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times as long as the width of the base may be used.During experiments on intravital staining of flaps

raised on the flank of a pig, Dr. Stuart Milton, inOxford, made the chance observation that the flaps hecut, though of very different widths, all survived toapproximately the same length.’ A flap consisting ofalmost the whole pig flank survived to the same lengthwhether left intact or split longitudinally into severalflaps of different widths. These flaps were not con-forming to the old concept relating width of base tolength, in that the viable length of the narrow oneswas equal to that of the broad ones. It was found

mathematically that they did, however, conform to aconstant width-of-base to area-of-flap ratio. This new

concept of relating width of base to flap area, ratherthan to flap length, makes sense in terms of physiology,as it is the actual amount of tissue that can be nourished

by a given width of base that matters. Doubling thewidth and length of a flap produces twice as muchblood-supply, but four times the amount of tissue to benourished. No matter how wide the base of a flap, itwill only survive to a constant length on any givenpart of the body. Doubling the width of the basedoubles the volume of tissue to be nourished, andtherefore does not allow any increase in length. Con-

versely, narrowing the base of a parallel-sided flapdoes not reduce the viable length unless-as happenedin some of Dr. Milton’s pigs-by so doing the surgeonexcludes a major feeding vessel.

Surgeons will continue to cut flaps in the light oftheir own experience. However, they will now thinkin terms of area-to-width, rather than length-to-width,and perhaps bear this in mind when cutting extremelylarge flaps which clinical experience has shown arenot always safe, even when designed " square".Dr. Milton has shown us why.

SEPTIC THROMBOPHLEBITIS AND VENOUSCANNULAS

A CONNECTION between septicaemia, thrombo-

phlebitis, and indwelling venous cannulas has beenestablished; and septic complications seem to becommoner now that these cannulas are being increas-ingly used.2,3 Septic thrombophlebitis is a seriouscondition and, even without cannulas, it is a well-knowndanger of many suppurative infections, such as septicabortion and osteomyelitis. Septic emboli may producelung abscesses, a generalised pyaemia or septicaemia,and even bacterial endocarditis.

During the past few years, disturbing reports havecome from the burns unit of the U.S. Army Instituteof Surgical Research in Texas concerning septicaemiaand septic thrombophlebitis.4-7 The thrombophlebitiswas traced to the use of indwelling venous cannulaswhich were being, or had been, used during the1. Milton, S. H. Br. J. Surg. 1970, 57, 502.2. Phillips, R. W., Eyre, J. D., Jr. New Engl. J. Med. 1958, 259, 729.3. Elfving, G., Hästbacka, J., Tanmisto, T. Am. Heart J. 1967, 73, 717.4. O’Neill, J. A., Pruitt, B. A., Foley, F. D., Moncrief, J. A. J. Trauma,

1968, 8, 256.5. Foley, F. D. Am. J. clin. Path. 1969, 52, 1.6. Pruitt, B. A., Stein, J. M., Foley, F. D., Moncrief, J. A., O’Neill,

J. A. Archs Surg. 1970, 100, 399.7. Stein, J. M., Pruitt, B. A. New Engl. J. Med. 1970, 282, 1452.

shock phase and later for fluid therapy or otherreasons. Large burns, of course, commonly becomeinfected and septicaemia has nearly always been dueto heavy infection; but this danger has been lessenedrecently by the topical use of antibacterial agents suchas mafenide (’Marfanil’, ’ Sulfamylon’) and silvernitrate. Mafenide is used in Texas, where, from aseries of necropsies, Foley 5 found that by 1966 septicphlebitis of cannulated veins had become a source offatal septicaemia equal in importance to the bumwound itself. Sometimes the cannula was still presentat necropsy, but in other cases it had been withdrawndays or even weeks before. This-point is emphasisedin the clinical reports, and Stein and Pruitt observedthat the cut-down incision was often healed by thetime septic thrombophlebitis was diagnosed. Septicthrombophlebitis was found in 4% of 1929 inpatientsduring 1960-68,6 and in 4.6% of 521 patients during1967 and 1968. In 7 of the 24 cases,’ complicatingsepticsemia was the cause of death. The textbookpicture of superficial phlebitis (pain, tenderness,swelling, and a red streak) was seldom seen, evenwhen the cannula had been inserted through unburnedskin, and the diagnosis was made only post mortemin more than half the cases. This silent onset suggeststhat infective thrombophlebitis following cannulationmay be commoner than is realised. When it was looked

for, it was found in 4 out of 60 surgical cases, in 2 ofwhich it was lethal. The septicaemia unheralded byclinical evidence of thrombophlebitis has a parallelwith pulmonary embolism from silent deep-veinthrombosis.

In the first report of the Texas workers,3 Staphylo-coccus aureus was the most common organism isolatedfrom the infected material in the veins, but gram-negative bacteria, especially Klebsiella, Providencia,and Proteus species, were dominant later. The exactmechanism of the infection is uncertain, but thecut-down incisions seemed an unlikely source becausethey were often healed at the time of diagnosis.Bacteria entering the lumen of the cannula may betrapped by the small thrombus which normally formsat its tip, and this is the seed from which the infectiveprocess may spread.As for clinical diagnosis, Stein and Pruitt advocate

exploration of venotomy sites in all suspected cases,milking the vein, and culturing any purulent or

thrombotic material which exudes. Section of thevein and antibiotic therapy are advocated by Hume etal.,9 who also warn against using heparin when thereis evidence of septicaemia. The Texas workers insistthat if septicaemia is present, the only certain cure isexcision of the whole of the affected vein and itsaffected tributaries. Prevention, however, is better.The connection between the indwelling duration ofthe cannula and thrombophlebitis is a guide to action,since the time was longer than two days in nearly allcases. Indwelling venous catheters should be used

only when strictly necessary and not when a needle issufficient; and they should be withdrawn as soon aspossible.8. Stein, J. M. Archs Surg. 1970, 100, 403.9. Hume, M., Sevitt, S., Thomas, D. P. Venous Thrombosis and

Pulmonary Embolism. Cambridge, Mass, 1970.