Patient description: 19 yo Hispanic female G1P1 at 32+ 6 weeks of gestational age presented to the OB clinic for routine follow up complaining of mild headache and leg swelling. Primary evaluation revealed BP of 146/99 and urine dipstick with 3+ proteins. Her BP and U/A were normal in previous visit. Repeat BP a few hours later is 150/100 mmHg. Laboratory studies showed a normal hematocrit, platelet count, and liver transaminase levels. She is complaining of fatigue but no fever or chills. She is also suffering of headaches with no vision changes. No shortness of breath, cough, chest pain, orthopnea and palpitations or skin rash were observed. Her physical exam was negative for abdominal pain, change in bowel habits, nausea, vomiting, dysuria, frequency, hematuria or frothy urine. Leg swelling was observed with no arthralgia or back pain. She has no specific past medical issues and only uses prenatal vitamins. Her family history is positive for DM type 2 and HTN. She is a social alcohol consumer with the negative history of smoking or drug use. She is only have one partner in past 2 years and didn't have any contraceptive methods since 2 years ago. Her BMI was 24 at the first

Clinical trial eligibility criteria: eligible ages (years): 18.0-55.0,  Traumatic Brain Injury (See Table 2 for more details) 1. Age 18 to 55 years, inclusive 2. Either gender 3. TBI subjects ONLY: Meets DoD for moderate or severe TBI and TBI occurred less than 30 days prior to study enrollment (Sustained a traumatically induced physiological disruption of brain function, as manifested by at least one of the following: 1. . Period of loss of consciousness > 30 minutes 2. . Loss of memory for events lasting> 24 hours after the accident 3. . Alteration of mental state lasting> 24 hours after the accident (e.g., feeling dazed, disoriented, and confused) 4. . Clinical neuroimaging intracranial abnormality Traumatically induced includes the head being struck, the head striking an object, or the brain undergoing an acceleration/deceleration movement (i.e. whiplash) without direct external trauma to the head. 4. Ability to undergo fNIRS testing with hypercapnia challenge serially 5. Ability to provide informed consent  Unstable respiratory or hemodynamic status 2. Evidence of penetrating brain injury 3. TBI requiring craniotomy or craniectomy 4. History of disabling pre-existing neurologic disorder, e.g. dementia, uncontrolled epilepsy, multiple sclerosis, strokes, brain tumors, prior severe TBI, or other disorder that confounds interpretation of NIRS testing or neuropsychological results 5. History of pre-existing disabling mental illness, e.g. major depression or schizophrenia 6 for sildenafil: History of melanoma; Current use of organic nitrate vasodilators; Use of ritonavir (HIV-protease inhibitor); Current use of erythromycin, ketoconazole, or itraconazole; Current use of cimetidine; Current use of Alpha-blockers such as doxazosin (Cardura), tamsulosin (Flomax), and terazosin (Hytrin) prazosin (Minipress); Resting hypotension (systolic BP <90); Severe renal insufficiency; Hepatic cirrhosis; Cardiac failure or coronary artery disease causing unstable angina; Retinitis pigmentosa; Pregnant or breastfeeding female; Known hypersensitivity or allergy to sildenafil. 7. Inability to read and communicate in English 8. criterion for healthy subjects only: History of TBI. 9. Current use of a PDE5 inhibitor (a drug such as Sildenafil, Tadalafil, Vardenafil, Avanafil, Udenafil,Dipyridamole, Vardenafil hydrochloride)