Preparation for Anasthesia

 
The only group for whom preoperative pulmonary function tests (PFTs) are mandatory is the group under consideration for lung resection. For all other types of surgery, there is no absolute minimal lung function, as assessed by PFTs, for avoidance of postoperative pulmonary complications. In other words, even patients with severe COPD can be managed perioperatively with a satisfactory outcome. Patients with unexplained pulmonary symptoms may benefit from preoperative PFTs, as well as those whose symptoms due to known COPD or asthma may not be optimally controlled.

·         PREOPERATIVE EVALUATION

1. Preoperative laboratory testing should be selective and individualized.
2. The American College of Cardiology/American Heart Association guidelines for cardiac testing for noncardiac procedures is the current gold standard for preoperative cardiac assessment.
3. The most important information obtained in a preanesthetic evaluation comes from a thorough, accurate, and focused history and physical examination.
4. Thorough preoperative airway examination is necessary to reduce and plan for anticipated episodes of difficult airway management.
5. A patient's baseline hemoglobin tends to predict the need for transfusion when large blood loss occurs.

·         Anesthesia machine is divided into three sections:

1. A gas mixer, or gas delivery system, that supplies at its outlet a defined mixture of gas chosen by the anesthesiologist.
2. The patient breathing system, which includes the patient breathing circuit, absorber head, ventilator, and often gas pressure and flow monitors.
3. A scavenger system that collects excess gas from the patient and the gas supply section itself and expels the gas outside the hospital to reduce the exposure of operating room personnel to anesthetic gases.

·         All wall supply gas connectors are keyed so that only the O2 supply hose can be plugged into the O2 connector on the wall, the N2O hose into the N2O outlet, and so on. This is known as a Diameter-Index-Safety-System (DISS) and by other similar proprietary names.

·         The gas cylinders are keyed using a Pin-Index-Safety-System (PISS) so that only the correct tank can be attached to the corresponding yolk on the anesthesia machine.

·         Ideally, O2 flowmeter must always be on the right. There is also an issue of mechanical safety-in this case, the O2 should enter the common manifold closest to the gas egress side. That way, most leaks will tend to selectively lose gases other than O2.

·         Fail-safe valves are a somewhat archaic method of providing a necessary, although not a truly fail-safe, safety feature. The device is designed to cut off the flow of all gases except O2 when the O2 pressure falls below a set value, usually about 25 psig

·         All E-cylinders contain approximately the same volume of compressed gas if the gas is at the same pressure. An E-cylinder contains approximately 600 L if the contents remain a gas when the cylinder is filled to 2200 psig. Two exceptions are N2O and CO2 because these molecules condense into a liquid at a pressure of approximately 750 psi, and these E-cylinders contains approximately 1600 L of gas when condensed to a liquid state.

·         Mapleson circuits:

Controlled: D > B > C > A (Dead Bodies Can't Argue)

Spontaneous: A > D > C > B (All Dogs Can Bite)

·         The Mapleson A circuit is known as the Magill circuit, named for Sir Ivan Whiteside Magill (1888-1986), who introduced endotracheal intubation and Magill's forceps and first described and used circuit A. Swapping the effective locations of the fresh gas inlet and the pop-off valve, the Mapleson A becomes a Mapleson D circuit, which is often called a Bain circuit. Sometimes this is commercially implemented by fresh gas tubing running within the corrugated tubing but the fresh gas is inserted functionally in an identical location. This design was first used by Sir Robert Macintosh (of laryngoscope fame) and E. A. Pask during World War II. The Mapleson E circuit is known as the Ayre's T-piece and was used primarily for pediatric anesthesia years ago; now it is used for weaning patients from mechanical ventilation. The Mapleson F circuit is known as the Jackson-Rees modification of the Ayre's T-piece and is functionally identical to a Mapleson A circuit.

·         ANESTHESIA CIRCUITS

1. The semiclosed circuit using a circle system is the most commonly used anesthesia circuit. Components include an inspiratory limb, expiratory limb, unidirectional valves, carbon dioxide absorber, gas reservoir bag, and a pop-off valve on the expiratory limb.
2. Advantages of a circle system include conservation of heat and moisture, the ability to use low flows of fresh gas, and the ability to scavenge waste gases.
3. Disadvantages include a complex design with approximately 10 connections, each of which has the potential for failure; and a large, bulky design that limits portability.
4. CO2 is eliminated from rebreathing (circle) systems by soda lime.

·         In modern anesthetic systems the vaporizers are located downstream from the flowmeters. Fresh gas passes from the flowmeters to the vaporizer, then on to the common gas outlet.

The desflurane vaporizer actively injects vapor into the fresh gas stream whereas all traditional vaporizers use a passive variable bypass system. The TEC 6 vaporizer is a unique vaporizer designed for desflurane, which has a vapor pressure of 664 mmHg at 20°C.

·         "beach chair" position: This position is used to improve access and mobility of the shoulder for surgery. When finally positioned, the anesthesiologist does not have easy access to the head or endotracheal tube, so these structures must be well secured and in a neutral position prior to initiation of the surgery. Perfusion pressure must be closely monitored since the head is well above the level of the heart.

·         Flexion (or extension) of the head may move the endotracheal tube toward (or away) from the carina. A general rule is that the tip of the endotracheal tube follows the direction of the tip of the patient's nose. The change in tube position is probably more problematic in a child than in an adult. Sudden increases in airway pressure or oxygen desaturation may be due to mainstem bronchial intubation.

·         MECHANICAL VENTILATION IN CRITICAL ILLNESS

1.       Three indications for mechanical ventilation: inadequate respiratory drive, inability to maintain alveolar ventilation, and hypoxia.

2.       Two indications for noninvasive positive-pressure ventilation: hypercapnic respiratory failure and comfort in terminally ill patients.

3.       Risk factors for auto-PEEP: high minute ventilation, small endotracheal tube, COPD, and asthma.

4.       The normal airway gradient between peak and static pressure is approximately 10 cm H2O.

5.       The first step in the care of the hypoxic patient fighting the ventilator is to ventilate the patient manually with 100% oxygen.

·         Split-lung ventilation (SLV) refers to ventilation of each lung independently usually via a double-lumen endotracheal tube and two ventilators. Patients with severe unilateral lung disease may be candidates for SLV. SLV has been shown to improve oxygenation in patients with unilateral pneumonia, pulmonary edema, and contusion. Isolation of the lungs can save the life of patients with massive hemoptysis or lung abscess by protecting the good lung from spillage. Patients with a bronchopleural fistula also may benefit from SLV. Different modes of ventilation may be applied to each lung individually. The two ventilators need not be synchronized, and in fact hemodynamic stability is better maintained by using the two ventilators asynchronously.