Increased Cerebral CO2 Reactivity After Heparin-Mediated Extracorporal LDL Precipitation (HELP) in Patients With Coronary Heart Disease and Hyperlipidemia
Background and Purpose—There is experimental and clinical evidence that hypercholesterolemia leads to an impairment of endothelial function in coronary and cerebral arteries. Using transcranial Doppler sonography, we examined CO2 reactivity as a marker of cerebral vasoreactivity in patients with coronary heart disease and hyperlipidemia before and after drastic lowering of LDL cholesterol, lipoprotein(a) [Lp(a)], and fibrinogen levels by heparin-mediated extracorporal LDL precipitation (HELP).
Methods—CO2 reactivity was determined in 13 patients with coronary artery disease and hyperlipidemia undergoing regular HELP therapy. Middle cerebral artery mean blood flow velocity (MFV) was detected by transcranial Doppler. CO2 reactivity was calculated as the percent change of MFV during hypercapnia, induced by ventilation of carbogene (5% CO2, 95% O2), to normocapnia. Patients with extracranial or intracranial stenoses were excluded. Other parameters such as blood viscosity, heart rate, and blood pressure were measured to control hemorheologic and systemic influences on CO2 reactivity.
Results—A single HELP treatment reduced total cholesterol, LDL cholesterol, Lp(a), triglycerides, and fibrinogen levels by >50% (P<0.001). Blood viscosity significantly decreased from 1.24±0.04 to 1.07±0.02 mPa (P<0.001). Blood pressure, heart rate, and MFV did not change significantly. CO2 reactivity increased from 22% ± 21% to 36% ± 18% (P<0.05).
Conclusions—Fast and drastic removal of LDL cholesterol, Lp(a), and fibrinogen from plasma results in an improvement of cerebrovascular reactivity in patients with coronary heart disease and hyperlipidemia. The clinical use of HELP in patients with impaired cerebrovascular reactivity might be promising.
Hypercholesterolemia is a risk factor for atherosclerotic disease. Lowering elevated LDL cholesterol levels is effective in primary and secondary prevention of myocardial infarction.1 2 3 Pravastatin, an HMG-CoA reductase inhibitor (statin) was recently shown to reduce the risk of stroke in patients with coronary artery disease and moderate hypercholesterolemia.4 Meta-analysis of clinical trials also suggests that treatment with statins is effective in the prevention of ischemic stroke.5 If pharmacological measures are not sufficient, heparin-mediated extracorporal LDL precipitation (HELP) offers an invasive but highly efficient method to reduce serum LDL cholesterol.6 In addition, plasma fibrinogen and lipoprotein(a) [Lp(a)], both independent risk factors for coronary heart disease and stroke,7 8 9 are substantially reduced by HELP. With this method, fibrinogen, LDL cholesterol, and Lp(a) levels can be lowered by 60% to 75% per single treatment.10 11 Acute effects of HELP apheresis on hemorheology are the reduction of red cell aggregability and plasma viscosity.10
Animal and in vitro studies have shown that experimental hypercholesterolemia enhances coronary vasoconstriction and vascular resistance.12 13 14 15 Recently, it was demonstrated that a single LDL apheresis shows a beneficial effect on the acetylcholine-induced endothelial-dependent vasoreactivity in coronary and peripheral arteries.16 17 18 However, the effects of LDL apheresis on the vasoreactivity of cerebral arteries are still unknown.
CO2 reactivity is a marker for cerebral vasoreactivity (CVR). Being a strong physiological stimulus, CO2 leads to vasodilatation of the cerebral arterioles, mediated by endothelial smooth muscle cell interactions.19 Assuming a constant diameter of the middle cerebral artery (MCA), increased blood flow in the precapillary system is correlated with increased MCA mean blood flow velocity (MFV).20 This parameter can easily be detected by transcranial Doppler and allows quantification of CO2 reactivity.21
The objective of our prospective study was to test the acute effect of a single LDL apheresis on CO2 reactivity as a marker of CVR in patients with coronary artery disease and hyperlipidemia.
Subjects and Methods
Thirteen male patients (mean age 57±7.5 years) with a history of coronary heart disease (7 patients with heart transplantation due to ischemic cardiomyopathy) and hyperlipidemia were included in the study. All of them were on regular HELP therapy (1- to 4-week intervals) because of elevation of LDL cholesterol or Lp(a) (threshold values: LDL cholesterol 180 mg/dL, Lp(a) 45 mg/dL) with insufficient response to diet and drug therapy. Most patients received antihypertensive therapy. All patients with a history of heart transplantation received immunosuppressive therapy with cyclosporine, azathioprine, and steroids. For detailed information on the clinical status of the patients before initiation of HELP therapy, see Table 1⇓. All patients had extracranial and intracranial Doppler sonography to exclude hemodynamically relevant stenosis (>50%) or occlusion of brain-supplying vessels and to ensure a detectable MCA signal.
For extracorporal LDL apheresis, the system Plasmat Secura (Braun) was used. The average treatment lasted 2 hours, 2.8 to 3.0 L of plasma were filtrated. Detailed description of this method has been published before.22
CO2 reactivity was determined 30 minutes before and 30 minutes after HELP apheresis. Informed consent for the Doppler examinations was obtained from all patients before the procedure. Transcranial Doppler studies were performed with the MultiDop X4 (DWL). While the patients were sitting in a comfortable position, 2-Mhz probes were bilaterally fixed over the temporal bone window when highest intensity and velocity of the MCA signal in a depth of 45 to 55 mm had been obtained. Blood pressure and pulse were measured before and after HELP apheresis. End-tidal CO2 partial pressure was continuously monitored in 5 of our 13 patients during the Doppler examination with a CO2 monitor (EGM I, Heyer).
Bilateral MCA MFV was continuously monitored and digitally recorded for later off line analysis. Patients breathed through a plastic mouthpiece, and a nose clamp kept their nostrils closed. A valve mechanism allowed for prompt switching from room air to carbogene (5% CO2, 95% O2). The patients started by breathing room air for 30 to 60 seconds, until a steady state in mean MCA blood flow velocity was obtained. They were then asked to hyperventilate for 30 to 60 seconds. Thereafter, patients were allowed to breathe normally for another minute. In the next step, patients were ventilated with carbogene for at least 1 minute, until mean MCA blood velocity and end-tidal CO2 partial pressure remained stable. Finally, patients breathed room air again until mean MCA blood flow velocity had normalized.
For off line analysis, mean MCA blood flow velocities were defined from the recorded Doppler curves at the following time points: resting (normocapnia), during hyperventilation after a steady state in flow velocity had been obtained (hypocapnia), and after at least 1 minute of carbogene ventilation, when end-tidal CO2 partial pressure and flow velocity had reached stable values (hypercapnia).
CO2 reactivity was calculated as the percent change of MCA MFV in hypercapnia compared with normocapnia (CO2 reactivity= (MFVHypercapnia−MFVNormocapnia)×100%/MFVNormocapnia). Changes in CO2 reactivity were presented as absolute percent differences (change in CO2 reactivity=CO2 reactivityafter HELP−CO2 reactivitybefore HELP). Blood viscosity was measured before and after HELP apheresis.
All data are presented as mean±SD. To compare CO2 reactivity and other parameters before and after HELP apheresis, a non parametric test was performed (Wilcoxon matched pairs signed ranks test). To test an association between CO2 reactivity changes and pretreatment CO2 reactivity or LDL cholesterol, a bivariate non parametric correlation (Spearman) was performed.
Total Cholesterol, LDL Cholesterol, HDL Cholesterol, Lp(a), Triglycerides, Fibrinogen, and Plasma Viscosity
HELP apheresis reduced total cholesterol, LDL cholesterol, Lp(a), and triglycerides by >50% (P<0.001). HDL cholesterol was reduced by 15% (P<0.01). For exact individual values and means, see Table 2⇓. Fibrinogen and plasma viscosity were reduced by 56% and 14%, respectively (P<0.001; see Table 3⇓).
Blood Pressure and Heart Rate
There were no significant changes in blood pressure or heart rate. Mean blood pressure values were 147/88 mm Hg before and 141/86 mm Hg after HELP (NS). Pulse rate values were 86 and 83 bpm, respectively (NS).
MCA Mean Blood Flow Velocity
Average pretreatment MFV was 54.8±6.4 cm/s for the left and 56.4±6.8 cm/s for the right MCA. After HELP apheresis MFV was 56.7±9.9 cm/s and 57.9±9.5 cm/s, respectively. Combining both sides, average MCA MFV was 55.6±6.0 cm/s before and 57.3±9.3 cm/s after HELP apheresis. These changes were not significant.
Pretreatment CO2 reactivity calculated as the percent change of MCA MFV in hypercapnia compared with normocapnia ranged from −12% to 54%, with a mean of 22%±21%. After HELP apheresis values ranged from −2% to 68%, with a mean of 36%±18% (Table 3⇑). The relative increase in CO2 reactivity of 14% was significant (P<0.05) (Figure 1⇓). Changes of CO2 reactivity in individual patients are presented in Figure 2⇓. Pretreatment LDL cholesterol, Lp(a), triglycerides, and fibrinogen, as well as pretreatment CO2 reactivity, were not significantly associated with change in CO2 reactivity. However, change of CO2 reactivity tended to be positively associated with pretreatment LDL cholesterol and negatively associated with pretreatment CO2 reactivity (Figures 3⇓ and 4⇓). Lack of significance might be due to the low number of patients. An estimated number of 28 patients would be necessary to possibly reach significance.
We showed for the first time that a single LDL apheresis with the HELP system significantly improves CO2 reactivity. This improvement tended to be more pronounced in patients with low pretreatment CO2 reactivity and elevated pretreatment LDL cholesterol levels. Significant effects might have been missed due to the small number of patients and the huge interindividual variability in the CO2 reactivity. Intraindividual reproducibility in detecting CO2 reactivity by transcranial Doppler is known to be high.21
The reduction rates of LDL cholesterol, Lp(a), triglycerides, and fibrinogen matched well the results from previous studies using the HELP system in patients with coronary heart disease.6 Although all patients had a history of hyperlipidemia, only some had pathologically elevated LDL cholesterol or Lp(a) levels at the time of the HELP apheresis. This is a consequence of repeated HELP apheresis in all and concomitant treatment with statins in some patients (with no history of heart transplantation). Previous studies have demonstrated a new steady state in LDL cholesterol and fibrinogen levels after 4 to 8 treatment sessions.23
As expected, blood viscosity was markedly reduced after HELP apheresis. This alteration had no influence on systemic hemodynamic parameters such as mean arterial blood pressure or heart rate. Interestingly, mean MCA blood flow velocity did not change after HELP apheresis.
This is in contrast to the investigation of Izumi et al,24 who found a small but significant increase in MCA blood flow velocity after pharmacological defibrination with the venom batroxobin. The latter study was performed on normal subjects with a mean age of 32 years and no history of vascular disease. There are 2 explanations for the discrepancy with our results. First, there was considerable variance regarding pretreatment MCA blood flow velocities in our patients. Thus, our patient number might have been too small to detect significant changes on flow velocities. Second, there might be specific limitations of hemorheologic effects on cerebral blood flow in our patients. These limitations include older age, reduced cardiac output due to ischemic heart disease, generalized vascular alterations due to atherosclerosis, and denervation of the graft in the transplanted patients.
Despite constant blood flow velocities, a significant increase in CO2 reactivity as a marker of CVR was observed. Izumi et al24 also observed increased CO2 reactivity after defibrination with batroxobin. However, in their (healthy) subjects, this increase was, as pointed out before, associated with increased cerebral blood flow. The independent increase in CO2 reactivity in our patients suggests a direct influence of HELP apheresis on CVR.
HELP apheresis in our patients seemed to have a stronger effect on CO2 reactivity if pretreatment values were low. A low initial CO2 reactivity may represent a state of endothelial dysfunction. In this context the effect of HELP apheresis could be explained as the normalization of disturbed endothelial function, as it was recently demonstrated for peripheral and coronary arteries.16 17 18 Patients with high initial CO2 reactivity and normal endothelial function may have no range for further improvement.
Considering a direct influence of HELP apheresis on endothelial function and CVR, several mechanisms may play a role. In experimental long-lasting hypercholesterolemia in pigs, vasoconstrictive effects of endothelin-1 are pronounced while vasorelaxing effects of endothelium-derived relaxing factor/nitric oxide are reduced. This imbalance leads to increased vascular tone.12
Regarding short-term effects, Andrews and coworkers13 demonstrated inhibition of endothelium-dependent relaxation after exposure to native but not to chemically modified LDL cholesterol in an in vitro model of rabbit aorta. The authors proposed that a receptor-dependent mechanism mediated the inhibition of endothelium-dependent relaxation. Kugiyama and coworkers,14 however, showed that inhibition of vascular relaxation only occurs after exposure to oxidized, but not to native, LDL cholesterol. These controversial results could be explained by insufficient protection of native LDL against oxidation in the study by Andrews et al, with subsequent intraexperimental development of oxidized LDL being the main factor for inhibited arterial relaxation. In a more recent study, Hein and Kuo25 investigated the influence of native and oxidized LDL on porcine coronary arterioles. They found an inhibition of nitric oxide–mediated vasorelaxation after 1 hour exposure to both forms of LDL. Oxidized LDL produced more severe inhibition than native LDL. The inhibitory effects of both LDLs were prevented by administration of a cell-permeable superoxide scavenger, suggesting a role of superoxide anions in LDL-mediated inhibition of vasorelaxation. Galle and coworkers15 used bovine arterial endothelial cells to investigate short-term effects of both forms of LDL. They showed that oxidized LDL, and to a lesser degree native LDL, inactivated endothelium-derived relaxing factor after its release from the endothelium, which suggests a further pathway of LDL-mediated vasomotor impairment.
The discussed mechanisms of acute impairment of arterial relaxation by LDL cholesterol may help to explain the observed increase in cerebrovascular reactivity detected in our patients as early as 30 minutes after HELP apheresis. One could speculate that reduction of LDL cholesterol acutely reverses LDL-dependent impairment of arterial relaxation and thus leads to normalization of CVR.
We conclude that HELP apheresis in patients with ischemic heart disease and hyperlipidemia directly affects CVR by normalizing endothelial function. In cases of compromised cerebral perfusion, endothelium-dependent CVR improvement may have beneficial effects on cerebral microcirculation.
The authors thank Mrs. Judy Benson for the language editing of this manuscript. The authors state that there are no conflicts of interest regarding the submission.
- Received February 2, 1999.
- Revision received May 27, 1999.
- Accepted June 21, 1999.
- Copyright © 1999 by American Heart Association
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