CHRONIC VENTRICULAR ASSIST DEVICECurrent left ventricular assistance devices (L.V.A.D's) are either aimed at short term support of the failing heart or as long-term implantable devices. All the L.V.A.D's currently in or approaching clinical use suffer from common design drawbacks. The most important of these in the long term is their reliance on extra-corporeal power with various methods for trans cutaneous energy transfer. Furthermore, all the L.V.A.D'S currently studied work from the blood sac principle, whereby blood enters a rigid chamber limited at each end by valves, one an inlet and one an outlet valve. Movement of blood through the chamber is produced by intermittent movement of a sac which effectively 'squeezes' blood out of the chamber.Methods of using skeletal muscle to augment the heart are currently being explored, and these generally use the latissimus dorsi muscle to either squeeze the heart rhythmically or to compress conduits be they natural or prosthetic. All these methods however require that most or all of the origin of the muscle, from the ribs and thoracolumbar fascia are divided and the tendon is divided and attached to a rib for stability. Most of these techniques require that the muscle be transposed into the thoracic cavity.
The current invention'is radically different from other chronically implantedV.A.D.'S. firstly the whole system is implantable within the body, without transcutaneous power delivery, secondly an Archimedes screw' is used to provide the blood motion, thirdly the latissimus dorsi muscle is left intact, with only its tendon divided, and power is transferred from the muscle to the V.A.D. directly through a mechanical linkage.
The V.A.D. can be discussed in terms of its components. Figure 1 shows a general view of the blood containing element: (1) is the housing, formed from either 'polycarbonate' or from 'sintered titanium'. (2) is the inlet conduit from the left atrium and (3) is the outlet conduit to the descending thoracic aorta both of these would be made from collagen impregnated dacron or similar material. (4) is a bileaflet mechanical heart valve such as a 'St Jude' or a sCarbomedics' which allows central flow with a relatively low gradient across it. (5) is the moving rotor, shaped as anArchimedes screw and manufactured from heparin bonded plastic. This rotor is mounted on a titanium, or similar, spindle which exits the casing through a seal [Figure 2, (6) ] . The spindle is stabilised by running through a series of bearings (7) and ends at a screw connector (8).Into the screw connector connects a co-axial cable (not shown), similar to a car speedometer cable, which-engages through a simple lug connector.
Figure 3 shows the gearbox through which the linear energy of latissimus dorsi, or other appropriate skeletal muscle, contraction is converted into rotary motion to turn the rotor. The gearbox is contained in a titanium box (9), the tendon of the muscle is surgically divided and securely joined to the tendon attachment arm (10). This arm attaches to a cranked arm (12) such as is found on spinning wheels which drives the primary gear wheel (13). The primary gear wheel turns the central shaft of the secondary gear wheel (14), the outer part of which drives a tertiary gear wheel (15).
The tertiary gear wheel drives the co-axial cable through another screw connector, which seals the cable to the box. The tendon attachment arm itself moves within a highly flexible rubber sleeve (16) to maintain the integrity of the box. The spring (17) returns the muscle to the stretched position during the muscle relaxation phase which must follow muscle contraction. It is the mechanical equivalent of a synergistic muscle.
The box is firmly stabilised by being wired to the underlying ribs through the attachment points (18).
Muscle contraction is controlled by a neuro-muscular pacing system, such as theMedtronic company's SP1005 or Prometheus systems.
The advantage of a separate coaxial cable to connect the gear box to the blood containing part of the V.A.D. is to allow the length of the cable to be tailored, so that the cable passes through the chest wall and then runs around the inside of the hemithorax in a smooth curve, without kinks.
In practical terms, a 4cm by 6 cm rotor would enclose a volume of approximately 75 mls. A primary gear of 4 cm turning a 1 cm spindle on a 3 cm secondary gear, in turn turning a 1 cm tertiary gear would produce 12 turns of the rotor for each muscle contraction.