Intens. Care Med. 5, 87 (1979) Intens ive Care Medic ine �9 by Springer-Verlag 1979

Comments on the Article: "Tracheostomy and Indwelling Central Venous Line"

Causality, Co inc idence and C o n s e q u e n c e G. Wolff

Michel, McMichan and Bachy (1979) report on 390 intra- venous subclavian catheters placed for 1 - 36 days (mean 8.4 days) in 327 patients. 74 catheters were left for an un- known number of days in an unknown number of patients with tracheostomy. In 201 patients with infectious loci, 39 tips of catheter showed positive cultures, but in 189 patients without infectious loci, positive cultures were only found in 8 tips of catheter. In the presence of bacteriemia, the incidence of catheter contamination was 45%. The authors present as the most important finding of the study that the incidence of contamination rose up to 81% when tracheostomy and bacteriemia were both present. Statistical methods have proved that this finding is very unlikely to be caused by chance. These findings are facts. What can be concluded from these facts is, however, another question. Since a patient is tracheostomized only if quite a long period of mechanical ventilation and / or tracheobronchial suction is to be expected, one must assume that tracheo- stomized patients were in the ICU for a longer period of time, that this prolonged care in the ICU was because of their especially bad condition, that more o f them simul- taneously had several catheters and that the catheters were in place for a longer period. However, there is good evidence that an important factor for increased rate of infectious complications is the number of days the in- vestigated catheter is left in the vein, and the number of days (and how many) additional catheters are simultane- ously in the same or in other veins or arteries (Frey et al. 1977). Therefore one must assume that the rate of contaminated catheters in the group of tracheostomized patients is in any case higher than in the group of patients without tracheostomy and that this difference is not necessarily caused by a corresponding difference in a third parameter as e.g. if the catheter were placed in the sub- clavian vein or in another vein; this correspondence could well be due to mere coincidence. The difference between causality and coincidence cannot be investigated by statistical methods, but solely by adap- ting the protocol to the new question. To make this point absolutely clear, we would like to re- count a well known local fable: Once upon a time a number of young ladies were reluctant to believe that

babies were brought by the stork, unless a very sophisti- cated study could prove that the largest number o f storks were sighted in September, that most babies were delivered in September and - cutting down any discussion - that p was less than 0.0001. After this paper was pub- lished this group of young ladies considered it advisable to leave the country for the whole of September. We have no empirical evidence as to whether their faith was justified! The common feature of all medical studies- as in any em- pirical science - is, that no statement can be proved to be true. But if a statement makes sense and if this statement is formulated as a falsifiable hypothesis (Popper 1976) being tested by conclusive investigation it must be accepted as being useful, and confirmed as long it also passes future tests. It cannot be proven to be true; but if only one test cannot affirm it, the hypothesis is proven to be false.

The falsifiable form of Michel's hypothesis could be: "In a tracheostomized patient the risk o f an intravenous catheter being contaminated is increased if this catheter is placed in the subclavian vein compared to the position in another vein." As long as this hypothesis is investigated and can be con- firmed, we adopt it. The paper o f Michel does not investi- gate this hypothesis, it is therefore not confirmed nor re- futed. So we remain free to place the catheter in a sub- clavian vein even in a tracheostomized patient.

References

1. Michel, L., McMichan, J.C., Bachy, J.L.: Tracheostomy and indwelling central venous line: A hazardous combination ? Intens. Care reed. 5, 83 (1979)

2. Frey et al.: Iatrogene Sch~iden in der Intensivmedizin, Rund- tischgespfiich an der Jahresversammlung der Schweiz. Gesell- schaft fiir Intensivmedizin, 11./12.11. 1977, Regensdorf/ZH (Publication in preparation)

3. Popper, K.R.: Logik der Forschung. Ttibingen: J.C.B. Mohr (Paul Siebeck), 6. verbesserte Auflage, 1976

PD Dr. G. Wolff Leiter der Abt. Intensivmedizin Departement flit Chirurgie der Universit/it Kantonsspital CH - 4031 Basel Switzerland

0342-4642/79/0005/0087/$ O1.00