Low flow anaesthesia - a need of tomorrow

Kyoto ProtocolAll inhalational- atmospheric pollution O3 layer depletion-ban by year 2030

(Excluding India)!!!.`greenhouse gas' capacity 10x CO2

Regulations to reduce such emissions in 1997. N2O –O3 +GWP so restriction placed.

What we can do as Anaesthesiologist?

1.RA2.Gases with less impact.3.Xenon.4.Avoid N2O as a carrier gas.Use Air &

Oxygen .5.Avoid unnecessarily high FGF rates.6.Use LOW FLOW as aroutine

DEFINITION…

Low Flow Anesthesia (LFA) inhalation technique in which a circle system with absorbent is used with a fresh gas inflow of :-less than 1 l/min-More than50% rebreathing

Dorsch and Dorsch

Fact

In 1994, in the United States, 90% of anesthesiologists utilized 3-5 Lt/min of FGF and only 2% of physicians used FGF less than 1 Lt/min.

Benefits …

Humidification and warmingReduction in wastage and occupational

hazardsEconomic Minimizing Environmental pollution

Misconceptions….Ideal technique in the present situation, very few

anesthetists are adopting this approach and most are uncomfortable.

Why? familiar with constant inspired anesthesia agent

concentration technique with high flow.?? loss of control over anaesthetic concentration a complex process requiring knowledge, skill, training and

sophisticated apparatus.

Reality….

Till 90’s only halogenated agents, which were found to be unsuitable for low flow.

Inadequate knowledge about physics laws and pharmacology of inhalational anaesthetics

Not so clear about interrelations between FGF and uptake

Technologically poor machines and monitors till 90’s

Technical requirements Independent of anaesthesia machines used.

1. Circle rebreathing system with CO2 absorption2. Accurate flow meters for adjustments of FGF below

1L/min3. Gas tight breathing system.( below 150 ml/min at

cm H2O test pressure.)4. Sensitive and reliable alarms5. Ascending bellows

Breathing system

1. Minimal internal volume , number of components and connections.

2. Continuous gas monitoring MUST be employed. Real time inspired (Fi )and expired (Et) gas concentrations.

3. preferable at Y piece

Safety with Low Flow….

Knowledge of the basic underlying pharmacological and physiological principles associated with LFA.1. Behavior of gases in circle and relation with FGF.2. The lower the FGF, more important to know these

facts 3. Knowledge of Risk benefit ratio particularly in low

and minimal flow anesthesia 4. Monitoring Fi and Et at Y piece.

O2 and N2O uptakeVO2 = 10 x KG [kg] 3/4 [mL/min](Brody)

(VO2 = 3.5 x BW [ml/min] –approximately)VN2O = 1000 x t-1/2 [mL/min](Severinghouse JW)

. – 1st minute uptake of 1000mL

- 200 mL/min uptake after 25 minutes - 140 mL/min uptake after 49 minutes - 90 mL/min uptake after 2 hours (120 minutes)

Source: Severinghouse JW, The rate of uptake of nitrous oxide in man. J Clin Invest 1954

Uptake of inhalational agentsIndependent of the agent employed uptake can be calculated using Lowe’s formula:

Van= f x MAC x λB/G x Q x t-1/2 [mL/min]

f = factor that defines the inhalation concentration that is sufficient for unresponsive skin incision at ~MAC 1.3

λB/G = blood/gas partition coefficient

Q = cardiac outputt = time

Mathematics… Using a FGF of 5 LPM and a vaporizer setting of 2% Sevo (1 MAC), : 5000mL/min x 0.02 = 100 mL vapor/min What would be the vaporizer setting if we would use a

FGF of 1 LPM for the same 100 Ml vapor to be delivered? 1000mL/min X ? = 100 mL/min ? = ……(10%)

Time constant in minutes is an inverse function of FGF

Time constant1 time constant - 67 % target con achieved2 time constant - 86 % target con achieved3 time constant - 95 % target con achieved e.g To raise FIO2 from 0 % to 100% with a system volume of 6 lt

18 min with FGF of 1 lt,3 min with 6 lt FGF(O2)90 sec with 12 lt FGF(O2)60 Sec with 18 lt FGF(O2)

Same applies to inhalational anaesthetics

Risks

The potential risks associated with LFA are accidental hypoxia, hypercapnia,inadequate depth of anaesthesia andthe accumulation of potentially toxic trace gases.

(can be minimized by pre use flushing of system with high flow air or O2 )

RGM

Real time monitoring possibleFIO2, SpO2- early detection of hypoxiaEtCO2- status of ventilation and perfusionFi and Et of Anesthetic gases helps in adjusting vaporizer

settings.MAC value –prevention of awareness

Disadvantages of LFA

1. Limitations of currently used vaporizers:

Modern vaporizers are not much different from those used in the ’60s. They are designed for use with high

FGF with a consequent requirement for high thermal capacity, temperature compensation and high

accuracy. The use of LFA makes these characteristics unnecessary but it also introduces the problem of delivering an inadequate quantity of volatile agent into the breathing system.

Frequent changes in vaporizer setting to prevent awareness.

To deliver same quantity of vapor one has to increase vaporizer output settings.

In the absence of RGM it is a difficult proposition.

2. Accumulation of unwanted gases into the breathing system:

If FGF into the breathing system is low, little (or none) gases will come out. This result s in failure to flush other gases out of the system, any gases introduced which are not taken up by the patient or absorbed chemically will tend to accumulate.

Concentration of such gases may increase, be a contami- nant of the medical gases or result from a reaction with the chemical agents used for CO2 absorption.

- Alcohol- Acetone- CO- CH4

Therefore the use of LFA is contraindicated in patients who are intoxicated, in uncompensated diabetic states or who are suffering from CO intoxication.

HOW TO ADJUST FGF AT DIFFERENT PHASES OF LFA

Premedication, pre-oxygenation and induction of sleep are performed according to the usual practice. Concerning adjustment of FGF anesthesia can be divided into 3 phases:

1. Initial HIGH flow2. Low flow3. Recovery

INITIAL HIGH FLOW

High FGF of 5-6 LPM to wash out nitrogen (N2) from body tissues.

This facilitates the filling of the breathing system with the desired gas composition in much less time (FA) ,which in turn influences patient uptake and distribution of the anesthetic agents and in turn the depth of anaesthesia. High flow for short period to increase FA/FI if felt

essential intraoperatively

After the high flow phase of 5-15 min, or when the target gas concentrations(0.9/1.3 MAC) has been achieved FGF can be reduced at the desired low flow level. The lower the FGF the greater the difference between the vaporizer setting and inspired concentration of the anesthetic agent in the breathing circuit will be.Those agents with high B/G solubility show wide differenceSimilarly, with low FGF, time to reach the desired concentrationof the inspiratory gas will be prolonged.Hence, monitoring of oxygen and anesthetic agent concentration is essential and necessary in LFA.

Low flow phaseAfter 5-10 min of high flow when Et reaches desired level

switch to 1 Lt /min (N2O-500 ml,O2-500 ml or even less!)Change vaporizer setting to higher level to achieve 0.9

MAC if N2O is used as carrier gasIf air +O2 as carrier gas then desired MAC is 1.3.For Des minimal changes in vaporizer settings are

required (Blender type vaporizer)Analgesics and NMB agents as per requirement

Recovery phase

After “ shutting off Gas” it takes 10-15 min for Et of gas to return to 0.

If high FGF (>10 lt O2 ) wash out then it is much faster 100 % O2 normally used Reversal of NMB as per protocol

Today, LFA is such a safe and simple procedure that there are no reasons not to use it

routinely. unnecessary highFGF - inappropriate. it can be used in spontaneous breathing patients or with those having an LMA. The LMA has been shown to be effective both

in pediatric and adult LFA.

Summary..The technological development of high-

performing anesthesia ventilators, supplied with feed-back control systems and high precision monitoring systems, make LFA a safe and feasible on a routine basis.

LFA represents a great advantage as far as safe clinical practice, environmental, pharmacological, technological and cost savings are concerned.

Desflurane and sevoflurane (low B/G & tissue /G solubility),better performance when used in low-flow anaesthesia.

Apart from the economic advantages, low-flow anaesthesia helps to reduce environmental pollution

Associated with several physiological benefits for the patient.

Low-flow anaesthesia is a simple but highly effective method of cost minimization that can be applied to a large number of patients without any compromise in patient care or safety.

So why not Break the fear and misconceptions….

Any Questions Please…..