Circuit+Principles

= = toc =**Circuit Principles**=

Resistance to Gas Flow
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 * **R = P/Q** (Resistance is calculated as forcing pressure divided by gas flow)
 * The **forcing pressure** for airway resistance is mouth (or nose) pressure minus alveolar pressure, whereas the other driving pressures will be part of the changes in transpulmonary and pleural pressure.
 * **Gas flow** can be **turbulent**, with a disorderly pattern signified by vortices, or **laminar**, with a streamlined unidirectional flow pattern.
 * Turbulent **flow is proportional to the square of the pressure, and** laminar **flow, which is less pressure demanding, is linearly related to the pressure.**
 * **Laminar flow** is characterized by **Poiseuille's law**:
 * Flow is **turbulent** in larger airways and at bifurcations and irregularities of the bronchi. It is **laminar** in the smaller airways.
 * Of note, respiratory resistance, in particular airway resistance , is dependent on lung volume. R esistance increases with decreasing lung volume and more so below functional residual capacity (FRC).

Physiologic Dead Space

 * **Dead space** - describes portion of the lung that is **//ventilated//** but **//not perfused.//**
 * **Physiologic Dead Space** - Total of both alveolar and anatomic dead space
 * **Alveolar Dead Space** - Volume of gas that is not perfused at the alveolar level.
 * **Anatomic Dead Space**- volume of gas ventilating the conduction portion of the airways not involved in gas exchange, from the oronasopharynx to the terminal and respiratory bronchioles. Approx 2ml/kg of ideal body weight. Can be modified to include apparatus dead space.
 * **Apparatus Dead Space** - Distance from the tracheal tube and the ventilator Y-piece. Increased dead space further by large lengths of ventilator tubing between the tracheal tube and the ventilator Y-piece.
 * Factors that decrease pulmonary perfusion and/or hyperinflate the alveoli can **increase dead space**.
 * Decreased cardiac output, hypovolemia, hypotension, hypothermia, hypoventilation, emphysema, pulmonary embolism, ARDS, smoking, advanced age, upright position, positive pressure ventilation and/or PEEP (decreased venous return --> decreased pulmonary blood flow), anticholinergics, and bronchodilators.

The Effect of Rebreathing

 * Inspired gas contains two gases: that delivered from the anesthetic machine and that previously exhaled by the patient and subsequently rebreathed.
 * An increase in uptake or rebreathing lowers the inspired concentration of a highly soluble gas more than the inspired concentration of a poorly soluble gas.
 * This effect of uptake can be diminished by increasing the inflow rate to decrease rebreathing. An inflow rate that equals or exceeds minute ventilation eliminates rebreathing.
 * High inflow rates (i.e., ≥5 L/min) have the advantage of increasing the predictability of the inspired anesthetic concentration but the disadvantages of being wasteful and increasing atmospheric pollution.
 * High inflow rates may also dry inspired gas and make it difficult to estimate ventilation from excursions of the rebreathing bag. These several disadvantages prompt the use of low inflow rates.

Gas Mixtures

 * The physical behavior of gases follows the ideal gas law **PV=nRT**
 * For gas mixtures, each gas in a closed container exerts a pressure proportional to its fractional volume according to the ideal gas law, referred to as **//partial pressure//**
 * **Dalton Law of Partial Pressures**- states that the total pressure of a mixture of gases equals the sum of the pressures that each gas would exert if it were present alone.
 * Pt = P1 + P2 + P3 + ...
 * Of note, fail-safe valves and proportioning systems help minimize delivery of a hypoxic gas mixture, but they are not foolproof. Delivery of a hypoxic gas mixture can result from (1) the wrong supply gas , (2) a defective or broken safety device, (3) leaks downstream from the safety devices, (4) administration of an inert gas , and (5) dilution of the inspired oxygen concentration by high concentrations of inhaled anesthetics.

Dilution of Inspired Oxygen Concentration by Volatile Inhaled Anesthetics

 * Volatile inhaled anesthetics are added to the mixed gases downstream from both the flow meters and the proportioning system.
 * Concentrations of less potent inhaled anesthetics such as desflurane may account for a larger percentage of the total fresh gas composition than is the case with more potent agents.
 * Because significant percentages of these inhaled anesthetics may be added downstream of the proportioning system, the resulting gas-vapor mixture may result in delivery of a hypoxic mixture and a massive overdose of inhaled desflurane anesthetic despite a functional proportioning system.

Low-Pressure Circuit Leak Test

 * The low-pressure leak test checks the integrity of the anesthesia machine from the flow control valves to the common gas outlet, including also the flowmeters and vaporizers. It evaluates the portion of the machine that is downstream from all safety devices except the oxygen analyzer. The components located within this area are //precisely// the ones most subject to breakage and leaks . Leaks in the low-pressure circuit can cause hypoxia and/or patient awareness.
 * A typical three-gas anesthesia machine has 16 O-rings in the low-pressure circuit. Leaks can occur at the interface between the glass flow tubes and the manifold and at the O-ring junctions between the vaporizer and its manifold. Loose filler caps on vaporizers are a common source of leaks
 * Several different methods have been used to check the low-pressure circuit for leaks . Generally speaking, the low-pressure circuit **without an outlet check valve** can be tested with a **positive-pressure leak test**, and machines **with check valves** must be tested with a **negative-pressure leak test**.
 * ** Oxygen Flush Positive-Pressure Leak Test -  **  but in modern machines with check valves,  the positive pressure from the breathing circuit results in closure of the outlet check valve, thus not testing a vulnerable area from the check valve back to the flow control valves
 * ** 1993 FDA Negative-Pressure Leak Test - ** universal, done via a simple suction bulb to create a vacuum.
 * The machine is free of leaks if the hand bulb remains collapsed for at least 10 seconds. A leak is present if the bulb reinflates during this period.
 * The test is repeated with each vaporizer individually turned to the “on” position because internal vaporizer leaks can be detected only with the vaporizer turned on.
 * [[image:http://web.squ.edu.om/med-Lib/MED_CD/E_CDs/anesthesia/site/content/figures/2007F30.gif]]

Circle System Tests

 * The circle system tests evaluate the integrity of the circle breathing system, which spans from the common gas outlet to the Y-piece. It has two parts—the // leak test// and the //flow test//. To thoroughly check the circle system for leaks, valve integrity, and obstruction, both tests must be performed preoperatively.
 * The // leak test// is performed by closing the pop-off valve, occluding the Y-piece, and pressurizing the circuit to 30 cm H2O with the oxygen flush valve. The value on the pressure gauge will not decline if the circle system is leak free, but this does not ensure valve integrity. The value on the gauge will read 30 cm H2O even if the unidirectional valves are stuck shut or the valves are incompetent.
 * The //flow test// checks the integrity of the unidirectional valves, and it detects obstruction in the circle system.

Temperature and Humidity

 * Inadvertent hypothermia can occur during general anesthesia due to a combination of anesthetic-impaired thermoregulation and exposure to a cold operating room environment
 * Heat can be transferred from a patient to the environment in four ways: radiation, conduction, convection, and evaporation. Among these mechanisms, radiation and convection contribute most to perioperative heat loss.
 * The circle system conserves respiratory moisture and heat
 * Of note, the materials that vaporizers are constructed of are chosen because they have a relatively high specific heat and high thermal conductivity. These factors help minimize the effect of cooling during vaporization.

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