Effect of Sodium Nitroprusside on Cerebral Blood Flow
Phase: NA
Conditions: Healthy Male Subjects
Sponsor: Rigshospitalet, Denmark

Protocol - Effect of Sodium Nitroprusside on Cerebral Blood Flow 
Brief Title: Effect of sodium nitroprusside on cerebral blood flow 
Official Title: Effect of sodium nitroprusside on cerebral blood flow 
Unique Protocol ID: H-17017045 as approved by the Regional Ethical Committee of the 
Copenhagen Region (De Videnskabsetiske Komiteer Region Hovedstaden, 004538666395, Email: 
vek@regionh.dk, Adress: Kongens Vænge 2, DK-3400 Hillerød, Denmark 
The study is monitored by the Danish Dataprotection Agency. 
Study is planned to be conducted from November 2017-February 2018 
FDA Regulated: No 
FDA Regulated Intervention: No 
Sponsor: Rigshospitalet, Denmark 
Principal investigator: Niels Damkjær Olesen, MD 
Sub-Investigator: Niels H. Secher, M.D., D.M.Sc., Prof. 
Contacts/Locations 
Central Contact: Niels D. Olesen, MD 
Telephone: 0045 35452674 
Email: nielsdolesen@gmail.com 
 
Central Contact Backup: Niels H. Secher, M.D., D.M.Sc., Prof.. 
Telephone: 0045 35452242 
Email: niels.secher@regionh.dk  
 
Study Officials: Niels D. Olesen, MD 
Study Principal Investigator 
Department of Anesthesia, Rigshospitalet 2043, DK-2100 Copenhagen Ø, Denmark 
 
Locations: Department of Anesthesia, Rigshospitalet 2043, DK-2100 Copenhagen Ø, Denmark 
Facility contact: Niels D. Olesen, M.D. Tel.: 0045 35452674 e-mail: nielsdolesen@gmail.com 
Short Description   
The brain has a high energy demand and requires continuous blood flow. The blood flow to the 
brain appears to be unaffected by small changes in blood pressure, but brain blood flow may be 
reduced by a large reduction in blood pressure. Large reductions in blood pressure are common 
during anesthesia or bleeding. It is unclear, however, how a given reduction in blood pressure 
affects blood flow to the brain.  

  
In this study, medicine called sodium nitroprusside is used to dilate blood vessels and reduce 
blood pressure in twenty healthy young men. The study will evaluate whether blood flow to the 
brain is affected when sodium nitroprusside is used to induce a moderate and a large reduction in 
blood pressure. Blood flow to the brain is evaluated using ultrasound on the neck. 
  
During breathing, oxygen is inhaled and carbon dioxide is exhaled. Carbon dioxide increases 
brain blood flow whereby changes in respiration can affect the blood flow to the brain. Sodium 
nitroprusside causes mild hyperventilation, whereby more carbon dioxide is exhaled, which will 
contribute to a reduction in brain blood flow. Thus, the study will also evaluate how brain blood 
flow is affected by hyperventilation and by breathing a mix of air and carbon dioxide. 
Background 
Cerebral autoregulation is generally considered to maintain cerebral blood flow (CBF) if mean 
arterial pressure (MAP) is between 60 and 150 mmHg. Yet, it is controversial whether there is a 
plateau for CBF. Thus, blood velocity of the middle cerebral artery (MCA Vmean), as an index of 
CBF, is affected by pharmacological changes in MAP between approximately 40-125 mmHg. In 
contrast, internal carotid artery blood flow is unaffected by a moderate decrease in blood pressure. 
  
The brain is supplied by the internal carotid and vertebral arteries that have different regulation 
with higher CO2 reactivity and larger orthostatic reduction for the internal carotid than the vertebral 
artery. The different regulation of the two arteries may reflect higher sympathetic innervation of 
arteries that originate from the internal carotid artery than those of the vertebral artery. The internal 
carotid artery may contribute to cerebral autoregulation as the vessel dilates during a moderate 
decrease in MAP while internal carotid artery blood flow is maintained. The arterial CO2 tension 
(PaCO2) is an important regulator of CBF and also maintenance of central blood volume and 
cardiac output is important for regulation of CBF. In the present study, internal carotid and vertebral 
artery blood flow is evaluated using duplex ultrasound in 20 healthy men when MAP is reduced by 
sodium nitroprusside, a potent short-lasting vasodilator. 
Objective 
The purpose of the study is to evaluate whether CBF is affected by 20% and 40% reductions in 
MAP by sodium nitroprusside. The CO2 reactivity of the internal carotid and vertebral arteries is 
evaluated in order to control for hyperventilation during sodium nitroprusside induced hypotension. 
Further, the study will evaluate whether changes in internal carotid and vertebral artery blood flow 
by a 40% reduction in MAP are different and whether the arteries have different CO2 reactivity. 
Lastly, the study will evaluate whether the slope of the linear regression of CBF and MAP is higher 
for the evaluations when MAP is reduced by 40% and 20% than that of the evaluations at baseline 
and when MAP is reduced by 20%. 

Hypotheses 
- 
Sodium nitroprusside-induced reduction in MAP by 40% reduces CBF 
- 
Sodium nitroprusside-induced reduction in MAP by 20% reduces CBF 
- 
Sodium nitroprusside-induced reduction in MAP by 40% causes a larger relative reduction 
in internal carotid artery blood flow than that of the vertebral artery 
- 
The CO2 reactivity of the internal carotid artery is higher than that of the vertebral artery 
- 
The slope of linear regression of MAP and CBF at the evaluations when MAP is reduced by 
20% and 40% by sodium nitroprusside is higher than that of the evaluations at baseline and 
when MAP is reduced by 20%  
Methods 
The study will include 20 healthy men. The experiment lasts for approximately three hours and the 
subject must be fasting for at least 4 hours, have abstained from alcohol and caffeine for 12 hours 
and rigorous exercise for 24 hours. Before the start of the experiment, the internal carotid and 
vertebral arteries are evaluated and in case the vessels can not be visualized, e.g. because of high 
carotid bifurcation, the subject will not be able to participate in the study. The subject rests in the 
supine position throughout the study.  
Measurements 
A catheter is placed in the radial or brachial artery on the non-dominant arm for evaluation of 
arterial pressure and gas variables. Total volume of blood sampled is less than 25 ml. A long line is 
placed in a cubital vein and advanced to the subclavian vein for infusion of sodium nitroprusside 
(Cavafix 20G 32 cm, B. Braun, Melsungen, Germany). The arterial pressure measurement is used for 
evaluation of stroke volume, cardiac output, and total peripheral resistance by modified pulse 
contour analysis (Finometer, Finapres Medical Systems, Amsterdam, The Netherlands). Heart rate 
is evaluated by electrocardiogram lead II. Forehead skin oxygenation and blood flow is evaluated 
close to the hairline using Laser Doppler flowmetry with an integrated oxygenation sensor at a 
depth of 1-2 mm in an area of approximately 9 mm^2 (MoorVMS-LDF and MoorVMS-OXY, 
Moor Instruments, Axminster, UK). Cerebral and biceps muscle oxygenation are evaluated using 
near-infrared spectroscopy (INVOS 5100C, Somanetics, Troy, MI, USA). Transcranial Doppler is 
used to evaluate MCA Vmean (Multidop X; DWL, Sipplingen, Germany). At a constant diameter of 

the MCA, changes in blood velocity reflect those of regional CBF, but the diameter may be affected 
by changes in MAP and PaCO2. 
  
Changes in central blood volume are evaluated by recording of thoracic electric admittance (C-
guard, DanMeter, Odense, Denmark). Central hemodynamics, MCA Vmean, electrocardiogram, and 
skin oxygenation and blood flow is recorded at 100 Hz (Powerlab 16/35 and LabChart 7, 
ADInstruments, Bella Vista, Australia), cerebral and muscle oxygenation is recorded at 0.1 Hz, and 
thoracic admittance at 0.25 Hz and saved on a pc. 
  
Internal carotid and vertebral artery blood flow is evaluated unilaterally on the neck using 
duplex ultrasound (Logiq E, GE Medical System, Jiangsu, China). Evaluation is in the longitudinal 
section 1-2 cm distal to the carotid bifurcation and the vertebral artery is evaluated between the 
transverse processes of C2-5 with the head turned approximately 30⁰ to the contralateral side. In 
order to limit the influence of ventilation, two recordings of approximately 20 s of both arteries are 
conducted at each level of MAP and the mean is reported. A frequency of 8-12 MHz is used and 
gain is set as high as possible while vessel lumen is echo-free. The angle-corrected time maximum 
flow velocity (TAVMAX), that corresponds to twice the mean blood velocity, is evaluated using 
pulsed-wave Doppler at stable angle ≤ 60º. Diameter is assessed using automatic software to track 
the vessel wall (Brachial Analyser for Research v. 6, Medical Imaging Applications LLC, 
Coralville, IA, USA) and blood flow is: 0.125*60*TAVMAX*π*diameter^2 and CBF is the sum of 
unilateral internal carotid and vertebral artery blood flow.  
  
The CO2 reactivity of the internal carotid and vertebral artery is: change in blood flow *100 / 
change in PaCO2 * baseline blood flow using individual linear regression for the evaluations during 
normo-, hypo-, and hypercapnia. The CO2 reactivity of MCA Vmean and cerebral oxygenation is 
evaluated similarly and measurements during sodium nitroprusside infusion are corrected for 
changes in PaCO2 from baseline using the CO2 reactivity to hypocapnia.  
Procedures 
Baseline evaluation at rest is conducted at least 30 min after placement of catheters. Subsequently, 
the CO2 reactivity is evaluated during hypo- and hypercapnia in random order. Hypocapnia is 
attained by hyperventilation for 6 min to provoke a 0.7-1.2 kPa reduction in PaCO2, and the 
evaluation is repeated if the reduction in PaCO2 is not within this interval. Hypercapnia is achieved 
by breathing 6% CO2 for 6 min.  
  
Thereafter, sodium nitroprusside is infused to reduce MAP by 20% (15%-25%) and 40% (35%-
45% and minimally MAP 50 mmHg at a maximal infusion rate of 10 µg/(kg*min)). Sodium 
nitroprusside is a short-acting, potent vasodilator that activates guanylate cyclase directly or by 
production of nitric oxide, causing relaxation of smooth muscle cells in arteries and veins. The 
effect of sodium nitroprusside is attained within 2 min, and the half-life is 2 min whereby the effect 
is short lasting. Sodium nitroprusside has no direct effect on CBF. Infusion of sodium nitroprusside 
is by an electronic infusion pump, at 0.25 µg/(kg*min) for one minute and increased by 0.25 
µg/(kg*min) each minute. When a level of MAP is reached, MAP is maintained for 120 s where 
after measurements are conducted during 2-3 min. When evaluations have been done at both levels 

of MAP, the infusion speed is reduced gradually in order to avoid so-called “rebound 
hypertension”. At this time the experiment is finished and catheters are removed.  
Statistics  
Trial size: In a similar study, sodium nitroprusside-induced reduction in MAP by 43% decreased 
CBF by 15% after correction for the decrease in PaCO2. A power calculation indicates that at least 
14 subjects are required to detect a 15% reduction in CBF with a standard deviation of 18% when 
MAP is reduced by 40% by sodium nitroprusside with a 5% significance level and a power of 80%. 
Inclusion Criteria  
- 
Informed consent 
- 
Male 
- 
Age 18-35 years 
Exclusion Criteria  
- 
Alcohol intake ≥ 420 g / week  
- 
Body mass index below 18 kg/m^2 and above 25 kg/m^2  
- 
Smoking 
- 
Beard on the neck 
- 
Chronic cardiac, lung, liver, kidney or metabolic disease that require medication 
- 
Vitamin B12 deficiency 
- 
Anemia 
- 
Leber’s hereditary optic neuropathy  
- 
Tobacco-alcohol amblyopia 
- 
Stenosis that obstructs ≥ 16% of the internal carotid artery  
- 
Intake of sildenafil or vardenafil for 24 hours and tadalafil for 48 timer prior to the 
experiment  

- 
Intake of monoamine oxidase inhibitors 
- 
Neurologic disease considered to affect cerebral blood flow, including epilepsy and 
multiple sclerosis 
Primary Outcome Measure: 
1. Change in cerebral blood flow from baseline to when MAP is decreased by 40% by sodium 
nitroprusside 
 
Secondary Outcome Measures: 
2. Change in cerebral blood flow from baseline to when MAP is decreased by 20% by sodium 
nitroprusside  
3. Relative reduction in internal carotid artery blood flow as compared to that of the vertebral 
artery from baseline to when MAP is reduced by 40% by sodium nitroprusside  
4. The CO2 reactivity of the internal carotid as compared to the vertebral artery  
5. Comparison of the slope of linear regression of MAP and internal carotid artery blood flow 
for the evaluations at baseline and when MAP is reduced by 20% by sodium nitroprusside 
and that of the evaluations when MAP is reduced by 20% and 40%  
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