Additional equipment used in anaesthesia and intensive care

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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Additional equipment used in anaesthesia and intensive care

Continuous positive airway pressure (CPAP)

CPAP is a spontaneous breathing mode used in the intensive care unit, during anaesthesia and for patients requiring respiratory support at home. It increases the functional residual capacity (FRC) and improves oxygenation. CPAP prevents alveolar collapse and possibly recruits already collapsed alveoli.

Mechanism of action

1. Positive pressure within the lungs (and breathing system) is maintained throughout the whole of the breathing cycle.

2. The patient’s peak inspiratory flow rate can be met.

3. The level of CPAP varies depending on the patient’s requirements. It is usually 5–15 cm H2O.

4. CPAP is useful in weaning patients off ventilators especially when positive end expiratory pressure (PEEP) is used. It is also useful in improving oxygentaion in type 1 respiratory failure, where CO2 elemination is not a problem.

5. Two levels of airway pressure support can be provided using inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP). IPAP is the pressure set to support the patient during inspiration. EPAP is the pressure set for the period of expiration This is commonly used in reference to bilevel positive airway pressure (BiPAP©). Using this mode, the airway pressure during inspiration is independent from expiratory airway pressure. This mode is useful in managing patients with type 2 respiratory failure as the work of breathing is reduced with improvements in tidal volume and CO2 removal.

Problems in practice and safety features

Haemofiltration

Haemofiltration is a process of acute renal support used for critically ill patients. It is the ultrafiltration of blood.

Ultrafiltration is the passage of fluid under pressure across a semipermeable membrane where low molecular weight solutes (up to 20 000 Da) are carried along with the fluid by solvent drag (convection) rather than diffusion. This allows the larger molecules such as plasma proteins, albumin (62 000 Da) and cellular elements to be preserved.

The widespread use of haemofiltration has revolutionized the management of critically ill patients with acute renal failure within the intensive therapy environment (Fig. 13.2). Haemofiltration is popular because of its relative ease of use and higher tolerability in the cardiovascularly unstable patient.

In the critical care setting, haemofiltration can be delivered by:

Components

1. Intravascular access lines. These can either be arteriovenous lines (such as femoral artery and vein or brachial artery and femoral vein) or venovenous lines (such as the femoral vein or the subclavian vein using a single double-lumen catheter). The extracorporeal circuit is connected to the intravascular lines. The lines should be as short as possible to minimize resistance.

2. Filter or membrane (Fig. 13.3). Synthetic membranes are ideal for this process. They are made of polyacrylonitrile (PAN), polysulphone or polymethyl methacrylate. They have a large pore size to allow efficient diffusion (in contrast to the smaller size of dialysis filters).

3. Two roller pumps, one on each side of the circuit. Each pump peristaltically propels about 10 mL of blood per revolution and is positioned slightly below the level of the patient’s heart.

4. The collection vessel for the ultrafiltrate is positioned below the level of the pump.

Mechanism of action

1. At its most basic form, the haemofiltration system consists of a circuit linking an artery to a vein with a filter positioned between the two.

2. The patient’s blood pressure provides the hydrostatic pressure necessary for ultrafiltration of the plasma. This technique is suitable for fluid-overloaded patients who have a stable, normal blood pressure.

3. Blood pressure of less than a mean of 60 or 70 mmHg reduces the flow and the volume of filtrate and leads to clotting despite heparin.

4. In the venovenous system, a pump is added making the cannulation of a large artery unnecessary (Fig. 13.4). The speed of the blood pump controls the maintenance of the transmembrane pressure. The risk of clotting is also reduced. This is the most common method used. Blood flows of 30–750 mL/min can be achieved, although flow rates of 150–300 mL/min are generally used. This gives an ultrafiltration rate of 25–40 mL/min.

5. The fluid balance is maintained by the simultaneous reinfusion of a sterile crystalloid fluid. The fluid contains most of the plasma electrolytes present in their normal values (sodium, potassium, calcium, magnesium, chloride, lactate and glucose) with an osmolality of 285–335 mosmol/kg. Large amounts of fluid are needed such as 2–3 L/h.

6. Pressure transducers monitor the blood pressure in access and return lines. Air bubble detection facilities are also incorporated. Low inflow pressures can happen during line occlusion. High and low post-pump pressures can happen in line occlusion or disconnection respectively.

7. Some designs have the facility to weigh the filtrate and automatically supply the appropriate amount of reinfusion fluid.

8. Heparin is added as the anticoagulant with a typical loading dose of 3000 IU followed by an infusion of 10 IU per kg body weight. Heparin activity is monitored by activated partial thromboplastin time or activated clotting time. Prostacyclin or low molecular weight heparin can be used as alternatives to heparin.

9. The filters are supplied either in a cylinder or flat box casing. The packing of the filter material ensures a high surface area to volume ratio. The filter is usually manufactured as a parallel collection of hollow fibres packed within a plastic canister. Blood is passed, or pumped, from one end to the other through these tubules. One or more ports provided in the outer casing are used to collect the filtrate and/or pass dialysate fluid across the effluent side of the membrane tubules.

Arterial blood gas analyser (Fig. 13.5)

In order to measure arterial blood gases, a sample of heparinized, anaerobic and fresh arterial blood is needed.

The measured parameters are:

From these measurements, other parameters can be calculated, e.g. actual bicarbonate, standard bicarbonate, base excess and oxygen saturation.

Polarographic (Clark) oxygen electrode

This measures the oxygen partial pressure in a blood (or gas) sample (Fig. 13.6).

Mechanism of action

pH electrode

This measures the activity of the hydrogen ions in a sample. Described mathematically, it is:

image

It is a versatile electrode which can measure samples of blood, urine or CSF (Fig. 13.7).

Carbon dioxide electrode (Severinghaus electrode)

A modified pH electrode is used to measure carbon dioxide partial pressure, as a result of change in the pH of an electrolyte solution (Fig. 13.8).