Vacuum extractor

A vacuum extractor for use in childbirth has an evacuable suction cup for engaging the child's scalp, the cup being connected to a source of a pulling force in a manner which ensures that the line of the force passes substantially through the center of the cup's lip, thereby minimizing the production of tilting moments on the cup on application of an oblique pulling force.

This invention relates to a vacuum extractor. 
The vacuum extractor has been well accepted as having a useful place in 
modern obstetrics since Malstrom's cup was introduced in 1954. John Yonge 
(1706) had first tried to use a suction device but his instrument met with 
very little success. The vacuum found a place in clinical practice again 
in 1947 with Couzigous' ventouse eutocique in France, and Koller's 
instrument in Norway in 1950. 
Since then the instrument has been improved by a number of workers. Lovset 
in 1965 suggested modifying the vacuum, by making one side of the vacuum 
cup thinner, moving the suction connection from the centre to the edge of 
the cup, and applying traction to the cup from four equally-spaced points 
at the peripheries of the cup. These points were attached to two loops of 
cord, each of which had its own block attached. The two blocks were then 
joined by a further loop which had a further block attached to it. However 
this arrangement was not considered practical, and did not gain wide 
acceptance. 
In 1969, Bird suggested applying traction to the cup by a chain llinked to 
a small, centrally situated half ring. The peripherally placed vacuum 
suction tubing was connected to his cup in a similar manner to Lovset's 
cup. 
In modern obstetrics the vacuum cups designed by Bird and Malstrom have 
gained wide acceptance. Both of these cups have the intrinsic disadvantage 
of producing a rotational force, causing one edge of the cup to lift while 
the other remains the fulcrum. The problem is illustrated in FIG. 1 of the 
accompanying drawings in which: 
F=Traction required to break the vacuum. 
A=Effective lifting force on fetal head. 
B=Sliding force. 
C=Rotational force acting on point E. 
P=Vacuum pressure attaching vacuum to fetal head. 
By vector analysis the force required to break the vacuum can be calculated 
using the formula: 
EQU F=[(.pi.R.sup.3 P)/(X Sin .phi.+R Cos .phi.)] 
The force (A) which effectively lifts the baby can be calculated using the 
formula: 
EQU A=F Cos .phi. 
This has to be computed for angles from 0.degree. to 45.degree. from the 
perpendicular. With a 5 cm Bird cup, the effective traction on the baby is 
approximately 50% when the direction of pull is 45.degree. from the 
perpendicular axis. 
Most clinicians are aware of the problem and so attempt to pull at 
90.degree. to the surface of the cup. As this is not always possible, the 
experienced clinician is able to apply force at the margin of the cup. 
These rotational forces introduce unnecessary tissue trauma and decrease 
the effective lifting force of the vacuum cup. 
According to the present invention there is provided a vacuum extractor 
having a cup shaped member for forming an air-tight seal around its lip in 
use and having means through which its interior can be evacuated in use, 
elongate connection means for transferring an applied force to the 
cup-shaped member, and adaptor means through which the connection means is 
attached to the cup-shaped member, the adaptor means being adapted and 
arranged so that the line of a force exerted on the cup through the 
connection means passes through or adjacent the centre of a closed figure 
defined by the lip of the cup-shaped member over a range of angles between 
the line of force and a line normal to the plane of the closed figure. 
The adaptor means may be attached to the connection means through a ring or 
hook for attachment of a chain, cord, string, plastics extrusion or the 
like. The ring or hook may be slidable on an arcuate rod, the centre of 
the rod's arc being at or adjacent the centre of the closed figure defined 
by the lip of the cup-shaped member. The rod itself may be rotatable on 
the cup-shaped member so that the rod describes part of the surface of an 
imaginary sphere centred at or adjacent the centre of the closed figure. 
The rod may extend rigidly from a ring or plate freely rotatable on the 
base of the cup-shaped member. 
Alternatively the adaptor means can be provided by a cord, string or other 
elongate flexible member extending from the cup-shaped member at its 
opposed sides. Again, the connection means is attached to the adaptor 
means so as to be movable along it. 
The adaptor means may comprise a second elongate flexible member secured at 
each end of the cup-shaped member at diametrically opposed locations 
intermediate the locations at which the first flexible member is secured, 
and the first and second flexible members are interconnected by a sliding 
connector to which the connection means is attached. 
The cup-shaped member may be of rubber, plastics, metal or other suitable 
material. The adaptor means is preferably in such a form that forces can 
be applied at angles between 0.degree. and 30.degree. in any direction to 
the line normal to the plane of the closed figure defined by the lip of 
the cup-shaped member, although greater or lesser ranges can be provided 
if required, for example by increasing or decreasing the radius of the 
sphere on whose surface the attachment between the connection means and 
the adaptor means may lie.

Referring to FIG. 2, the extractor of this embodiment of the invention has 
a rubber cup-shaped member 1 having a round annular lip 2. Securely 
attached to the side faces of the cup 1 are a pair of cords 3 and 4 
disposed so as to extend across the base 5 of the cup 1 at right-angles to 
one another. First and second metal rings 6 and 7 encircle the cords 3 and 
4 respectively, and also pass through one another, and a length of chain 9 
is attached at one end to the ring 7, its other end being free. 
The first cord 3 is 4 mm shorter than the second cord 4; the first ring 6 
is of 3 mm diameter while the second ring is of 6 mm diameter. 
A tube 8 leads through the side wall of the cup-shaped member 1 and 
provides a connection for a vacuum pump (not shown). 
In use, the cup 1 is held with its annular lip 2 on the surface of a baby's 
head prior to delivery, with the lip 2 forming an air-tight seal against 
the head. The tube 8 has already been connected to a vacuum pump, and this 
is now switched on, evacuating the interior of the member 1 to seat it 
firmly on the baby's head. The obstetrician grips the free end of the 
chain 9 and pulls it, applying traction to the ring 7 in a direction F1, 
F2 or F3. The rings 6 and 7 slide along their respective cords 3 and 4 
under the effect of the traction in each case to the positions shown in 
FIG. 2 so that the line of applied force F1, F2, F3 passes through a point 
D lying at the centre of the circle defined by the lip 2. In this way 
traction is applied to the baby's head through the cup 1 without causing 
rotational forces on the cup 1, even though the line of applied force is 
not at right-angles to the base of the cup. The interaction of the cords 3 
and 4 ensures that the rings 6 and 7 are constrained to move on the 
surface of an imaginary sphere centred on the point D, as shown by the 
dotted line A. 
Referring now to FIG. 3, the cup 1 is of stainless steel and its side wall 
is generally of the same shape as that in FIG. 2. However, its base is 
recessed at 10, with a peripheral inwardly facing flange 11 around it. An 
annular bearing 12 is located in the recess 10 below the flange 11 by 
which it is held in place, and a circular metal plate 13 has its periphery 
in engagement with the bearing 12 so as to be rotatable within the recess 
10. 
The plate 13 has a diametrically-directed arcuate rod 14 formed integrally 
with it, the centre of the rod's arc being at point D, the centre of the 
circle defined by the annular lip 2 of the cup 1. A ring 15 is loosely 
held on the rod 14 and is slidable along it, and a metal chain 9 is 
attached to the ring 15 for applying traction. 
The extractor of FIG. 3 is used in similar fashion to that of FIG. 2. The 
tube 8 is connected through a rubber hose to an air pump and the cup 1 is 
placed on the head of a baby to be delivered, so as to form an air-tight 
seal around its lip 2. Suction is applied from the pump to reduce the 
pressure of air within the cup 1, and traction is applied by the 
obstetrician pulling on the free end of the chain 9 in a direction F. 
Under the effect of this force F the ring 15 slides along the rod 14, and 
the plate 13 rotates on its bearing 12, until the line of the force F 
passes through the point D. In this position equilibrium is achieved, and 
if the angle of the line of force F is changed, by the obstetrician 
altering the position of the free end of the chain 9, the ring 15 slides 
on the rod 14 and the plate 13 rotates until equilibrium is re-established 
by the new line of force passing through the point D. The arrangement is 
therefore self-aligning. 
By providing an arrangement which ensures that the line of applied force F 
always passes through the centre D of the closed figure defined by the lip 
2 of the cup-shaped member 1, rotational forces about an axis in the plane 
of the closed figure are minimised or eliminated, and the tendency for the 
cup 1 to tilt and break the air-tight seal with the baby's head is 
correspondingly reduced. The ease and success of traction during delivery 
is thus greatly enhanced, so the extractors of this invention as shown in 
FIGS. 2 and 3 provide a valuable tool for the obstetrician. Further, this 
result has been achieved in a simple and inexpensive apparatus which can 
be made either disposable or re-usable depending on the materials of 
manufacture selected. 
Using such vacuum apparatus of the invention the effective lifting force 
resulting from the vacuum within the cup 1 is maintained at all angles of 
the line of force F; the applied force required to break the seal between 
the lip 2 and the baby's head increases with the angle of the line of 
force F from a line normal to the baby's head, or vacuum surface. 
FIG. 4 is a plan view of the extractor of FIG. 3. 
Modifications and improvements may be made without departing from the scope 
of the invention.