Table 3Metabolic Equivalent (MET) Values for Various Physical Activities56x56Wallis, RM, Corbin, JD, Francis, SH, and Ellis, P. Tissue distribution of phosphodiesterase families and the effects of sildenafil on tissue cyclic nucleotides, platelet function, and the contractile responses of trabeculae carneae and aortic rings in vitro. Am J Cardiol. 1999; 83: 3C–12C
In the past 6 years, the FDA has approved three oral agents for the treatment of ED: sildenafil, vardenafil, and tadalafil. All three are phosphodiesterase type 5 (PDE-5) inhibitors and work by potentiating the effect of nitric oxide in the penis. In particular, they block the hydrolysis of cyclic guanosine monophosphate to guanosine 5'-monophosphate, thus enhancing nitric oxide–mediated smooth muscle relaxation, increasing blood flow to the penis and facilitating erection.
Due to their vasorelaxing effect, administration of PDE-5 inhibitors in hypertensive individuals was initially confronted with great suspicion. A wealth of clinical data however has proven that PDE-5 inhibitors are associated with few side effects and provoke a small and insignificant reduction in blood pressure with minimal heart rate alterations in both normotensive and hypertensive patients as well. As a matter of fact, they can be safely and effectively administered to hypertensive individuals even when they are already taking multiple antihypertensive agents[51-56]. The sole exception to the rule is co-administration with organic nitrates, which is an absolute contraindication due to profound and possibly hazardous hypotension effect[57,58]. Moreover, precaution should be taken when PDE-5 inhibitors are combined with a-blockers where, due to possible orthostatic hypotension effect, lower starting doses should be implemented in the therapeutic regime[59-62].
The initial event for normal erectile function is sexual stimulation. Subsequent to processing in the central nervous system neural impulses are conveyed along the spinal cord, exiting through the pelvic parasympathetic preganglionic nerves. These pelvic nerves form the pelvic plexus and send their message through first messenger, acetyl choline, to the cavernosal nerves. The cavernosal nerves enter erectile bodies (corpora cavernosa) (Figure 1). Here, their nerve endings release a second messenger, nitric oxide. Nitric oxide activates the enzyme guanylyl cyclase, which lyses guanosine triphosphate (GTP) to produce a third messenger, the intracellular cyclic guanosine monophosphate (cGMP). Ultimately, the result is a decrease of intracellular calcium and an opening of potassium channels with the resultant relaxation of vascular smooth muscle in the arteries, aterioles, and sinusoids of the corpora cavernosa. The sinusoids open and rapidly fill with blood. Finally, the distended sinusoids compress their drainage pathways (venules) against the fibroelastic covering of the cavernosal bodies (tunica albuginea) and trap the blood in cavernosal bodies. The combination of an increased inflow of blood into the penis and coincident markedly diminished outflow results in rapidly increasing intracavernosal pressure that ultimately approximates systolic pressure. At this pressure the penis has sufficient axial rigidity to permit vaginal penetration.
To understand what happens in ED, it's helpful to know some anatomical basics. When aroused by either sensory or mental stimuli, the brain sends a signal through the nerves to the penis, causing the muscles there to relax. This opens up space for blood to flow in and engorge the penis. A membrane within the penis traps blood inside to help maintain the erection, which subsides when the penile muscles contract, forcing blood back into the rest of the body. Any number of things can go wrong in this process, leading to erectile dysfunction.

Abstract | Full Text PDF | PubMed | Scopus (3562) | Google ScholarSee all References During a 9-year follow-up study of 513 of these men who had no ED at the first study, the risk of new-onset ED was analyzed.5x5Feldman, HA, Johannes, CB, Derby, CA et al. Erectile dysfunction and coronary risk factors: prospective results from the Massachusetts Male Aging Study. Prev Med. 2000; 30: 328–338
When it comes to scientific development, in Western medicine, an analytic approach is often used to identify and resolve medical challenges. A hypothesis is first derived through general observations of a phenomenon. A research plan is then carefully designed and data collected. Once sufficient data is collected, critical statistical evaluations are done and conclusions are drawn (4). Every aspects of a disease entity are studied from macroscopic to microscopic views, down to the cellular and molecular levels. The deep understanding of the role of cGMP-specific phosphodiesterase type 5 enzymes in ED and the use of phosphodiesterase-5 inhibitors in treatment of ED exemplifies the success of this approach.
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
There are no studies specifically assessing the effectiveness of intraurethral suppositories of prostaglandin E1 (PGE-1) in diabetic men. A single randomized clinical trial of the effectiveness of this agent in the general population of men with ED documented that 60% of those who tried this agent were able to achieve successful sexual intercourse.53 Unfortunately, in clinical practice, this agent appears to be considerably less effective.54
Penile arterial supply (top) and venous drainage (middle), longitudinal views. Bottom, Transverse and longitudinal views of venous return. From Lue TF. Physiology of penile erection and pathophysiology of erectile dysfunction and priapism. In: Walsh PC, Retik AB, Vaughan ED Jr, Wein AJ, eds. Campbell's Urology. Vol 2. 7th ed. Philadelphia, Pa: WB Saunders Co; 1998:1157-1179. With permission from Elsevier.

Smoking is an independent risk factor for ED. Tobacco smoking causes direct toxicity to endothelial cells, including decreased eNOS activity, increased adhesion expression and impaired regulation of thrombotic factors.6 A meta-analysis of 19 studies by Tengs and Osgood suggested that 40 % of the impotent men studied were current smokers compared with 28 % who had never smoked.49
The development of PDE5-inhibitors is a clear example of how Western medicine approached the problem of ED differently from Eastern medicine. The erectogenic effect of sildenafil (Viagra®) was discovered by accident when patients undergoing heart clinical trials reported better erections as a side effect after taking sildenafil. This observation led to further elucidation of the NO/cGMP signalling pathway and development of PDE5-inhibitors as a first-line therapy in ED (5).
Following the breakthrough in ED treatment using PDE5-inhibitors, Western medicine has now moved on to a new frontier of regenerative medicine, with stem cell and gene therapy leading the way (25). There is a practical need for novel therapy as a significant portion of diabetic or post-prostatectomy ED patients do not respond to oral pharmacotherapy. To date, stem cells derived from different sites including adipose tissue-derived stem cells, bone marrow mesenchymal stem cells and muscle-derived stem cells have been investigated using animal models for ED, to study their effects on neural, vascular, endothelial or smooth muscle regeneration (25,26).
Experimental in vivo studies have implicated central and peripheral neuropathy, impaired neurotransmission, and endothelial dysfunction in the pathogenesis of diabetic ED.26,27 Copulatory behavior and penile reflexes are uniformly impaired 4–12 months after the onset of diabetes in the BB rat.26,27 McVary et al.26 found that peripheral neuropathy accounts for only part of the dysfunctional findings, and that spinal sexual reflexes were also severely impaired.
ED is easily and successfully treated! If your sex drive is unaffected, but you experience problems achieving or sustaining erection for a period of four to five weeks, you may have ED. Talk to your doctor immediately. Don’t delay—erectile dysfunction doesn’t “just go away!” Additionally, ED could be a sign of a serious, even life-threatening complication, such as congestive heart failure or kidney disease. Ignoring your ED because it’s embarrassing could jeopardize your health.
Abstract | Full Text | Full Text PDF | PubMed | Scopus (24) | Google ScholarSee all References Withdrawal of sexual stimulation causes a return of sympathetic tone and degradation of cGMP, predominantly by phosphodiesterase type 5 (PDE-5) within the trabecular smooth muscle.11x11Nehra, A. Intracavernosal therapy: when oral agents fail. Curr Urol Rep. 2001; 2: 468–472
Abstract | Full Text | Full Text PDF | PubMed | Scopus (272) | Google ScholarSee all References No data suggested adverse interactions between sildenafil and other drugs commonly used in the treatment of coronary artery disease, such as aspirin, heparin, or narcotics.10x10Kloner, RA and Zusman, RM. Cardiovascular effects of sildenafil citrate and recommendations for its use. Am J Cardiol. 1999; 84: 11N–17N
Organic nitrates are drugs that widen arteries by increasing their supply of nitric oxide; that's how they open the partially blocked coronary arteries in patients with angina. But because nitrates and ED pills both act on nitric oxide, the drugs don't mix; healthy volunteers given Viagra followed an hour later by nitroglycerin see their blood pressures drop by 25–51 mm Hg, a potentially dangerous amount. All experts agree that men who are taking nitrates cannot use ED pills; this includes all preparations of nitroglycerin (short-acting, under-the-tongue tablets or sprays), long-acting nitrates (isosorbide dinitrate or Isordil, Sorbitrate, and others, and isosorbide mononitrate, Imdur, ISMO, and others), nitroglycerin patches and pastes, and amyl nitrite or amyl nitrate (so-called poppers, which some men use for sexual stimulation).

Table 3Metabolic Equivalent (MET) Values for Various Physical Activities56x56Wallis, RM, Corbin, JD, Francis, SH, and Ellis, P. Tissue distribution of phosphodiesterase families and the effects of sildenafil on tissue cyclic nucleotides, platelet function, and the contractile responses of trabeculae carneae and aortic rings in vitro. Am J Cardiol. 1999; 83: 3C–12C
Nebivolol seems to have an advantage over other beta-blockers when used to treat men with hypertension and ED. It has additional vasodilating effects because it stimulates endothelial release of nitric oxide (NO), resulting in relaxation of smooth muscle in the corpus cavernosum, allowing penile erection.25 Despite limited studies, nebivolol does not seem to worsen erectile function and some studies have demonstrated significant improvement in erectile function with nebivolol compared with second-generation cardioselective beta-blockers.23,26–28
Testosterone therapy (TTh) should be reserved for patients who (i) are symptomatic (ED or reduced libido) of testosterone deficiency45 and (ii) they have biochemical evidence of low testosterone (TT <8 nmol/L or 2.3 ng/mL). In men with borderline TT (8–12 nmol/L or 2.3–3.5 ng/mL), a TTh trial (for 3–6 months and continuation if effective) may be envisaged. While adding a PDE5 inhibitor can be considered in men who have not improved with TTh, the usual clinical scenario is to add TTh in patients who have not responded to PDE5 inhibitors. Improvement is dependent on the testosterone levels with better results being obtained at lower levels of TT.45 Despite evidence of benefit in patients with pre-existing cardiovascular conditions (angina or heart failure), it should be emphasized that TTh is not a medication with cardiovascular indications.
Abstract | Full Text PDF | PubMed | Scopus (29) | Google ScholarSee all References After controlling for diabetes mellitus, tobacco use, and hyperlipidemia, hypertension was not found to be an independent predictor of vasculogenic ED in 440 impotent men as measured by the PBI.7x7Virag, R, Bouilly, P, and Frydman, D. Is impotence an arterial disorder? a study of arterial risk factors in 440 impotent men. Lancet. 1985; 1: 181–184

The natural history of ED in people with diabetes is normally gradual and does not occur overnight. Both vascular and neurological mechanisms are most commonly involved in people with diabetes. Atherosclerosis in the penile and pudendal arteries limits the blood flow into the corpus cavernosum. Because of the loss of compliance in the cavernous trabeculae, the venous flow is also lost. This loss of flow results in the inability of the corpora cavernosae to expand and compress the outflow vessels.
Abstract | Full Text | Full Text PDF | PubMed | Scopus (95) | Google ScholarSee all References Rates of severe cardiovascular adverse effects were also similar at 1.7 per 1000 person-years of treatment with sildenafil compared with 1.0 events per 1000 personyears with placebo treatment.10x10Kloner, RA and Zusman, RM. Cardiovascular effects of sildenafil citrate and recommendations for its use. Am J Cardiol. 1999; 84: 11N–17N
Abstract | Full Text | Full Text PDF | PubMed | Scopus (95) | Google ScholarSee all References In conclusion, sildenafil, when used alone, seems to produce minimal decreases in blood pressure level, which are well tolerated in healthy patients and in those with stable ischemic coronary disease.3x3Zusman, RM, Morales, A, Glasser, DB, and Osterloh, IH. Overall cardiovascular profile of sildenafil citrate. Am J Cardiol. 1999; 83: 35C–44C
PubMed | Google ScholarSee all References Unlike sildenafil, vardenafil has been associated with a slight prolongation of the QT interval and thus should not be used by patients with a congenital QT prolongation or by any patient currently taking antiarrhythmic medications.67x67Vardenafil (levitra) for erectile dysfunction. Med Lett Drugs Ther. 2003; 45: 77–78
The natural history of ED in people with diabetes is normally gradual and does not occur overnight. Both vascular and neurological mechanisms are most commonly involved in people with diabetes. Atherosclerosis in the penile and pudendal arteries limits the blood flow into the corpus cavernosum. Because of the loss of compliance in the cavernous trabeculae, the venous flow is also lost. This loss of flow results in the inability of the corpora cavernosae to expand and compress the outflow vessels.
Mancia G,  Laurent S,  Agabiti-Rosei E,  Ambrosioni E,  Burnier M,  Caulfield MJ,  Cifkova R,  Clément D,  Coca A,  Dominiczak A,  Erdine S,  Fagard R,  Farsang C,  Grassi G,  Haller H,  Heagerty A,  Kjeldsen SE,  Kiowski W,  Mallion JM,  Manolis A,  Narkiewicz K,  Nilsson P,  Olsen MH,  Rahn KH,  Redon J,  Rodicio J,  Ruilope L,  Schmieder RE,  Struijker-Boudier HA,  van Zwieten PA,  Viigimaa M,  Zanchetti A. European Society of HypertensionReappraisal of European guidelines on hypertension management: a European Society of Hypertension Task Force document, J Hypertens , 2009, vol. 27 (pg. 2121-2158)https://doi.org/10.1097/HJH.0b013e328333146d
Crossref | PubMed | Scopus (72) | Google ScholarSee all References This study found that the mean PSV was a better predictor of the presence of cardiovascular disease than stratification by standard cardiac risk factors such as diabetes mellitus, hypertension, obesity, and smoking. The researchers recommended that persons with no history of prior perineal trauma and with a PSV lower than 35 mL/s should undergo exercise testing before receiving treatment of ED because these patients had a 42% risk of having ischemic heart disease. However, other investigators questioned the utility of using penile arterial flow to predict the presence of ischemic heart disease.18x18Chiu, AW, Chen, KK, Chen, MT, Chang, LS, and Chang, MS. Penile brachial index in impotent patients with coronary artery disease. Eur Urol. 1991; 19: 213–216
The recommended dosage of sildenafil is 50 mg/day, usually taken 1 hour before sexual activity. This dose may be increased to 100 mg or decreased to 25 mg based on side effects.6 PDE5 inhibitors also have a beneficial effect in the treatment of heart failure with reduced ejection fraction as well as pre- and post-capillary pulmonary hypertension. The use of PDE5 inhibitors in the treatment of right heart failure and left ventricular failure associated with combined pre- and post-capillary pulmonary hypertension has been well studied.71,72
Logically, ED secondary to testosterone deficiency should be treated by testosterone replacement. Testosterone levels in men decrease with age.4 Both epidemiological and observational studies have demonstrated that reduced testosterone is associated with increased cardiovascular risk. One meta-analysis showed lower testosterone and higher 17β oestradiol as significant risk predictors despite adjustment for age and body mass index.4 Patients with coronary artery disease (CAD) have been found to have lower testosterone levels than controls, and there is inverse correlation between testosterone and the incidence of major cardiovascular disease (CVD).4 A significant negative correlation has been reported between total testosterone levels and Framingham risk score.4 However, it has been pointed out that ‘It is unclear if this is a causal association or due to low testosterone being a biomarker of poor health’.4 Testosterone replacement as a treatment for …
Some blood pressure medicines can also cause erectile dysfunction. Thiazide diuretics and beta-blockers are most likely to cause problems, but this is not a common effect of these medicines and will not happen to everyone. If you are taking either of these medicines and are worried about erectile dysfunction, your GP may be able to change your medicines.

Another common reason for failures of oral therapy is the absence of sexual or genital stimulation prior to attempting sexual intercourse. These medicines facilitate an erection by increasing blood flow to the penis, but they do not act as an aphrodisiac or as an initiator of the erection. A man who is not “in the mood” or does not have adequate physical stimulation will not respond with an erection.
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