation from dynamic CT

Myocardial blood flow (MBF) can be estimated from dynamic contrast enhanced (DCE) cardiac CT acquisitions, leading to quantitative assessment of regional perfusion. The need for low radiation dose and the lack of consensus on MBF estimation methods motivates this study to refine the selection of acquisition protocols and models for CT-derived MBF. DCE cardiac CT acquisitions were simulated for a range of flow states (MBF = 0.5, 1, 2, 3 ml (min g)(-1), cardiac output = 3, 5, 8 L min(-1)). Patient kinetics were generated by a mathematical model of iodine exchange incorporating numerous physiological features including heterogenenous microvascular flow, permeability and capillary contrast gradients. CT acquisitions were simulated for multiple realizations of realistic x-ray flux levels. CT acquisitions that reduce radiation exposure were implemented by varying both temporal sampling (1, 2, and 3 s sampling intervals) and tube currents (140, 70, and 25 mAs). For all acquisitions, we compared three quantitative MBF estimation methods (two-compartment model, an axially-distributed model, and the adiabatic approximation to the tissue homogeneous model) and a qualitative slope-based method. In total, over 11 000 time attenuation curves were used to evaluate MBF estimation in multiple patient and imaging scenarios. After iodine-based beam hardening correction, the slope method consistently underestimated flow by on average 47.5% and the quantitative models provided estimates with less than 6.5% average bias and increasing variance with increasing dose reductions. The three quantitative models performed equally well, offering estimates with essentially identical root mean squared error (RMSE) for matched acquisitions. MBF estimates using the qualitative slope method were inferior in terms of bias and RMSE compared to the quantitative methods. MBF estimate error was equal at matched dose reductions for all quantitative methods and range of techniques evaluated. This suggests that there is no particular advantage between quantitative estimation methods nor to performing dose reduction via tube current reduction compared to temporal sampling reduction. These data are important for optimizing implementation of cardiac dynamic CT in clinical practice and in prospective CT MBF trials.

Effects of acute normovolemic hemodilution on T2*-weighted images of rat brain

Acute normovolemic hemodilution (HD) was induced in anesthetized rats to assess the effect of changes in hematocrit (Hct) on signal intensity in T2*-weighted magnetic resonance (MR) images. Other relevant physiological parameters were maintained invariant. Two degrees of HD were induced: mild (Hct reduced from 42.6+/-2.2% to 33.4+/-2.1%) and moderate (Hct reduced from 44.6+/-2.7% to 26.2+/-1.7%). A two-dimensional gradient-echo sequence was used to monitor signal changes with high temporal resolution before, during, and after HD protocols. The time course of signal intensity change was closely related to that of changes in Hct. Corresponding changes in R2* (deltaR2*) with respect to the pre-HD state were calculated for the brain parenchyma. Average deltaR2* values of -0.24+/-0.06 s(-1) and -0.40+/-0.07 s(-1) were obtained for the mild and moderate HD groups, respectively, during the final 2 min of MR imaging (proximal to correlative measurements of Hct). MR measured deltaR2* values were in close agreement with the expected changes in R2* predicted from theory when the measured changes in Hct were used as independent variables. These data are in good agreement with the current understanding of the effects of changes in the intravascular concentration of deoxyhemoglobin on induced magnetic susceptibility and hold promise for quantitative measurement of brain oxygenation in vivo.

Myocardial oxygen supply-demand relations during isovolemic hemodilution

The findings presented in this article pertain strictly to the specific conditions of the studies and should be extrapolated to human patients with caution for several reasons. First, the experimental model of a single acute coronary stenosis is a rare clinical occurrence, and it ignores the influence of the well-developed collateral circulation and impaired endothelial function which may accompany long-standing coronary artery disease (22). Second, the absolute hematocrit at which cardiac dysfunction arises would likely vary if a different anesthetic technique or diluent were used, or if the study were carried out in the absence of thoracotomy and positive-pressure ventilation or at a reduced body temperature. Nevertheless, the present findings provide broad guidelines for assessing when hemodilution may be used safely as a method of blood conservation. They suggest (1) that relatively severe hemodilution alone or moderate hemodilution combined with controlled hypotension with either SNP or ADEN may be used safely if cardiac function is normal and the coronary circulation is not obstructed, but (2) that even moderate hemodilution alone may be unsafe in patients in whom the coronary vasodilator reserve is exhausted or severely depleted by a proximal stenosis.

Pitfalls in quantitative contrast echocardiography: the steps to quantitation of perfusion

Current methods used clinically to assess myocardial perfusion are invasive and expensive. As the technology of ultrasound imaging improves, CE may provide a relatively inexpensive, noninvasive means of quantitating myocardial perfusion. Issues regarding stability of microbubble contrast agents must be studied more closely under physiologic conditions. As such, encapsulated microbubbles may provide more stability under physiologic pressures than free gas microbubbles. Introducing high concentrations of contrast, either by hyperconcentrating the contrast agent or by increasing the injection rate, may provide greater stability under physiologic conditions. Further, before quantitative statement of tissue perfusion can be made, the relationship between tracer concentration and system response must be established. Further, a "linear" postprocessing ultrasound setting does not eliminate this requirement as data must still undergo nonlinear transformation during log compression and time-gain compensation. Additionally, issues regarding "electronic thresholding" must be explored more extensively in vivo. Commercial ultrasound scanners, in their present form, may not offer adequate sensitivity for absolute quantitative studies. Further development of modified ultrasound systems may provide sufficient sensitivity for quantitative perfusion imaging. CE offers a potentially powerful tool in the clinical management of patients with ischemic heart disease. Conventional coronary angiography provides information on the size of a lesion, but accompanying tissue perfusion distal to the lesion cannot be determined. Doppler ultrasonography determines velocity of blood flow in large vessels but does not offer the potential to quantitate tissue perfusion. Clearly, CE has a place in the future of diagnostic imaging. The recent work of Ito et al. demonstrated the qualitative potential of CE in the identification of "areas at risk" in patients who had undergone thrombolysis or percutaneous transluminal coronary angioplasty after an acute myocardial infarction. With further improvement in the ultrasound imaging techniques and microbubble stability, CE may offer an inexpensive, noninvasive means of assessing myocardial perfusion.

Microwave fixation for the determination of cerebral blood volume in rats

The cerebral blood volume (CBV) is sensitive to changing hydrostatic pressures. Thus, measurement methods that rely on removing tissue from unfixed brain may lead to underestimates of the CBV due to the loss of blood from the tissue. In situ fixation of tissue before removal may offer improved accuracy. We employed a triple-label method to measure simultaneously whole brain CBF and CBV in halothane-anesthetized Sprague-Dawley rats, which were then killed either by focused microwave irradiation (approximately 8 kW of incident power x 770 ms) or by decapitation. CBF was measured with [3H]nicotine while the CBV was determined as the sum of the cerebral red cell volume (CRCV--measured with 99mTc-labeled red cells) and the cerebral plasma volume (CPV--measured with [14C]dextran). Animals were studied during hypocarbic (PaCO2 approximately 25 mm Hg), normocarbic, or hypercarbic (PaCO2 approximately 70 mm Hg) conditions. Added studies were performed to verify that the microwave irradiation scheme used was capable of fixing previously administered tracers in place, and also halting the entry of tracer given after irradiation. Results indicate that the method of killing had no effects on CBF measurements, as assessed either by absolute values during normocarbia or responsiveness to changing PaCO2. However, all three volume measurements made using nondiffusible tracers (CRCV, CPV, and CBV) were significantly lower in animals killed by decapitation. Furthermore, CO2 responsiveness for all three variables (as assessed by the slope of the PaCO2/volume) was not evident in decapitated animals. We conclude that in situ fixation offers significant advantages when examining the cerebral distribution space of nondiffusible tracers.

Postocclusive reactive hyperaemia of cutaneous blood flow in premature newborn infants

Reactive hyperaemia, which occurs after a period of arterial occlusion is the result of an autoregulatory mechanism, involving local factors and autonomic nerve system control. To determine this autoregulatory mechanism in cutaneous blood flow in premature newborns measured the cutaneous reactive hyperaemia response after a 1 min occlusion, using a diode laser Doppler flowmeter applied to the skin. Twenty-four infants with a gestational age ranging from 25 to 37 weeks and a postnatal age of 0.3 to 72 days were studied. The reactive hyperaemia response is described by the parameters maximal cutaneous blood flow (Vmax), the time to reach maximal flow (tmax), the time taken for blood flow to return to baseline following Vmax (t(end)) and the % increase in cutaneous blood flow above preocclusional level (overshoot). A cutaneous reactive hyperaemia response could be elicited in all infants. Following occlusion the Vmax was 2,640 +/- 1,050 mV (mean +/- SD), the tmax was 7.6 +/- 3.9 sec (mean +/- SD), the t(end) was 35.6 +/- 14.6 sec (mean +/- SD) and the overshoot was 74.6 +/- 34% (mean +/- SD). A negative correlation was found between hematocrit and tmax (r = -0.62, p less than 0.01). No influence of postconceptional age, postnatal age, skin and rectal temperature, incubator temperature, weight, and transcutaneous oxygen tension on the reactive hyperaemia parameters could be found. We conclude that reactive hyperaemia of cutaneous blood flow can be elicited in premature newborn infants irrespective of postconceptional age, and that the hematocrit is inversely related to the time to reach maximal flow after occlusion.