Additional equipment used in anaesthesia and intensive care
Continuous positive airway pressure (CPAP)
Components
1. A flow generator producing high flows of gas (Fig. 13.1), or a large reservoir bag may be needed.
Fig. 13.1 A CPAP breathing system set-up.
2. Connecting tubing from the flow generator to the inspiratory port of the mask. An oxygen analyser is fitted along the tubing to determine the inspired oxygen concentration.
3. A tight-fitting mask or a hood. The mask or hood has both inspiratory and expiratory ports. A CPAP valve is fitted to the expiratory port.
4. If the patient is intubated and spontaneously breathing, a T-piece with a CPAP valve fitted to the expiratory limb can be used.
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
1. CPAP has cardiovascular effects similar to PEEP but to a lesser extent. Although the arterial oxygenation may be improved, the cardiac output can be reduced. This may reduce the oxygen delivery to the tissues.
3. A loose-fitting mask allows leakage of gas and loss of pressure.
4. A nasogastric tube is inserted in patients with depressed consciousness level to prevent gastric distension.
5. Skin erosion caused by the tight-fitting mask. This is minimized by the use of soft silicone masks or protective dressings or by using a CPAP hood. Rhinorrhoea and nasal dryness can also occur.
6. Nasal masks are better tolerated but mouth breathing reduces the effects of CPAP.
Haemofiltration
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:
1. Continuous low flow therapy, which is typically run 24 h per day for more than 1 day, but may be stopped for procedures and filter or circuit changes. Continuous treatments are said to offer better cardiovascular stability and more efficient solute removal because of the steady biochemical correction and gradual fluid removal. Continuous therapy is also better suited to the frequently changing fluid balance situation in patients with multiorgan dysfunction in critical care.
2. Intermittent high flow systems, often 4-hour sessions with a new filter and circuit for each session.
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.
a) They are highly biocompatible causing minimal complement or leucocyte activation.
b) They are also highly wettable achieving high ultrafiltration rates.
c) They have large pore size allowing efficient diffusion.
d) The optimal surface area of the membrane is 0.3–1.9 m2.
e) Both small molecules (e.g. urea, creatinine and potassium) and large molecules (e.g. myoglobin and some antibiotics) are cleared efficiently. Proteins do not pass through the membrane because of their larger molecular size.
Problems in practice and safety features
1. The extracorporeal circuit must be primed with 2 L of normal saline prior to use. This removes all the toxic ethylene oxide gas still present in the filter. (Ethylene oxide is used to sterilize the equipment after manufacture.)
2. Haemolysis of blood components by the roller pump.
3. The risk of cracks to the tubing after long-term use.
4. The risk of bleeding must be controlled by optimizing the dose of anticoagulant.
Arterial blood gas analyser (Fig. 13.5)
1. Heparin should be added to the sample to prevent clotting during the analysis. The heparin should only fill the dead space of the syringe and form a thin film on its interior. Excess heparin, which is acidic, lowers the pH of the sample.
2. The presence of air bubbles in the sample increases the oxygen partial pressure and decreases carbon dioxide partial pressure.
3. An old blood sample has a lower pH and oxygen partial pressure and a higher carbon dioxide partial pressure. If there is a need to delay the analysis (e.g. machine self-calibration), the sample should be kept on ice.
Polarographic (Clark) oxygen electrode
This measures the oxygen partial pressure in a blood (or gas) sample (Fig. 13.6).
Mechanism of action
1. Oxygen molecules cross the membrane into the electrolyte solution at a rate proportional to their partial pressure in the sample.
2. A very small electric current flows when the polarization potential is applied across the electrode in the presence of oxygen molecules in the electrolyte solution. Electrons are donated by the anode and accepted by the cathode, producing an electric current within the solution. The circuit is completed by the input terminal of the amplifier.
3. The oxygen partial pressure in the sample can be measured since the amount of current is linearly proportional to the oxygen partial pressure in the sample.
pH electrode
This measures the activity of the hydrogen ions in a sample. Described mathematically, it is:
It is a versatile electrode which can measure samples of blood, urine or CSF (Fig. 13.7).
Fig. 13.7 Mechanism of action of the pH electrode.
Components
1. A glass electrode (silver/silver chloride) incorporating a bulb made of pH-sensitive glass holding a buffer solution.
2. A calomel reference electrode (mercury/mercury chloride) which is in contact with a potassium chloride solution via a cotton plug. The arterial blood sample is in contact with the potassium chloride solution via a membrane.
3. A meter to display the potential difference across the two electrodes.
Mechanism of action
1. The reference electrode maintains a constant potential.
2. The pH within the glass remains constant due to the action of the buffer solution. However, a pH gradient exists between the sample and the buffer solution. This gradient results in an electrical potential.
3. Using the two electrodes to create an electrical circuit, the potential can be measured. One electrode is in contact with the buffer and the other is in contact with the blood sample.
4. A linear electrical output of about 60 mV per unit pH is produced.
5. The two electrodes are kept at a constant temperature of 37°C.
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).
Mechanism of action
1. Carbon dioxide (not hydrogen ions) diffuses in both directions until equilibrium exists across the membrane between the sample and the electrolyte solution.
2. Carbon dioxide reacts with the water present in the electrolyte solution producing hydrogen ions resulting in a change in pH:
3. The change in pH is measured by the glass electrode.
4. The electrode should be maintained at a temperature of 37°C. Regular calibration is required.
Intra-aortic balloon pump (IABP)
This is a catheter incorporating a balloon which is inserted into the aorta to support patients with severe cardiac failure. Its core principle is synchronized counterpulsation. It is usually inserted using a percutaneous femoral approach, over a guidewire, under fluoroscopic or transoesophageal echo guidance. The correct position of the pump is in the descending aorta, just distal to the left subclavian artery (Fig 13.9).
Fig. 13.9 The intra-aortic balloon pump in situ.
