Determination of preload-recruitable stroke work and elastance by the relationship of arterial blood flow velocity to left ventricular area

OBJECTIVE: The authors investigated the feasibility of an online sampling and display of LV flow-area loops for the determination of LV elastance and preload-recruitable stroke work (PRSW). Automated LV area measurements by echocardiography may be combined with flow velocity measurements in the internal carotid artery to construct LV flow-area loops as estimates of the systolic pressure-volume relationship. SETTING: University hospital. DESIGN: Open chest model. PARTICIPANTS: Eight anesthetized minipigs. INTERVENTIONS: Inferior vena cava occlusion was performed to simultaneously obtain parameters of the LV flow-area relationship and the LV pressure-area relationship. MEASUREMENTS and MAIN RESULTS: Parameters were obtained at baseline and during sequential administration of dobutamine (5 microg/kg/min) and halothane (0.8 vol%). Linear regression analysis and analysis of variance were performed to investigate an underlying linear relationship between the corresponding variables. Highly linear elastance and PRSW curves were derived from the flow-area and pressure-area loops (n = 24, R >/= 0.85). Changes of the curve slopes indicated inotropic changes as well as model independent dP/dt(max). Elastance from the pressure-area relationship was expressed by elastance from the flow-area relationship by the term y = 0.52 + 0.04. x (R(2) = 0.84; p < 0.0001). Linear regression of PRSW as derived from the flow-area relationship with PRSW as derived from the pressure-area relationship was expressed by y = 0.43 + 0.02. x (R(2) = 0.77; p < 0.0001). CONCLUSION: Indices of the LV pressure-area relationship can be derived from real-time loops constructed from arterial flow velocity and LV area.     

Single-beat estimation of ventricular end-systolic elastance-effective arterial elastance as an index of ventricular mechanoenergetic performance

BACKGROUND: The ratio of ventricular end-systolic elastance (Ees) to effective arterial elastance (Ea) is known to reflect not only ventricular mechanical performance but also energetic performance. Despite these useful features, technical difficulties associated with estimating Ees make the clinical application of Ees/Ea impractical. We developed a framework to estimate Ees/Ea without measuring ventricular volume or altering the loading condition. METHODS: To achieve this goal, we approximated the ventricular time-varying elastance curve with two straight lines, one for the isovolumic phase and the other for the ejection phase, and characterized the curve with the slope ratio, k, of these two straight lines. Using the concept of the pressure-volume relationship, Ees/Ea is algebraically expressed as Ees/Ea = Pad/Pes (1 + k. ET/PEP) - 1, where Pes is end-systolic pressure, Pad is aortic diastolic pressure, ET is ejection time, and PEP is pre-ejection period. In 11 anesthetized dogs, we recorded arterial and ventricular pressures and ventricular volume and estimated Ees and Ea under various contractile states and loading conditions. RESULTS: An empirical relation between k and Ees/Ea was found as k = 0.53 (Ees/Ea)0.51. Simultaneous solution of these two equations yielded Ees/Ea as a function of Pad/Pes and ET/PEP. The estimated Ees/Ea values correlated well with the measured Ees/Ea values ([Measured Ees/Ea] = 0.96 [Estimated Ees/Ea] + 0.098, r = 0.925, SEE = 0.051). CONCLUSIONS: The proposed framework is capable of estimating Ees/Ea from ventricular and aortic pressure.     

Chaos attractors of ventricular elastance to evaluate cardiac performance

Quantitative evaluation of cardiac function is very important in the clinical application of a ventricular assist device. This article reports a new evaluating method of E max, which is the most reliable parameter to evaluate cardiac function. Fluctuation in the E max time series data was evaluated by the nonlinear mathematical analyzing method including chaos and fractal theory. Experimental goats were anesthetized with halothane inhalation, and left ventricular volume and pressure were measured with other hemodynamic parameters to evaluate E max during various drug administrations. E max was evaluated by two methods. One was the conventional pressure volume loop evaluation and the other was the parameter optimization method without left ventricular volume data. As a result, E max evaluated by the parameter optimization method correlated well with the E max with conventional PV curve. Furthermore, interesting results were obtained. There were rhythmical fluctuations in the E max time series data. By the methodology of Takens, E max time series data was embedded into the phase space and a strange attractor was observed. These results may be important when considering E max evaluation during left ventricular assistance.     

Operative assessment of carotid endarterectomy: Internal carotid arterial back pressure, carotid arterial blood flow, and carotid arteriography.

Determination of ICA back pressure, electromagnetic measurement of internal carotid arterial blood flow, and performance of operative carotid arteriography have been assessed in a series of elective carotid endarterectomies. If ICA back pressure is used to determine the need for an internal shunt, our data suggest that a minimal pressure of 60-70 mmHg is required to insure adequacy of collateral cerebral blood flow in all patients. Internal carotid arterial blood flow was not significantly increased after endarterectomy. Although flow measurements are of some interest, their routine measurement does not appear to have significant clinical usefulness. Operative carotid arteriography is a valuable technique in identifying unsuspected technical errors at the site of endarterectomy and should be used routinely.     

Effect of N2O on segmental left ventricular function and effective arterial elastance in pigs when added to a halothane-fentanyl-pancuronium anesthetic technique

The interaction of various concentrations of N2O and a stable halothane-fentanyl-pancuronium anesthetic technique was examined in nine pigs. Segmental myocardial contractility was measured with the end-systolic pressure-length relationship (Ees), and the effective arterial elastance (Ea) was quantified based on the Windkessel model. The addition of 30, 50, and 70% N2O did not change myocardial contractility or the effective arterial elastance. During the 30 and 70% N2O challenge, however, arterial capacitance decreased significantly from a mean (+/- SEM) 0.86 +/- 0.15 to 0.71 +/- 0.0.11 mL/mm Hg with 30% N2O (P less than 0.05) and from 0.90 +/- 0.09 to 0.71 +/- 0.07 mL/mm Hg (P less than 0.05) with 70% N2O. A dose-response relationship for the effect on the arterial capacitance could not be demonstrated. We concluded that in the presence of halothane, fentanyl, and pancuronium, N2O does not depress the normal myocardium or change left ventricular afterload. The decrease in arterial capacitance that occurred when 30 and 70% N2O were given was not sufficient to change the effective afterload and appears to be of no importance to normal left ventricular function.     

The effects of changes in lumen and flow on the arterial velocity waveform

Pulse Doppler signals were recorded from the midstream of the abdominal aorta of seven dogs both 5 cm proximal and distal to a caliper-noose constricting device. Heart rate and peripheral resistance were controlled at several (three to five) different levels while progressive increments of stenosis were produced. Analysis of the data of 44 experimental runs over a 100–1100/ml flow range appear to justify the following conclusions: Significant changes in almost every velocity waveform (VWF) variable studied were consistently produced with less than 40% reduction in diameter, and in some with as little as 5% stenosis. No flow measurement, even maximum systolic flow, was significantly reduced until at least a 50% reduction in diameter, the usual point of “critical stenosis,” was produced. Some VWF variables, especially diastolic dimensions and those recorded distally, increased before declining in the period before critical stenosis was reached. This initial response was often clearly biphasic in pattern. Because of this bidirectional initial change, distally recorded VWFs were not as consistently changed at lesser degrees of stenosis as the same parameters recorded proximally. Nondimensional or ratio variables were as sensitive as the most sensitive (VWF) dimensions in detecting luminal narrowing. The sensitivity of these changes in VWF dimensions and ratios in detecting stenosis was not significantly reduced at lower flow rates.     

Cerebral blood flow and cerebral blood flow velocity during angiotensin-induced arterial hypertension in dogs

Pressure-passive perfusion beyond the upper limit of cerebral blood flow (CBF) autoregulation may be deleterious in patients with intracranial pathology. Therefore, monitoring of changes in CBF would be of clinical relevance in situations where clinical evaluation of adequate cerebral perfusion is impossible. Noninvasive monitoring of cerebral blood flow velocity using transcranial Doppler sonography (TCD) may reflect relative changes in CBF. This study correlates the effects of angiotensininduced arterial hypertension on CBF and cerebral blood flow velocity in dogs. Heart rate (HR) was recorded using standard ECG. Catheters were placed in both femoral arteries and veins for measurements of mean arterial blood pressure (MAP), blood sampling and drug administration. A left ventricular catheter was placed for injection of microspheres. Cerebral blood flow velocity was measured in the basilar artery through a cranial window using a pulsed 8 MHz transcranial Doppler ultrasound system. CBF was measured using colour-labelled microspheres. Intracranial pressure (ICP) was measured using an epidural probe. Arterial blood gases, arterial pH and body temperature were maintained constant over time. Two baseline measures of HR, MAP, CBF, cerebral blood flow velocity and ICP were made in all dogs (n = 10) using etomidate infusion (1.5 mg · kg^?1 · hr^?1) and 70% N₂O in O₂ as background anaesthesia. Following baseline measurements, a bolus of 1.25 mg angiotensin was injected iv and all variables were recorded five minutes after the injection. Mean arterial blood pressure was increased by 76%. Heart rate and ICP did not change. Changes in MAP were associated with increases in cortical CBF (78%), brainstem CBF (87%) and cerebellum CBF(64%). Systolic flow velocity increased by 27% and Vmean increased by 31% during hypertension (P < 0.05). Relative changes in CBF and blood flow velocity were correlated (CBF cortex — Vsyst: r = 0.94, CBF cortex — Vmean: r = 0.77; P < 0.001; CBF brainstem — Vsyst: r = 0.82, CBF brainstem — Vmean: r = 0.69; P < 0.05). Our results show that increases in arterial blood pressure beyond the upper limit of cerebral autoregulation increase CBF in dogs during etomidate and N₂O anaesthesia. The changes in CBF are correlated with increases in basilar artery blood flow velocity. These data suggest that TCD indicates the upper limit of the cerebral autoregulatory response during arterial hypertension. However, the amount of CBF change may be underestimated with the TCD technique.     

Cerebral blood flow in relation to contralateral carotid disease, an MRA and TCD study.

OBJECTIVE: to describe redistribution of cerebral blood flow in patients with severe internal carotid artery (ICA) stenoses in relation to contralateral ICA disease. METHODS: sixty-six patients scheduled for carotid endarterectomy (CEA) were grouped according to severity of contralateral stenosis (<30% [group I]; 30-69% [group II]; 70-99% [group III]; occlusion [group IV]. Transcranial Doppler (TCD) and magnetic resonance angiography (MRA) investigations were performed preoperatively. RESULTS: TCD demonstrated a reversed flow in the contralateral anterior cerebral artery (A(1)segment) and ophthalmic artery in three-quarters of group IV patients (p <0.0001). Group IV patients also exhibited decreased blood flow velocity in the contralateral middle cerebral artery (p =0.001). MRA showed increased ipsilateral ICA and basilar artery (BA) blood flow volumes (Q-flows) in group IV patients when compared to the other groups (p <0.001). No changes in total Q-flow (ICAs+BA) were found. CONCLUSIONS: in patients considered for CEA, the severity of the contralateral ICA disease is an important determinant of the pattern of blood flow redistribution through the anterior communicating pathway and ophthalmic artery. Significant flow redistribution through the posterior communicating pathway occurs especially in patients with contralateral ICA occlusion.     

Acute cardiovascular responses to internal carotid artery occlusion during carotid endarterectomy and to the restoration of internal carotid flow

Heart rate, central venous pressure, radial artery pressure and electrocardiograph were recorded in 14 patients undergoing carotid endarterectomy under general anaesthesia supplemented with fentanyl and halothane and muscle relaxation as required. Induction of anaesthesia was followed by a significant reduction in systolic arterial pressure and a rise in central venous pressure (CVP). Thereafter CVP did not vary significantly and heart rate did not change significantly at any stage. After internal carotid artery occlusion, mean systolic pressure at one minute, 143 (SD 17) mmHg, and three minutes, 160 (SD 27) mmHg, were both significantly higher than prior to occlusion, 132 (SD 17) mmHg (P less than 0.01 and P less than 0.002 respectively). Similar significant changes occurred in diastolic pressure after carotid occlusion (P less than 0.02 and P less than 0.002 respectively). The restoration of flow through the internal carotid artery in patients operated on without a shunt was associated with a significant reduction in mean systolic pressure. The mean systolic pressure at one minute, 145 (SD 20) mmHg, and three minutes, 135 (SD 19) mmHg, were both significantly lower than that before restoration of flow, 159 (SD 17) mmHg (P less than 0.02 and P less than 0.05 respectively). Changes in mean diastolic pressure in this group at these times, while in the same direction, were not significant. The observed hypertensive response to carotid occlusion may assist in preserving cerebral perfusion while the internal carotid artery is occluded, but may be hazardous for patients with ischaemic heart disease.     

Changes in middle cerebral artery flow velocity and pulsatility index after carotid endarterectomy.

Transcranial Doppler sonography (TCD) was used in 33 patients undergoing carotid endarterectomy (CEA). Mean flow velocity (MCA MV) and the pulsatility index in the middle cerebral artery (MCA PI) were measured pre- and on six occasions postoperatively. The MCA MV was reduced by anaesthesia but was increased postoperatively (+43%, p less than 0.001) compared to the preoperative value and was still increased at late follow-up after several months. The MCA PI, which was lower preoperatively than normally reported in this age group, was not changed by anaesthesia but then rose and remained elevated, i.e. within normal limits 72 h postoperatively (+30%, p less than 0.01) as well as at late follow-up. No significant changes in MCA MV or MCA PI were noted on the contralateral side. The results from this study support earlier findings from invasive studies showing that CEA results in an increased flow in the middle cerebral artery on the operated side and an increased MCA PI suggests an increased resistance on the operated side. The findings are compatible with an increased cerebral blood flow during the first days after CEA and with the operated side supplying a greater part of the cerebral blood flow even several months after surgery.     

Recovery of the blood pressure - cerebral flow relation after carotid stenting in elderly patients

BACKGROUND: As a sensitive and convenient means for the cerebral hemodynamic monitoring, dynamic cerebral autoregulation testing could be especially useful in medical conditions where less invasive diagnostics and therapies are preferred. This study analysed the effect of carotid stenting on dynamic autoregulation in elderly patients focussing on the relation between blood pressure and cerebral blood flow velocity. METHODS: We examined 20 patients age 69 +/- 8 years with coexisting cerebrovascular and medical risk factors before and at least six month after stenting of severe carotid stenoses. Data were compared to 24 age-matched healthy controls. Slow spontaneous oscillations were studied in continuous recordings of Transcranial Doppler and beat-to-beat blood pressure. Analysis was based on the "high-pass filter model", which predicts a positive phase relationship between these oscillations. FINDINGS: Whereas phase shift angles were diminished (20.4 +/- 14.1 degrees ) before stenting, after stenting these values were significantly increased to normal (48.1 +/- 16.6 degrees ), to the level of controls (46.7 +/- 15.9 degrees ). Medical conditions such as coronary artery disease, arterial hypertension, and dyslipidemia did not diminish this recovery. The level of increase was inversely correlated with the initial autoregulatory deficit (r = -0.68) which was largest with insufficient collateral blood supply and symptomatic carotid stenoses. CONCLUSIONS: The study showed that an impaired cerebral autoregulation may recover after stent-guided carotid angioplasty even in the elderly with co-existing medical conditions. In this respect to regain vasomotor capability, patients with cerebrovascular risk factors seemed to benefit particularly.     

Recovery of the blood pressure – cerebral flow relation after carotid stenting in elderly patients

Summary Background. As a sensitive and convenient means for the cerebral hemodynamic monitoring, dynamic cerebral autoregulation testing could be especially useful in medical conditions where less invasive diagnostics and therapies are preferred. This study analysed the effect of carotid stenting on dynamic autoregulation in elderly patients focussing on the relation between blood pressure and cerebral blood flow velocity. Methods. We examined 20 patients age 69 ± 8 years with coexisting cerebrovascular and medical risk factors before and at least six month after stenting of severe carotid stenoses. Data were compared to 24 age-matched healthy controls. Slow spontaneous oscillations were studied in continuous recordings of Transcranial Doppler and beat-to-beat blood pressure. Analysis was based on the “high-pass filter model”, which predicts a positive phase relationship between these oscillations. Findings. Whereas phase shift angles were diminished (20.4 ± 14.1°) before stenting, after stenting these values were significantly increased to normal (48.1 ± 16.6°), to the level of controls (46.7 ± 15.9°). Medical conditions such as coronary artery disease, arterial hypertension, and dyslipidemia did not diminish this recovery. The level of increase was inversely correlated with the initial autoregulatory deficit (r = −0.68) which was largest with insufficient collateral blood supply and symptomatic carotid stenoses. Conclusions. The study showed that an impaired cerebral autoregulation may recover after stent-guided carotid angioplasty even in the elderly with co-existing medical conditions. In this respect to regain vasomotor capability, patients with cerebrovascular risk factors seemed to benefit particularly.