A practical approach to prosthetic valve endocarditis

2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease: Executive Summary: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines

AIM

This executive summary of the valvular heart disease guideline provides recommendations for clinicians to diagnose and manage valvular heart disease as well as supporting documentation to encourage their use.

METHODS

A comprehensive literature search was conducted from January 1, 2010, to March 1, 2020, encompassing studies, reviews, and other evidence conducted on human subjects that were published in English from PubMed, EMBASE, Cochrane, Agency for Healthcare Research and Quality Reports, and other selected database relevant to this guideline. Structure: Many recommendations from the earlier valvular heart disease guidelines have been updated with new evidence and provides newer options for diagnosis and treatment of valvular heart disease. This summary includes only the recommendations from the full guideline which focus on diagnostic work-up, the timing and choice of surgical and catheter interventions, and recommendations for medical therapy. The reader is referred to the full guideline for graphical flow charts, text, and tables with additional details about the rationale for and implementation of each recommendation, and the evidence tables detailing the data considered in developing these guidelines.

Early and late outcomes after reoperation for prosthetic valve endocarditis

OBJECTIVE

Prosthetic valve endocarditis remains a challenging complication after heart valve replacement. To identify predictive risk factors, we have reviewed 30 patients who underwent surgery for prosthetic valve endocarditis between March 1986 and May 1999.

METHODS

There were 15 men and 15 women (mean age 51 years). Prosthetic valve endocarditis was classified as early (< or = 1 year after operation) in 10 cases, and as late in the other 20 cases. The most common indication for surgery was moderate to severe congestive heart failure due to prosthetic valve dysfunction in 21 (70%) patients. The average follow-up period was 6.5 years, with a range of 0.3 to 14.1 years.

RESULTS

The most common microorganism was Staphylococcus epidermidis in both patients with early (50%) and late prosthetic valve endocarditis (25%). The in-hospital mortality was 13.3% (4/30). There were six late deaths. The actuarial survival at 5 years was 78% and 66% at 10 years. An early onset of prosthetic valve endocarditis was the only significant determinant of both in-hospital mortality (p = 0.005) and overall mortality (p = 0.021). Emergency surgery had a statistically significant relationship with in-hospital mortality (p = 0.045). No significant influence on mortality after reoperation for prosthetic valve endocarditis was found in age, sex, valve position, antecedent native valve endocarditis, or in the type of pathological findings (ring abscess, valve dehiscence, and vegetation).

CONCLUSION

Early onset of prosthetic valve endocarditis and emergency surgery were important risk factors for mortality due to prosthetic valve endocarditis.

Physicochemical and antimicrobial properties of boron-complexed polyglycerol–chitosan dendrimers

A polyglycerol with dendritic structure (PGLD) was synthesized by ring-opening polymerization of deprotonated glycidol using a polyglycerol as core functionality in a step-growth process. Then, PGLD reacted with O-carboxymethylated chitosan to obtain PGLD-chitosan dendrimer (PGLD-Ch). After the reaction of PGLD-Ch with boric acid, there was a marked increase in the bulk viscosity evidencing physically that boron can initiate a charge transfer complex formation, (PGLD-Ch)B. Gel permeation chromatography analysis was used to characterize the molecular weight and the polydispersivity of the synthesized PGLD-Ch. A dendritic structure with a molecular mass of 16.7 kDa and a narrow polydispersity (Mw/Mn = 1.05) was obtained. 1H-NMR and 13C-NMR measurements were employed to assess the degree of branching in PGLD. The obtained value of 0.85 indicates the tendency toward a dentritic structure for PGLD. The glass transition temperature values of (PGLD-Ch)B membranes containing 10% and 30% PGLD were -19 degrees C and -26 degrees C, respectively, which favor its potential use as surface coating of several polymers. The in vitro cytotoxicity was evaluated using the minimum essential medium elution test assay. Extracts of boron-complexed PGLD exhibited lower cytotoxicity than the controls, suggesting that the material has an improved biocompatibility. Antibacterial studies of (PGLD-Ch)B against Staphylococcus aureus and Pseudomonas aeruginosa showed a significant activity. Our study confirms and supports the effectiveness of (PGLD-Ch)B as an antimicrobial coating due to its capacity in suppressing the bacterial proliferation. The best in vivo response was found for (PGLD-Ch)B-30 membranes, which exhibited higher synthesis of collagen fibers than PGLD-ChB-10.

Using 18-fluoro-2-deoxyglucose positron emission tomography in detecting infectious endocarditis/endoarteritis

RATIONALE AND OBJECTIVES

We evaluated the effectiveness of positron emission tomography (PET) with 18-fluoro-2-deoxyglucose (FDG) in the detection of infectious endocarditis/endoarteritis.

MATERIALS AND METHODS

For this study, we recruited 6 patients (4 women, 2 men; age range, 35 - 78 years; mean age, 55.8 +/- 16.8 years) who were clinically diagnosed as having infective endocarditis/endoarteritis by their echocardiographic findings and by Duke criteria.

RESULTS

For all 6 patients, we also found increased FDG uptakes in the corresponding areas detected in echocardiography.

CONCLUSION

FDG-PET appears to be a promising tool in diagnosing infective endocarditis/endoarteritis, and further prospective studies on a large scale to fully exploit the usefulness of FDG-PET for infective endocarditis/endoarteritis are needed.

Aortic root replacement using a homograft for recurrent valve endocarditis

Prosthetic valve endocarditis is a relatively rare condition associated with high mortality. Endocarditis affecting 2 successive mechanical valves at the aortic position has not, to the best of our knowledge, been described. We reported such a patient whose condition was further complicated by mitral regurgitation, pulmonary hypertension, worsening heart failure, and cardiac conduction abnormalities. Considering the failure of 2 previous mechanical valves, we conducted a homograft replacement of the aortic root with coronary reattachment. Mitral regurgitation was treated by annuloplasty. The patient's early postoperative course was uneventful and he was doing well 16 months after surgery. We discuss the overall treatment strategy for recurrent prosthetic valve endocarditis and potential homograft advantages.

Efficacy of silver-coated fabric to prevent bacterial colonization and subsequent device-based biofilm formation

Efficacy of silver-coated poly(ethylene terephthalate) to prevent bacterial attachment and subsequent infection was quantified in vitro, in both batch- and flowing-fluid experiments. Kinetic analysis of batch suspended cell cultures of Staphylococcus epidermidis (SE), at various growth-limiting nutrient concentrations, in the absence of any fabric, indicated a maximum culture growth rate constant micro(max) = 0.78 +/- 0.02 h(-1). Batch experiments for Control fabric samples indicated that SE cultures exhibited about the same suspended cell growth rate (0.72 +/- 0.02 h(-1)) as observed in batch suspended cultures without fabric. Suspended SE cultures in the presence of silver-coated fabric grew at a considerably lower rate, 0.15 +/- 0.01 h(-1), indicating the inhibitory effect of Ag(+2) ion released from the fabric. Growth rates of suspended SE cultures were 5-6 times higher in the fluid phase in contact with the Control fabric compared to cultures exposed to silver-coated fabric. Maximum suspended cell concentrations attained at time = 24 h were 1-2 orders of magnitude higher for Control fabrics vs. silver-coated fabric. In all batch colonization experiments, both live and dead SE bacterial cells accumulate on the surfaces of both silver-coated and Control fabrics. Adherent viable SE cells accumulated to 1-2 orders of magnitude more ( approximately 5 x 10(+8) cells/cm(2)) on Control fabric than SE cells on the silver-coated fabric ( approximately 1.1 x 10(+6) cells/cm(2)), respectively. Between 70-95% SE cells on the Control fabric were viable, while on the silver-coated fabric samples, at 24 h, viable cells were less than 10% of the adherent community (i.e., greater than 90% nonviable cells). In flow cell colonization experiments, SE cells accumulated on Control fabric to a maximum adherent cell concentration of 6 x 10(+7) - 8 x 10(+7) cells/cm(2) by 24 h with the proportion of viable cells remaining relatively constant at 76% throughout an experiment. Both noninvasive microscopic enumeration and destructive assays gave the same results for adherent cell numbers. Using silver-coated fabric, total cells numbers (live + dead) reached a level of approximately 1.1 x 10(+7) - 3.0 x 10(+7) cells/cm(2) after about 6 h and remained constant. However, while the proportion of viable cells initially on the surface was 63-75%, this fraction dropped continuously during each experiment to less than 6% viable cells at 24 h. Regardless of the criteria, the number of viable or nonviable cells attached to silver-coated fabric were significantly lower than on Control fabric.

Fungal endocarditis at the aortotomy site after aortic valve replacement

This brief report presents a patient with fungal endocarditis involving the suture location in the ascending aorta after aortotomy to replace a stenotic aortic valve. It emphasizes the importance of expanding the diagnostic investigation for endocarditis beyond the prosthesis, which was normal in this case. It also reiterates the value of transesophageal echocardiography during the evaluation of prosthetic valves and reminds the echocardiographer to visualize the aortotomy site carefully.

Early fungal endocarditis in homograft recipients

Retrospective analysis of 200 homograft valve recipients at our institution revealed two cases of fungal endocarditis. Pathogenesis appears to be related to either recipient seeding in one elderly immunocompromised patient or a previously contaminated donor valve implanted in an otherwise healthy recipient. Therefore, our experience underscores the need for both meticulous prevention of fungal infection preoperatively in the recipient and elimination of previously contaminated homograft valves from the donor pool.

Does a focal neurologic deficit contraindicate operation in a patient with endocarditis?

BACKGROUND

As many as 40% of patients with left-sided bacterial endocarditis will sustain a neurologic insult. The importance of a neurologic change as an indication or a contraindication for valve replacement remains controversial.

METHODS

We performed a retrospective analysis of the records of 33 patients admitted to the University of Virginia Health Sciences Center between January 1, 1978, and June 30, 1996, with a diagnosis of endocarditis and a neurologic change.

RESULTS

All 33 patients had echocardiographic or pathologic evidence of left-sided endocarditis; 23 were seen with focal neurologic findings and had a mortality rate of 22% (5 of 23), and 10 patients were seen with nonfocal, diffuse encephalopathy and had a mortality rate of 60% (6 of 10) (p<0.05). Of the 33 patients, 14 underwent operation and 19 were treated medically. The mortality rate was 21.4% (3 of 14) in the surgical group and 42.1% (8 of 19) in the medical group (p = not significant). In 71% (10 of 14) of the surgical patients, the operation was done within 1 week of the neurologic event. Additional neurologic deterioration occurred in 18.2% (2 of 11) of survivors in the surgical group and 9.1% (1 of 11) in the medical group (p = not significant).

CONCLUSIONS

Choosing therapy for a patient with endocarditis and a neurologic change remains a difficult challenge. Initial findings of nonfocal, global dysfunction on examination are a predictor of a poor outcome. By comparing surgical and medical groups derived from the same series of patients, it is clear that patients with bacterial endocarditis and central nervous system changes face substantial mortality regardless of intervention. However, these data demonstrate that when compared with a similar group of medical patients, surgical patients who require and receive operation early in the course of their illness do comparatively well. Improving outcomes by delaying surgical intervention may serve to "select out" hardier patients but will lead to the death of patients who might benefit from such intervention.

A study of 47 bacteremic Staphylococcus aureus endocarditis cases: 23 with native valves treated surgically and 24 with prosthetic valves

A retrospective review of medical records from the Staphylococcus Laboratory, Copenhagen, 1982-1991, was carried out at the Department of Clinical Microbiology, Statens Serum Institut, 1994-1995, to investigate the clinical features and outcome of two subgroups of bacteremic Staphylococcus aureus endocarditis cases in non-drug addicts: patients with prosthetic valve endocarditis (PVE) and patients with native valve endocarditis treated surgically. Twenty-four cases of PVE were included. Six cases were early (within 60 days of valve implantation) and 18 were late. The overall in-hospital mortality was 42%. Surgical treatment resulted in a non-significantly lower mortality as compared with medical treatment alone (0% vs 50%, p = 0.19). Medical treatment of aortic and mitral valve endocarditis resulted in similar mortality rates (44% and 50%, respectively). Twenty-three cases of native valve infective endocarditis had the valve replaced surgically. The in-hospital mortality was 22%, which was significantly lower as compared with medical therapy (69%, p < 0.0001). The treatment changed significantly during the study period: 6 of 112 patients (5%) were treated surgically in the first half of the period (1982-1986) compared to 17 of 124 patients (14%) in the second half (1987-1991, p = 0.049). Severe congestive heart failure was the main indication for cardiac surgery in 21 patients. In conclusion, a shift towards a more aggressive surgical approach has taken place in the 10-year period. This development should be strengthened in the future as surgical intervention may improve survival in patients with infective endocarditis caused by S. aureus whether the infected valve is prosthetic or native.

Indications for and timing of surgical intervention in infective endocarditis

Infective endocarditis, especially when it involves prosthetic valves, is a serious, often fatal illness. Although antibiotics are essential in management, surgery is required in many patients who develop even incipient heart failure and structural complications. Early identification and referral results in improved mortality and morbidity rates, and there is evidence that surgery should play a larger role in managing infective endocarditis. Patients with intracardiac pacemakers and cardioverting devices represent a growing reservoir of patients with the potential to develop endocarditis.

Echocardiography and Doppler assessment of prosthetic heart valves with transesophageal echocardiography

This article examines the use of transesophageal echocardiography (TEE) in the assessment of prosthetic heart valves. A summary of the commonly used artificial valves and their physiologic regurgitant flow patterns, as identified by color Doppler imaging, is presented. The hemodynamic evaluation of prosthetic valve stenosis using Doppler techniques is reviewed, and the diagnostic utility of TEE in identifying the complications of cardiac prostheses is discussed.

Valve replacement for acute left heart endocarditis

Between January 1982 and June 1993, 66 patients (48 men and 18 women of mean (range) age 50 (21-77) years) underwent valve replacement for acute infective endocarditis. There were 45 aortic valve and 17 mitral valve infections. Four patients had bivalvular involvement. Fifty-three patients suffered from native valve endocarditis, with underlying valvular lesions documented for 31 patients. Staphylococci and Streptococci spp. were responsible for 68% of infection, and 20% of blood and valve cultures were negative. Refractory congestive heart failure was the leading surgical indication in 86% of patients. The mean follow-up period was 44 months. The perioperative mortality rate was 6%. Actuarial survival rates were 88.5% at 1 year and 83% at 3 years. No early recurrence of infection was noted. Six patients (9%) needed reoperation. Satisfactory results confirm that early surgical management should be considered in patients with complicated infective endocarditis.

Antibiotic prophylaxis in dermatologic surgery

Antibiotic prophylaxis is generally administered either to prevent wound infection or to hinder the development of endocarditis. Although the use of antibiotics in certain circumstances to prevent wound infection can be straightforward, there are other circumstances in which the decision to use antibiotics is much less clear. Endocarditis prophylaxis has traditionally been based on the American Heart Association's guidelines, which do not cover dermatologic surgery. This article discusses the rationale and controversies surrounding the use of antibiotic prophylaxis for prevention of both wound infection and endocarditis, reviews the few studies that pertain to dermatology, and provides recommendations for antibiotic prophylaxis on a case-by-case basis for those who perform dermatologic surgery.

Postoperative cardiothoracic infection: diagnostic value of indium-111 white blood cell imaging

During a 6.5-year period, 50 indium 111-labeled white blood cell scans were obtained in 49 selected patients who had undergone sternotomy. Retrospectively, six sternal wounds and eight cardiovascular prostheses were found to be infected in 13 patients. For cardiothoracic surgical sites, the sensitivity, specificity, and accuracy of indium-111-labeled white blood cell scintigraphy were 86%, 97%, and 95%, respectively. Whole-body imaging identified the presence of major extramediastinal sites of infection in 10 patients. In patients with complicated cardiothoracic conditions, indium-111-labeled white blood cell imaging provides valuable diagnostic information regarding a variety of postoperative infections.

Prosthetic valve endocarditis: superiority of surgical valve replacement versus medical therapy only

The objective of our study was to assess the long-term outcome of patients with prosthetic valve endocarditis. We used a multicenter, prospective, observational study design. Six university teaching hospitals with high volume cardiothoracic surgery participated. Seventy-four patients with prosthetic valve endocarditis as defined by explicit, objective criteria were selected for participation. All patients were followed up prospectively for 1 year. Thirty-one percent and 69% had development of endocarditis within 60 days of valve insertion ("early") and after 60 days ("late"), respectively. The most common causes were Staphylococcus epidermidis (40%), Staphylococcus aureus (20%), streptococcal species (18%), and aerobic gram-negative bacilli (11%). Physical signs of endocarditis (new or changing murmur, stigmata, emboli) were seen in 58%. At 6 months and 12 months, mortality was 46% and 47%, respectively. Surgical replacement of the infected valve led to significantly lower mortality (23%) as compared with medical therapy alone (56%), as assessed by both univariate and multivariate analyses (p < 0.05). Improved outcome was seen for the surgical group even when controlling for severity of illness at time of diagnosis. From these findings we conclude that accurate assessment of outcome in prosthetic valve endocarditis requires long-term follow-up of at least 6 months following diagnosis. Surgical therapy warrants greater scrutiny; evaluation in controlled clinical trials is appropriate.

Fibrous skeleton endocarditis: repair using Konno procedure

Information about the surgical management of fibrous skeleton endocarditis is incomplete, as the vast majority of current literature describes the isolated repair of either the aortic or the mitral annulus, the combination rarely being addressed. Annular destruction, in the presence of endocarditis, demands extreme ingenuity for surgical treatment and cure. We describe and illustrate the Konno procedure for replacement of both infected mitral and aortic valves and repair of mycotic ventricular septal defects.

St. Jude prosthesis for aortic and mitral valve replacement: a ten-year experience

From January 1, 1979 through December 1990, 456 adult patients underwent isolated aortic (AVR) (254) or mitral (MVR) (202) valve replacement with the St. Jude prosthesis at the Medical University of South Carolina. Age ranged from 21 to 84 years (mean: 54 +/- 15 years for AVR; 51 +/- 13 years for MVR). Male sex predominated in the AVR group (66%) and female sex in the MVR group (64%). Ninety-two patients (20%) had associated coronary artery bypass grafting (AVR, 25%; MVR, 14%). There were 17 deaths (3.7%) occurring during the same hospitalization or within 30 days (AVR, 10/254 [3.9%]; MVR, 7/202 [3.5%]). Follow-up is 94.5% complete and ranges from 1.0 to 131 months (mean, 55 +/- 37 months; total, 2,073 patient-years). In the AVR group, 53 late deaths have occurred and actuarial survival is 80% +/- 3% at 5 years and 47% +/- 9% at 10 years. Twenty-one patients have sustained thromboembolic episodes (1.8%/patient-year), and the probability of remaining free of thromboembolism at 10 years is 67% +/- 13%. The mean improvement in New York Heart Association functional class from preoperative to postoperative is 3.1 +/- 0.76 to 1.6 +/- 0.84 (p < 0.0001). In the MVR group, there have been 41 late deaths, and the actuarial survival was 80% +/- 3% at 5 years and 63% +/- 5% at 10 years. Twenty-eight patients have sustained thromboembolic complications (2.9%/patient-year), and the probability of remaining free of thromboembolism at 10 years is 77% +/- 5%.(ABSTRACT TRUNCATED AT 250 WORDS)

Utility of transesophageal echocardiography in the conservative management of prosthetic valve endocarditis

Prosthetic valve endocarditis is a formidable complication following cardiac valve replacement. Surgical intervention has resulted in a significant reduction in mortality when certain complications prevail. We report two such cases of prosthetic valve endocarditis in which the use of transesophageal echocardiography permitted close surveillance during medical therapy and thus avoided the need for surgical intervention. Therefore, with the improved ability to monitor disease progression with transesophageal echocardiography, nonsurgical management of prosthetic valve endocarditis remains an option.

Improved sterile technique diminishes the incidence of positive line cultures in cardiovascular patients

In order to determine the incidence of positive line cultures, especially as affected by differing protocols for line insertion, cultures were obtained from lines in residence for up to 4 days from cardiac patients (who received Cefuroxime) and vascular patients (who received Cefazolin) as prophylaxis perioperatively. Positive cultures were obtained from 95 (19%) of 496 lines in cardiac patients and 83 (31%) of 261 lines in vascular patients. There was a linear relationship between duration of line residence and the incidence of positive line cultures, increasing from 14% on Day 1 to 33% on Day 4. The use of full sterile technique at the time of insertion halved the incidence of subsequent positive line cultures. Four of the 403 (1%) patients each had a single episode of postoperative line sepsis. Another four patients developed wound infections with the same organisms as cultured from their lines. One patient has had a vascular graft infection with the same organism cultured as was isolated from a Swan-Ganz line 1 year previously. These data suggest that monitoring lines should be inserted using full sterile technique and removed as soon as the patient is hemodynamically stable.

Treatment of complicated prosthetic aortic valve endocarditis with annular abscess formation by homograft aortic root replacement

The outcome of 30 consecutive patients with active aortic prosthetic valve endocarditis and root abscesses treated by the technique of homograft aortic root replacement with reimplantation of the coronary arteries is detailed. The principles of this technique are the removal of all abscesses and infected areas likely to drain into the infected mediastinum, excision of infected tissues down to healthy noninfected tissue and replacement with an antibiotic-impregnated homograft aortic root. All patients had evidence of progressive cardiac failure and ongoing sepsis. Mean patient age (+/- SD) at the time of operation was 42 +/- 18 years. The mean number of previous aortic valve replacements per patient was 1.6 +/- 0.7; 14 patients (47%) had undergone greater than or equal to 2 previous replacements. At operation, aortic root abscesses were found in all patients; abscess extension to adjacent structures and partial valve dehiscence had occurred in 23. In-hospital death occurred in 9 (30%) of the 30 patients. The 21 hospital survivors have been followed up for a mean of 66 +/- 42 months (range 9 to 144). Overall, 17 (81%) of the 21 hospital survivors have remained free of major adverse events (recurrence of endocarditis, need for reoperation or death). The results of our study suggest that homograft aortic root replacement should be considered favorably in the treatment of patients with aortic prosthetic valve endocarditis and root abscesses.

Heart valve operations in patients with active infective endocarditis

Sixty-two consecutive patients underwent heart valve operation for active infective endocarditis. There were 42 men and 20 women whose mean age was 49 years (range, 21 to 79 years). The infection was in the aortic valve in 37 patients, the mitral valve in 18, the aortic and mitral valves in 5, and the tricuspid valve in 2. Twenty-four patients had prosthetic valve endocarditis. Staphylococcus and Streptococcus were responsible for 86% of the infections. Annular abscess was encountered in 33 patients. Complex valve procedures involving reconstruction of the left ventricular inflow or outflow tract or both were performed in 31 patients. There were three operative deaths (4.8%). Predictors of operative mortality were prosthetic valve endocarditis, preoperative shock, and annular abscess. Patients were followed for 1 month to 130 months (mean follow-up, 43 months). Only 1 patient required reoperation for persistent infection. There were ten late deaths. Most survivors (96%) are currently in New York Heart Association class I or II. The 5-year actuarial survival was 79% +/- 7%. These data demonstrate excellent results in patients with native valve endocarditis, and support the premise that patients with prosthetic valve endocarditis should have early surgical intervention.

Enhanced morphological diagnosis in infective endocarditis by transoesophageal echocardiography

Thirty three consecutive patients with clinically suspected endocarditis were studied by both precordial cross sectional echocardiography and transoesophageal echocardiography. The diagnostic value of both techniques was assessed. The data were compared with findings at operation in 25 patients. In 21 patients with native valve endocarditis precordial echocardiography showed evidence of vegetations in six patients and suggested their presence in nine. Transoesophageal echocardiography identified vegetations in 18 patients. Complications were seen in four patients at precordial echocardiography and in nine patients at transoesophageal echocardiography. Precordial echocardiography did not show vegetations in any of the 12 patients with prosthetic valve endocarditis whereas transoesophageal echocardiography showed vegetations in four. Complications were seen in four patients at precordial echocardiography and in 10 at transoesophageal echocardiography. Echocardiographic findings were confirmed at operation in all 25 operated patients. In two patients both echocardiographic techniques had missed the perforation of the cusps of the aortic valve that was seen at operation, but this had no effect on patient management. Transoesophageal echocardiography is the best diagnostic approach when infective endocarditis is suspected in patients with either native or prosthetic valves.

Effect of electrical stimulus parameters on the development and propagation of action potentials in short excitable fibres

Intracellular action potentials (IAPs) produced by short fibres in response to their electrical stimulation were analysed. IAPs were calculated on the basis of the Hodgkin-Huxley (1952) model by the method described by Joyner et al. (1978). Principal differences were found in processes of activation of short (semilength L less than 5 lambda) and long fibres under near-threshold stimulation. The shorter the fibre, the lower was the threshold value (Ithr). Dependence of the latency on the stimulus strength (Ist) was substantially non-linear and was affected by the fibre length. Both fibre length and stimulus strength influenced the IAP amplitude, the instantaneous propagation velocity (IPV) and the site of the first origin of the IAP (and, consequently, excitability of the short fibre membrane). With L less than or equal to 2 lambda and Ithr less than or equal to Ist less than or equal to 1.1Ithr, IPV could reach either very high values (so that all the fibre membrane fired practically simultaneously) or even negative values. The latter corresponded to the first origin of the propagated IAP, not at the site of stimulation but at the fibre termination or at a midpoint. The characters of all the above dependencies were unchanged irrespective of the manner of approaching threshold (variation of stimulus duration or its strength). Reasons for differences in processes of activation of short and long fibres are discussed in terms of electrical load and latency. Applications of the results to explain an increased jitter, velocity recovery function and velocity-diameter relationship are also discussed.