TECNOLOGIA TRADICIONAL, MEDIA Y MECANIZADA EN LA PRODUCCION DE QUINUA ORGANICA 16

Necessary information and influencing factors.

The methods of investigation of the ischaemic limb are legion but involve the study of four basic parameters:

1. Total limb blood flow. 2. Cutaneous blood flow.

3. Nutritional state of the limb. 4. Skin temperature.

All of the above may be influenced by external factors such as ambient temperature, the time a l l o w e d for equilibration, air speed, humidity, and the amount and nature of clothing worn. These are quite apart from the constraints imposed by the measuring techniques themselves and the r e l i a b i l i t y of the i n s t r u m e n t s i nvolved. Furthermore, the subjects* internal environment may also affect the results. Drugs, i n c l u d i n g n i c o t i n e and ethanol, age, fitness, exercise habits and intercurrent disease such as diabetes will all influence the outcome. Similarly local infection, ulceration or gangrene will all affect the interpretation of results.

In each of the four main areas of assessment, several methods have been proposed in the past. These will be outlined and the particular methods used in this study then described in detail with reasons for their selection.

1. Total limb blood flow: methods of assessment.

i) Calorimetry;

ii) Plethysmography? - volume displacement of air or water segmental / capacitance

gravimetric - impedance - photoelectric - strain-gauge iii) Duplex ultrasound scan;

iv) Magnetic resonance imaging.

i) Calorimetry.

This method employs the Fick principle (Fick, 1870) which maintains that if the concentrations of a substance

'X* in the blood entering and leaving an organ (or limb) are known, and the rate of usage (or production) of the substance 1X 1 by the organ be likewise known, then the rate of blood flow through that organ can be calculated

(Wright, 1982).

In 1911 Stewart, using this method, where the substance 1X 1 was the heat content of the blood, measured blood flow in the hand. He immersed the hand in an i n s u l a t e d container full of water and measured the heat given off from the hand to the water. Previous calculation had been made of the temperature difference between arterial and v e n o u s blood. F r e s h c a d a v e r e x p e r i m e n t a t i o n h ad determined the specific heat of the hand. But the method had its drawbacks. The assumption that venous blood reflected skin temperature was not valid (Harris & Marvin, 1927? Pickering, 1936), and incomplete insulation of the

water in the calorimeter could result in a variable heat loss or heat gain (Bierman, 1939) . The m e t h o d was developed to measure the blood flow in the extremities (Sheard, 1926) and is of little value when total limb flow is required. Finally, it involves immersion of the part in w a t e r w h i c h is u n a c c e p t a b l e in the p r e s e n c e of ulceration or gangrene. All methods involving immersion of the limb have been discarded as inappropriate to this work.

ii) Plethysmography: theory and types.

This technique is based on enlargement of a limb following venous occlusion. The term was first coined in 1872 (The S h o r t e r Oxford English Dictionary, 1973) although the principle had been elaborated earlier by Glisson in 1622 and Swammerdam in 1737 (cited by Johnson, 1940). Adapting Archimedes* Principle, they placed an organ in a rigid box filled with air or water. Any subsequent swelling of the organ caused displacement of fluid from the box. This swe l l i n g m i g h t be due to increased blood flow to the organ which could be measured, being equal to the amount of fluid displaced from the box. Although there have been many refinements over the last 50 years, the present procedure is based on the system devised by Schafer and Moore (1896) and subsequently adapted for the human limb by Brodie and Russell, (1905), and Hewlett and van Zwaluwenburg (1909) . In this system, the limb, or a segment of it, is sealed in a rigid jacket so that volume changes in the enclosed part will displace

the fluid in the space between the limb and jacket. These changes are measured by a volume recorder.

To obtain reliable, reproducible measurements of limb blood flow, certain essential principles of the venous occlusion method need to be observed: - i) that the venous occlusion (not affecting arterial inflow) be as complete and i n s t a n t a n e o u s as pos s i b l e so that the r e is an immediate increase in limb volume (Landowne & Katz, 1942); ii) that the part of the limb to be measured is clearly defined; iii) that calculations are made using the initial slope of the curve obtained on the recorder. (Fig 1).

This is essential because the rate of the volume increase indicates the rate of (arterial) blood inflow from which can be determined the limb blood flow below the tourniquet. From Figure 1 it can be seen that after a few seconds this rate of volume increase declines. This is due to several factors. First, that with continuing application of the cuff, capillary and venous pressures rise and finally exceed the pressure of the occluding cuff. This results in an escape of blood through the veins under the cuff. Second, there is a slowing of inflow into the limb distal to the cuff due to increasing resistance in the distended limb. Third, the flattening of the slope after the initial rise r e p r e s e n t s the capillary filtration curve. The assumption is that the rate of vo l u m e inc r e a s e at the start of the v e n o u s occlusion is equal to the rate of unimpeded arterial blood flowing into the limb just before the venous occlusion

FIGURE 1.

Venous occlusion plethvsmoaraoh trace.

The s t r a i n - g a u g e was p l a c e d around the greatest circumference of the calf, and the distance measured from the superior border of the patella with the quadriceps femoris muscle relaxed. (See p.108).

The arrows indicate the points of mechanical inflation of the thigh cuff, and the small inflation artifact can be seen as a tiny vertical spike before the trace begins to rise. The cuff was rapidly inflated automatically to 50 mm Hg and held at that pressure for 5 seconds. Swift automatic deflation followed. After a further 5 second interval the cycle was repeated.

The size (gain) control on the SPG 16 unit (Vasculab, Medasonics, California) was x 2 , the unit was DC coupled

for venous recording, the paper speed on the R12B chart

. — l

recorder (Vasculab, Medasonics, California) was 5mm.sec , and the size switch on the R12B unit was set to xl.

A tangent was drawn to the initial rise in the trace (see p.52), and the vertical height measured after 3 seconds. This value appears above the curve of each period of occlusion, and the average height (12.9) noted. Blood flow was calculated from the formula mentioned in the text, page 109.

A v e r a g e 1 2 * 9

ism

There are several types of plethysmograph but the volume displacement types (either of air or water) were not suitable for this study due to the nature of the ischaemic limb.

S ecrmen t a 1 p 1 e thvsmogr aoh v .

W h e n d i s c u s s i n g s e g m e n t a l p l e t h y s m o g r a p h y , a distinction is usually made between an oscillometer and a s e gmental p l e thysmograph. The former r e c o r d s the pulsation of a limb or digit in arbitrary units, and the latter volume changes in millilitres (Woodcock, 1976) . The principle of operation of both types, however, is the same, in that air pressure is measured in a cuff which is wrapped around the limb. As the limb swells the pressure in the cuff rises and this can be calibrated in terms of blood flow. The pulsation volume which is recorded by the oscillometer is, however, for our purposes too inaccurate, although it has application in the normal limb under strictly controlled conditions. The volume fluctuations are caused by the phasic inequalities during the cardiac cycle of arterial inflow and venous outflow (Greenfield et al., 1963). A segmental plethysmograph is represented d i a g r a m a t i c a l l y in Figure 2. It is c o n n e c t e d to a pneumatic system and the volume change in the limb is derived from the pressure changes over the cardiac cycle t r a n s m i t t e d to one side of the d i a p h r a g m in the differential chamber (Raines, 1978). The shape of the arterial wave form (whether it is sharply rising or

FIGURE 2.

Pneumatic system of a Segmental Plethysmograph. (After a design by Winsor, 1957).

A cuff is placed around the limb, and the change in air pressure across the diaphragm in the differential chamber is measured following venous occlusion. As the limb swells, the pressure in the cuff rises, and this can be c a l i b r a t e d in terms of blood flow and e x p r e s s e d in millilitres.

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