3.5  Practical Conclusions in Order to Manage the Optimal Chemical Decontamination of an Eye

Fig. 3.80  Comparative diffusion of sodium hydroxide

Comparison of diffusion of NaOH at 20°C and 55°C

Time (S)

On the other hand, proportionally, the influence of temperature is less important for an already very concentrated substance.

Pressure

The pressure can sharply facilitate the penetration of the corrosive into the depth of the eye. By a physical phenomenon, the pressure entails a mechanical disintegration of the tissues. This will aggravate all the lesions.

3.5  Practical Conclusions in Order to Manage the Optimal Chemical Decontamination of an Eye

The gravity of an ocular chemical burn is due to multiple factors.

It is, of course, in touch relative to the intrinsic chemical nature of the substance but it also depends on the physical conditions of use.

These practical notions can have consequences at several levels:

•Primary prevention in the workplace or in domestic environment

•Precocity, relevance, and efficiency of the various parameters of the indispensable and specific decontamination to set up within the seconds following an eye projection

By optimizing each of these parameters, the initial burn can be avoided or its importance be minimized.

Considering the fundamental notions developed above, what conclusions can be stated using these theoretical and experimental data in order to define an optimal process for the decontamination of an ocular chemical projection?

What are the technical means indispensable for the emergency care of an ocular chemical burn? How to be rational and efficient just after contact?

3.5.1  The First Reflex: The Passive

Wash Including Dilution and

Mechanical Draining

The first reflex is dilution and mechanical draining with water. It is still efficient for the least aggressive chemicals and the least concentrated substances.

What happens when considering the mechanistics developed in the previous paragraphs?

A simple in vitro experiment enables the observation of the effect of dilution by water of concentrated corrosives such as soda or sulfuric acid (Fig. 3.81):

In spite of adding a big quantity of water, pH remains around 2, which is very corrosive.

During the contact between the chemical aggressor and the biological molecules constituting the cellular structures of an eye, the reactivity develops from molecules to molecules, until complete consumption of all the corrosive molecules. Thus, the less concentrated the corrosive, the smaller the number of biological molecules consumed and the smaller the cellular destruction. Similarly, during the passive dilution of a water wash, less and less entities are available to cause lesions. Nevertheless, even in a diluted solution,

Volume (mL)

pH

Physiologically acceptable pH

Fig. 3.81  Evolution of the pH of sulfuric acid

a corrosive molecule keeps all of its own reacting potential.

Thus the lesions to tissues become less and less noticeable but they will still develop. This is the main argument supporting the need of a prolonged 15–20 min wash.

3.5.2  Consequences of a Passive Washing: A Longer Time of Action

The reasoning developed in the previous paragraphs justifies the old international consensus on the recommendation of a 15–20 min prolonged wash when using water.

But why is it essential to continue the wash after 5 or 10 min? Because there is still a high destructive potential and lesions are still developing. This evidence shows the insufficiency of a simple effect of mechanical draining and passive dilution of a water wash.

Figure 3.82 illustrates this with a logarithmic curve ending with an asymptote.

Knowing well such dilution techniques, the scientist in chemistry will search for a better solution than such a slow, gradual, and partial phenomenon.

Of course the effect of mechanical draining from the tissue’s surface gradually eliminates about 90% of the corrosive. But the remaining 10% are still aggressive enough to cause the evident severeness of some chemical burns, which had first been early and widely washed with water anyway [5]. Moreover, it is well known that about 250 mL remain on the surface of a naked body after complete immersion. For an eye, the winking reflex

Evolution of the pH of 35% sodium hydroxide

with an increased volume of water

14

Fig. 3.82  pH evolution of sodium hydroxide

drives almost all the substance out except for a volume of about one drop that reaches the cornea. Therefore, it is easy to understand that chemical burns are due to very small quantities of chemicals.

3.5.3  The Concept of Active Wash

While keeping the principle of washing as a dilution, the chemist would search for a solution to get an immediate and complete neutralization. This is the actual goal of searching for solutions of active wash.

However, about 200 years ago, Thénard and GayLussac were the pioneers who decided to concentrate an HF solution and to study the very specific characteristics of both its physical and its chemical properties. They have noticed that the use of a solution of diluted potash could stop both pains and burns caused by HF. Why was this brand-new concept abandoned so quickly? Is the exothermic characteristic of the reaction the only cause of this abandon and is it only a myth? The limit of effectiveness of passive washing is probably the principal explanation.

Since a very long time, the chemists are using solutions of weak acids or weak bases to neutralize, respectively, strong bases and strong acids. They equipped their laboratories with bottles of weak acids and weak bases to neutralize ocular or skin projections.

Despite good intentions, the results have not always been remarkable! At first, it would have been necessary to know the nature of the chemical, then to know exactly the strength of the reaction, while also considering the exothermic character of the reaction, the lack

Fig. 3.83  pH evolution of different corrosives with addition of Diphoterine®

Evolution of the pH or the potential of different corrosive with an increased volume of Diphoterine R

Volume (mL)

of sterility of the solution, and, more especially, the risk of errors.

For these reasons one thinks of using buffer solutions. What is a buffer solution?

A buffer solution is a solution which resists change of pH upon addition of small amounts of acid or base, or upon dilution. It is made with material pairs:

•Weak acid and a soluble salt containing the conjugate base of the weak acid

•Weak base and a soluble salt containing the conjugate acid of the weak base

For example, the resistive action is the result of the equilibrium between the weak acid (HA) and its conjugate base (A−):

HA(aq) + H2O « H3O+ (aq) + A-(aq)

The best effectiveness of a buffer is reached when the pH of the solution is close to the pKa of the weak acid and conversely for the weak base.

If one washes, by error, a strong acid projection with a weak acid like, for example, the boric acid, the benefit will be null. The strong acid will not be washed at all.

All these problems can be solved in a modern way by the conception and the realization of amphoteric solutions available in sterile containers. (Amphoteric solutions can react with antagonistic couples of corrosives such as acid/base, oxidizing/reducing agents). These methods, moreover a chemical washing and dilution at the surface of the cornea, propose a dynamic

additive effect via chemical reactivity of the offending chemical.

The idea is to bind the aggressive chemical agent quicker than it could react with the biological component of the cornea, preventing and induced damage.

Beyond the simple polyvalent dilution of a water wash, the wash (Fig. 3.83) of various types of corrosives by Diphoterine® an amphoteric agent shows the need and profit of the use of an active answer.

The experimental simulation of a chemical burn (Figs. 3.84 and 3.85) is stated on a corrosive such as some 1M soda in contact for 3 min with the semipermeable membrane representing the cornea. It favorably shows the time gained, thanks to a solution that physically and chemically helps the pH of the aqueous humor to get back to the safe zone. Here, the point is not only to show the mechanical draining effect at the surface of the membrane/cornea (which quickly restores the pH to a normal value), but also to show how the internal decontamination of a corrosive is important and difficult. The internal decontamination will deal with the amount of corrosive that might have already penetrated into the anterior chamber of the eye.

It is noticeable that an increase of the osmotic pressure of the wash solution that is used can lower the time required to restore the pH to a physiologically acceptable zone. Therefore, it shortens the required washing time (the gap is due to osmolarity) (Fig. 3.86):

The use of a solution that is able to react with the corrosive will, in an even more remarkable way, reduce the time needed to restore the pH to a safe zone (the gap is