Patient support apparatus including stabilizing mechanism

A patient support apparatus such a surgical table or a stretcher is provided. The apparatus includes a top assembly for supporting a patient in a variety of selected positions. The top assembly is supported by first and second hydraulic column assemblies which are, in turn, mounted to a base assembly. A stabilizing mechanism is provided for exerting a constant downward force upon the hydraulic column assembly supporting the head end of the top assembly. The stabilizing mechanism counteracts the lifting force at the head end of the apparatus when a patient's weight is located at or beyond the foot end thereof. The stabilizing mechanism includes a gas spring which is mounted to the base assembly and a cable connected between the base assembly and top assembly. The gas spring exerts a constant force upon the cable in all elevational positions of the top assembly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention relates to patient support apparatus such as 
stretchers and other patient handling equipment, and particularly to such 
apparatus including stabilizing mechanisms for preventing tipping thereof 
when a patient's weight is towards one end of the apparatus. 
2. Brief Description of the Related Art 
Patient handling equipment, such as stretchers and surgical tables and the 
like, have been designed for many uses. Such equipment may be capable of 
providing a full range of stretcher and chair positions, thereby providing 
a great deal of versatility. 
Height adjustment of patient handling equipment is often provided through 
the use of one or more hydraulic column assemblies. Controls are provided 
for operating these assemblies. It is important that, upon use of the 
controls, the hydraulic column assemblies cause a steady ascent or descent 
of the patient support portion of the apparatus, whichever is desired. 
The stability of patient handling equipment is another important 
consideration, particularly when a patient is supported by such equipment. 
By moving such equipment between stretcher and various other 
configurations, such as chair or obstetric/gynecological positions, the 
patient's weight is shifted towards one end of the equipment. This can 
cause the telescoping of the hydraulic column assembly at the opposite end 
of the equipment, typically the head end, unless it is in the fully 
elevated position. The negative load at the head end may also prevent this 
end from lowering properly upon actuation of the controls. 
The problem resulting from the application of unequal forces upon a patient 
support apparatus have been recognized by the industry, and various 
approaches have been taken for addressing them. One approach for lowering 
a column assembly having a negative load thereon has been to manually 
exert a downward force at one end of the apparatus. The inconvenience 
associated with this procedure is readily apparent. A second approach has 
been to add weight to the head end of the unit for exerting a 
counterbalancing force. The mobility of the apparatus, however, is 
compromised by the addition of such weight. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a patient support apparatus 
which is capable of supporting a patient in a variety of positions. 
It is another object of the invention to provide such an apparatus which is 
stable regardless of where the patient's weight is concentrated on the 
apparatus. 
A still further object of the invention is to provide an apparatus which 
ensures steady ascent or descent of the top portion of the apparatus in 
all positions thereof. 
A still further object of the invention is to provide an apparatus which is 
relatively easily maneuvered by medical personnel. 
In accordance with these and other objects of the invention, which will 
become apparent from the detailed description which follows, a patient 
support apparatus is provided which includes a top assembly, a base 
assembly, first and second hydraulic column assemblies mounted to the base 
assembly for supporting the top assembly, and means connected between the 
base assembly and top assembly for exerting a downward force on one of the 
hydraulic column assemblies. In a preferred embodiment of the invention, 
the means for exerting a downward force include a gas spring connected by 
a cable to the top assembly. The gas spring resiliently urges the 
hydraulic column assembly adjoining the head end of the apparatus towards 
a contracted position, thereby substantially counteracting the lifting 
force at the head end of the apparatus when a patient's weight is 
concentrated towards the foot end thereof. The hydraulic column assembly 
does not actually move towards a contracted position until control means 
are actuated by an operator. The force exerted by the gas spring insures 
that the hydraulic column assembly does, in fact, contract, even when a 
patient's weight is towards or beyond the foot end of the apparatus.

DETAILED DESCRIPTION OF THE INVENTION 
A patient support apparatus in accordance with the invention is shown in 
FIG. 1 in the form of a mobile surgical table 10. The table 10 is capable 
of infinite positioning capabilities from sitting to supine, and can 
accordingly be used for a wide variety of applications. 
The surgical table includes a base assembly 12, a top assembly 14, a pair 
of side rails 16 mounted to the top assembly, and a pair of hydraulic 
column assemblies 18 mounted to the base assembly. The hydraulic column 
assemblies support the top assembly 14. 
Each hydraulic column assembly includes an elongate member 20 telescopingly 
coupled to a hydraulic actuator 22. Telescoping movement of one or both 
members 20 with respect to the actuator 22 allows the top assembly to be 
lowered, raised or moved to a Trendelenburg or other position. The 
hydraulic column assemblies are preferably the type which include flow 
control valves so that the top assembly can be lowered at a constant rate 
regardless of the weight supported thereby or the force exerted thereon. 
Such hydraulic column assemblies are commercially available to the 
industry. One such assembly is sold by Applied Power Inc. as the MEDIJACK 
Model DSC5. It is sold under the trademark POWER-KER. The control 
system for the hydraulic column assemblies include foot pedals 24A, 24B, 
24C to adjust the height or to provide Trendelenburg or reverse 
Trendelenburg positions. 
The top assembly 14, also known as a litter top, includes a Fowler backrest 
26, a leg rest 28, and a seat portion 30. The Fowler backrest and leg rest 
are both pivotably mounted with respect to the seat portion. The positions 
thereof may be adjusted using any one of several well known adjustment 
mechanisms, including pneumatic spring assemblies. A pneumatic spring 32 
for adjusting the Fowler backrest 26 is shown in FIG. 4. A top frame 
assembly 34 is provided for supporting the Fowler backrest 26, leg rest 28 
and seat portion 30. The pneumatic spring assembly 32 is pivotably 
connected between the Fowler backrest 26 and the frame assembly 34 defined 
by angle members 34 a and 34b 
The base assembly 12 includes a base frame 36 as shown in FIG. 3, to which 
the hydraulic column assemblies 18 are mounted. A four wheel brake and 
steer caster system is mounted to the frame. Brake and steer pedals 38 are 
located on all four casters 40. A similar arrangement is disclosed in 
commonly assigned U.S. Pat. No. 4,858,260, which is incorporated by 
reference herein. The '260 patent also discloses a pneumatic spring used 
for maintaining the angular position of a backrest with respect to a seat. 
A cover 42 is mounted to the frame for functional and aesthetic purposes. 
The cover includes a pair of hollow columns 44 which protect the hydraulic 
actuators. It further includes trough-like upper surface which may be used 
for transporting articles beneath the litter top. The cover also covers a 
large portion of the frame 36 as well as a stabilizing mechanism 46 
mounted in part to the frame. 
The stabilizing mechanism 46 is provided for constantly exerting a downward 
force to the hydraulic column assembly at the head end of the surgical 
table 10. In accordance with the preferred embodiment of the invention, 
the stabilizing mechanism includes a constant extension force gas spring 
48 capable of providing a force of about 510 Newtons. The spring 48 is 
attached to a lower plate assembly 50 by a clevis pin 52. The lower plate 
assembly is, in turn, bolted to the base frame 36 of the surgical table 
10. A pair of L-shaped brackets 53 are also secured to the base frame 36, 
and are positioned between the lower plate assembly and the base frame. A 
guide support 54 is also bolted to the base frame 36. The guide support 
includes a cylindrical guide portion 56 through which the body portion of 
the gas spring extends. The gas spring is accordingly mounted in a 
substantially horizontal position upon the base frame. 
A pulley 58 is secured to the end of the piston rod 60 of the gas spring. 
The pulley is preferably made from a polymeric material such as nylon or 
an acetal resin (e.g. DELRIN). A second pulley 62 is rotatably mounted to 
the lower plate assembly 50 by a clevis pin 64. A pivot shaft 66 having an 
opening extending diametrically therethrough is also mounted to the lower 
plate assembly 50. 
A top plate assembly 68 is bolted to the top assembly 14 of the surgical 
table in adjoining relation to the head end hydraulic column assembly. 
Specifically, it is bolted to a pair of opposing angle members 34A, 34B of 
the frame assembly 34. The top plate assembly includes a pivot shaft 70 
secured thereto which is substantially identical in construction to the 
pivot shaft 66 mounted to the lower plate assembly. 
A cable 72 having a pair of threaded ends 72A, 72B is coupled to the lower 
and top plate assemblies. One threaded end 72A extends through the opening 
in one of the pivots shafts and is coupled thereto by a hex nut 74. The 
other end 72B is coupled to the other pivot shaft 70 by a second hex nut 
76. The cable extends from the first pivot shaft 66, around the pulleys 
58, 62, through one of the hollow columns 44, and through thee second 
pivot shaft 70 to which it is coupled. 
The stabilizing mechanism 46 is designed to prevent a lifting or tipping 
upward of the hydraulic column assembly at the head end of the surgical 
table 10 when a patient's weight is over the foot end of the apparatus. 
When the litter top is at its lowest elevational position, the gas spring 
48 is extended in length, thereby taking up the cable length to the top 
surface mounting at the top plate assembly 68. The gas spring accordingly 
exerts a constant force to the cable and top assembly 14. As the litter 
top is elevated, the cable 72 forces the piston rod 60 of the gas spring 
inwardly. The gas spring accordingly continues to exert a downward force 
near the head end of the litter top. Such a downward force continues 
through the full elevation of the litter top. As the litter top is 
lowered, the piston rod of the gas spring extends under force to maintain 
the cable 72 in a taut condition, thereby exerting a constant downward 
load or pulling force upon the top assembly 14. Such a load is exerted 
regardless of where the patient's weight is concentrated. The pulleys 58, 
62 contribute to the smoothness of the movements of the load-bearing cable 
72. 
The stabilizing mechanism 46 is designed for relatively simple adaptation 
to many existing surgical tables and stretchers having dual hydraulic 
column assemblies. It can be incorporated within many such tables and 
stretchers either during original manufacture or by retrofitting through 
the use of bolt-on hardware the same as or similar to that described 
above. The mechanism 46 occupies little space, and accordingly requires, 
at most, minimal modifications to existing equipment in order to employ it 
therewith. 
By constantly exerting a downward force upon the head end hydraulic column 
assembly, the stabilizing mechanism ensures that the head end of the 
surgical table or stretcher will lower upon actuation of the assembly even 
when a patient's weight is concentrated towards the foot end thereof. In 
addition, lifting or telescoping of the head end hydraulic column assembly 
will be prevented regardless of where a patient's weight is concentrated. 
As the stabilizing mechanism weighs less than ten pounds, these benefits 
are achieved with virtually no sacrifice of mobility. No manual force is 
necessary for lowering the litter top. As the hydraulic column assemblies 
are preferably of the type which raise or lower a litter top at 
substantially the same speed regardless of the force applied to the litter 
top, the stabilizing mechanism does not affect the normal operation 
thereof. 
The benefits of the invention become particularly apparent when the 
stabilizing mechanism is used in a stretcher or surgical table having a 
top assembly capable of supporting a patient in a number of different 
positions. This capability inherently causes the shifting of a patient's 
weight such that, in units having dual hydraulic support assemblies, one 
of the hydraulic assemblies would be subjected to a negative force under 
certain circumstances. A patient's weight, for example, would be 
sufficiently distributed when the surgical table 10 is in the position 
shown in FIG. 1 that a negative force would not be exerted upon the 
hydraulic support assembly at the head end thereof. When the surgical 
table is moved to the position shown in FIG. 2, however, there is a 
likelihood that such a negative force would be exerted upon the head end 
hydraulic support assembly, and that this force will prevent proper 
operation of the assembly should the control pedals 24A, 24B or 24C be 
operated. The stabilizing mechanism provides a force to the hydraulic 
column assembly most likely to be subjected to such a negative force. This 
pulling force is sufficient to overcome the negative force anticipated 
under most operating conditions. A single gas spring capable of exerting a 
force of about 510 Newtons has been found to provide satisfactory results. 
It will be appreciated that the stabilizing mechanism can be designed to 
include more than one spring. 
While the stabilizing mechanism 46 according to the preferred embodiment of 
the invention is secured directly to the upper frame assembly 34 and base 
frame 36, respectively, it will be appreciated that such a mechanism can 
be designed for mounting, at least in part, to hydraulic column assembly 
18 itself. The mechanism would be, under such circumstances, only 
indirectly connected to the upper frame assembly and/or base frame, 
respectively. The particular points at which a stabilizing mechanism are 
secured must, however, be such that the mechanism provides a downward 
force upon the extensible portion of the hydraulic column assembly. 
As discussed above, a gas spring is employed in accordance with the 
preferred embodiment of the invention for providing a downward force upon 
the hydraulic column assembly at the head end of the surgical table. Such 
a spring is preferred as it exerts substantially the same force upon the 
hydraulic column assembly regardless of the position of the litter top. 
Alternatively, a mechanism could be employed which exerts a variable 
downward force upon the hydraulic column assembly depending upon the 
position of the litter top. An elastic member, coil spring(s), or the like 
could be directly or indirectly connected between the upper frame assembly 
and base frame in order to exert a constant, though variable force upon 
the hydraulic column assembly, depending upon the position of the litter 
top. Since the hydraulic column assemblies are preferably of the type 
which expand or contract at a substantially constant rate regardless of 
load, such approaches for counteracting the negative forces thereon are at 
least feasible. 
Although illustrative embodiments of the present invention have been 
described herein with reference to the accompanying drawings, it is to be 
understood that the invention is not limited to those precise embodiments, 
and that various other changes and modifications may be effected therein 
by one skilled in the art without departing from the scope or spirit of 
the invention.