Volume I Number I H A R R I S : AN IMPROVED MANOMETER 25

AN IMPROVED CENTRAL VENOUS PRESSURE MANOMETER

D. HARRIS

Institute of Orthopaedics, The Robert Jones and Agnes Hunt Orthopaedic Hospital, Oswestry

Monitoring of the central venous pressure is a diagnostic and therapeutic aid in traumatic shock and other conditions. It is increasingly recognized as a valuable guide to the rapidly changing dynamics of the circulation. Practical difficulty exists in calibrating the apparatus at the level of the great veins. An accurate self-calibrating manometer, readily modified from existing apparatus, is described.

M E A S U R E M E N T S of the central venous pressure, advocated by B6rst and Molhuysen (1952), are a valuable guide in the diagnosis and treatment of the shocked and injured patient. Measure- ment is quick and easy (Sykes, 1963) by means of a disposable drip set attached to an intravenous cannula whose tip lies in one of the great veins, to which a simple saline manometer is attached as a side arm. By means of a four-way tap the manometer may be filled with infusion fluid and the drip bottle temporarily excluded, leaving the manometer attached to the cannula. The pressure head in the manometer then falls until it is supported by that in the vein and this pres- sure may be read in centimetres of saline from a scale on the manometer.

THE PROBLEM As it is the pressure relative to hydrostatic

pressure at the level of the vena cava which is required, the scale of the manometer must be adjusted to 0 cm. at this level (5 cm. posterior to the manubriosternal junction) or at a level constantly related to it (the manubriosternal junction itself). It is here that practical difficulties arise (Wolff, 1968) and clinical methods such as spirit levels or gun-sights trained on these points have been used (Fig. 1). An improved method of calibrating the manometer is descrided.

THE PRINCIPLE This depends upon the hydraulic characteris-

tics of U-tubes containing fluid (Fig. 1, inset).

Where both limbs are open at the same (atmos- pheric) pressure, the fluid levels in each limb will be the same. If the limbs are of unequal length the level in the higher can never exceed that in the lower, regardless of how much fluid is introduced.

Fig. 1.--The standard manometer. Inset: Hydraulic characteristics of U-tubes (see text).

THE APPARATUS

The standard disposable set is modified by introducing a Y-connector and side tube (refer- ence tube) supplied with a roller clip, into the base of the manometer. The tubing here may be cut with a sterile scalpel blade and the connector interposed as a push fit. The length of tube is conveniently the same as that connecting the tap to the intravenous cannula but is not critical. Thus when the manometer is isolated from the drip set and the clip is open it represents a U-tube (Fig. 2).

26 Injury INJURY: THE BRITISH JOURNAL OF ACCIDENT SURGERY July 1969

Method of Use The drip is set up in the usual way and the

manometer tube and scale taped to the drip- stand. The reference tube is held parallel to it (Fig. 2, Posit ion 1) and the tap adjusted to fill both tubes with fluid, and then to isolate them

Fig. 2.--Calibrating manometer. Position 1: position to fill. Position 2: in position to cali- brate.

f rom the drip. The reference tube is now lowered and taped to the manubriosternal junct ion (Fig. 2, Position 2) and the sterile fluid it contains will overflow until the level in the manometer falls to the level at the manubrium. This repre- sents 0 cm. on the scale, i.e., it calibrates the manometer . The reference tube is then clipped off and pressure readings may be taken. To recalibrate the manometer if the patient 's level or posture is altered, the manometer is filled, isolated as above, and the clip on the reference tube released to discharge fluid back to the new zero.

Advantages

1. Simple accurate calibration without cum- bersome apparatus.

2. Easy recalibration with change of patient 's level or posture. Under these circumstances the apparatus can be managed remote f rom the patient leaving the immediate field clear for resuscitation, operative procedures, etc., even if the distal end of the reference tube is obscured, e.g., by sterile drapes.

3. The whole apparatus may be readily trans- ported with the patient to the ward bed, operat ing table, or radiograph table with rapid recalibration.

Notes The fluid lost is sterile but it is advisable to

cover a manubriosternal sited tube with a water- p roof sheet when sterile drapes are applied. The amounts of fluid lost f rom the drop in the mano- meter system are negligible but may be easily calculated (12 cm. of tubing contains 1 ml. fluid measured from the manometer scale).

Materials used A suitable Y-connector is manufactured in

polythene by Portex Ltd., Hythe, Kent (Cata- logue No. 7/140/000), and may be sterilized by autoclaving in a C.S.S.D. pack ready for use. Reference tubing may be obtained f rom a sterile drip set, with a disposable scalpel blade.

The apparatus is under consideration by Baxter Laboratories Ltd. for commercial produc- tion.

REFERENCES B6RST, J. G. A., and MOLHUYSEN, J. A. (1952), ' Exact

Determination of Central Venous Pressure by a Simple Clinical Method ', Lancet, 2, 304.

SYKES, M. K. (1963), ' Venous Pressure as a Clinical Indication of Adequacy of Transfusion ', Ann. R. Coll. Stn'g., 33, 185.

WOLFF, H. S. (1968), 'Future Developments in Patient Monitoring ', Br. J. Hosp. Med., 1, Supple- ment I, 5.

Requests for reprints should be addresred to:--D. Harris, The Robert Jones and Agnes Hunt Orthopaedic Hospital, Oswestry, Shropshire.

ABSTRACTS

Carpal Tunnel Syndrome One hundred and fifty operations on 113 patients were

reviewed after 2-7 years. A quarter of the operations had done no good but when the decompression was carried out within 6 months of the onset of symptoms it was successful in nearly every case. Transverse incisions were liable to cause a painful neuroma on the palmar branch of the median nerve; a longitudinal incision is recommended.

SEraPLE, J. C., and CARGILL, A. O. (1969), ' Carpal Tunnel Syndrome ', Lancet, 1,918.

Trendelenburg Tilt and Endotracheal Tubes Reports in the literature suggest that it is not as

well known as it should be that it is quite easy to insert an endotracheal tube so far that it enters the right bronchus and obstructs the left. The authors show that tilting a lying patient head downwards and some other actions could cause the carina to shift percep- tibly towards the inlet of the thorax, even to the point at which the tip of a tube that had lain in the trachea came to enter the right bronchus far enough to obstruct the flow of air into the left.

HEINONEN, J., TAKKI, S., and TAMMISTO, T. (1969), 'Effect of the Trendelenburg Tilt and Other Pro- cedures on the Position of Endotracheal Tubes ', Lancet, 1, 850.