Abstract:
The injection syringe which is particularly used for medical purposes. The syringe has a body, a piston which is disposed in the syringe body for displacement therein and an injection needle. The piston has at least two piston elements, 3 which are displaceable in relation to each other along the longitudinal axis of the syringe body.

Description:
This application is a continuation of application Ser. No. 07/851,391, filed Mar. 16, 1992 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to an injection syringe which is particularly used for medical purposes, the injection syringe including a syringe body, a piston being disposed in the syringe body for displacement therein, and an injection needle being located at the front end of the syringe body. 
     What is known is an ordinary injection syringe usually used for intravascular injections (for example, a product of the firm &#34;Becton Dickinson&#34; having a volume of 20 ml) and having a cylindrical syringe body, a piston with a piston rod and an injection needle. In most cases a mixture of medical substances is injected with this injection syringe, the mixture containing a minor amount of an active ingredient and a large amount of a diluent. 
     A disadvantage of this injection syringe is that it is not possible to precisely draw up a low dose of an active ingredient (for example 0.3-0.5 ml) due to the large volume of the injection syringe. 
     What is also known is an ordinary injection syringe (for example, a product of the firm &#34;Becton Dickinson&#34; having a volume of 1 ml) which also has a cylindrical body, a piston and a piston rod and a injection needle, and which enables a user to draw up a very precise amount of an active ingredient. However, intravascular injections cannot be carried out with this injection syringe, as for most intravascular injections it is necessary to draw up 0.3-0.5 ml of an active ingredient and to mix it with 10-20 ml of a diluent. 
     The problem underlying the invention is to provide an injection syringe by simple construction with which precise doses of an active ingredient and of a diluent can be drawn up. 
     SUMMARY OF THE INVENTION 
     Starting out from the injection syringe of the generic kind, this problem is solved by a piston including at least two piston elements which can be displaced in relation to each other along the longitudinal axis of the syringe body. 
     One rod is mounted on the side of each piston element which is remote from the injection needle so that the corresponding piston element can be pulled out or pushed in, respectively. 
     In order to be able to read the precise amount of a dose two scales having different gradations can be provided, the more precise scale being used for reading the amount of an active ingredient and the less precise for reading the amount of diluent. 
     In one embodiment of the invention the piston rod of one piston element has a central channel, the inner diameter thereof corresponding to the outer diameter of the other piston element arranged in the central channel, the less precise scale being provided on the syringe body and the more precise scale being provided on one of the piston rods. 
     In another embodiment of the invention both piston elements are arranged in the syringe body in such a manner that their outer surfaces are in contact with the inner surface thereof, both piston elements forming a cylinder, when they are disposed adjacent to each other, the outer diameter of the cylinder corresponding to the inner diameter of the syringe body. The scales of different gradation are provided on the cylindrical syringe body. 
     The injection syringe according to the invention has a high dosing accuracy when drawing up and diluting a substance. It can be easily produced and is reliable. The costs for producing the injection syringe according to the invention hardly exceed the costs for producing the syringes which are presently known. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, embodiments of the invention are further explained by means of the accompanying drawings, wherein 
     FIG. 1 shows a first embodiment of an injection syringe in disassembled form; 
     FIG. 2 shows a second embodiment of an injection syringe in  disassembled form; 
     FIG. 3 shows a longitudinal cross-section of the injection syringe of FIG. 1; 
     FIG. 4 shows a longitudinal cross-section of the injection syringe of FIG. 2; 
     FIG. 5 shows a longitudinal cross-section of the injection syringe of FIG. 1 at the moment of drawing up a small dose of an active ingredient; 
     FIG. 6 shows a longitudinal cross-section of the injection syringe of FIG. 1 at the moment of drawing up a diluent. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The injection syringe shown in FIGS. 1, 3, 5 and 6 includes a cylindrical or tubular syringe body 1 and a cylindrical piston 10 which is arranged in the syringe body 1 in such a manner that it can be displaced therein. A conical holding device 7 for an injection needle 6 is located at the front end of the syringe body 1. The injection needle 6 can either be detachable or fixed. 
     The piston 10 consists of two piston elements 2 and 3 which can be displaced in relation to each other along the longitudinal axis of the syringe body 1. The outer diameter of the first piston element 2 corresponds to the inner diameter of the syringe body 1. A piston rod 4 is mounted on the side of the first piston element 2 which is remote from the injection needle 6. A central channel 8, in alignment with the longitudinal axis of the syringe body 1 and having a constant diameter, runs through the piston rod 4 and the first piston element 2. The second cylindrical piston element 3 is arranged in this central channel 8 in such a manner that it can be displaced. The outer diameter of the second piston element 3 corresponds to the inner diameter of the central channel 8. A piston rod 5 is also mounted on the side of the second piston element 3 which is remote from the injection needle 6. 
     On the syringe body 1 there is a coarse scale (FIG. 1) having a gradation of 1 ml for example. The piston rod 5 of the second piston element 3 is marked with a fine scale having a gradation of 0.05 ml for example (FIG. 1). 
     If an injectable solution is drawn up with this injection syringe, the end of the injection needle 6 is immersed into an ampoule or vial containing an active ingredient, and the second piston element 3 is slightly pulled out of the central channel 8 of the piston rod 4 of the first piston element 2. Thus, a vacuum is created in the central channel 8 and the active ingredient flows from the ampoule through the injection needle 6 into the central channel 8 (FIG. 5). This guarantees that the active ingredient can be drawn up with a precision of 0.05 ml since the piston rod 5 has such a gradation. If more than one milliliter of the strong active ingredient, for example 1.25 ml, are to be drawn up, this scale and the scale on the body 1 will be used together. 
     After the active ingredient has been drawn up, the injection needle 6 is immersed into a diluent, and the first piston element 2 is displaced by pulling the corresponding piston rod 4 in a direction opposite to the injection needle 6. Thus, the necessary amount of diluent can be drawn up (FIG. 6). 
     In the embodiment of the injection syringe shown in FIGS. 2 and 4 the conical holding device 7 for the injection needle 6 is located at the area of edge of the front end of the syringe body 1. The piston consists of two adjacent piston elements 2a and 3a both of which have the form of a cylindrical segment. The front part thereof, which in general is circular, is limited by a straight chord 11 and 13, respectively. When both piston elements 2a and 3a and their straight chord 11, 13 are adjacent to each other they form a complete cylinder, with the outer diameter thereof corresponding to the inner diameter of the syringe body 1. Thus, the outer surfaces of both piston elements 2a, 3a are in contact with the inner surface of the syringe body 1. The cylindrical syringe body 1 is marked with scales of different gradation (not shown). 
     A piston rod 4a and 5a, respectively, is arranged at the side of each of the piston elements 2a, 3a which is remote from the injection needle 6. Each of the piston rods 4a and 5a have a surface 14 and 15, respectively lying in extension of the corresponding chord, the surface being parallel to the longitudinal axis of the syringe body 1. By these surfaces 14, 15 it is possible that both piston elements 2a and 3a can be disposed in relation to each other along the longitudinal axis of the syringe body 1. 
     The piston element 3a, the front surface of which is considerably smaller than the front surface of the first piston element 2a, is located directly before the conical holding device 7. 
     An injection solution is drawn up with the injection syringe shown in FIGS. 2 and 4 in the same manner as with the injection syringe shown in FIG. 1.