RESPIRATORY SYSTEM CHANGES IN NEONATE AND ADULT

Anatomical differences in the airway and the differences in respiratory physiology between a neonate and an adult

Neonate

Adult

Anatomical Changes: They have a larger head and tongue Narrow nasal passages Cephalad Larynx Large Epiglottis Short Trachea and Neck Narrowest part of airways is the cricoid cartilage Ribs are pliable and horinzontal,diaphragmatic muscles are weaker and abdomen protuberant so the respiratory rate is increased to compensate for less efficient ventilation. Small airways and limited number of alveoli reduces their lung compliance Alveolar maturation is not complete So the work of breathing is increased and respiratory muscles fatigue easily Tidal volume and dead space are per kilogram of body weight and are constant in all age groups Physiological Changes Chest wall collapse during inspiration and relatively low residual volume at expiration There is reduction in Functional Residual Capacity which limits oxygen reserves during periods of apnoea and predisposes neonates to hypoxemia and atelactasis Relatively higher rate of oxygen consumption Ventilation is not hypoxia and hypercapnia driven on the contrary the reduce ventilation

Anatomical Changes Propotionate smaller head and tongue Broader nasal passages Caudad Larynx Smaller Epiglottis Relatively elongated neck and trachea Narrowest part of the airway is the glottis Ribs are more more curved with stronger muscles making ventilation more efficient Airways better developed Physiological Changes Increased compliance of chest wall and better residual volume increase the functional residual capacity which gives a better oxygen reserve Ventialtion is hypoxia and hypercapnia driven ,partial pressure of oxygen and carbon dioxide is a feedback to respiratory centres in brain

weaning criteria from ventilator

Principles involved in weaning a patient from ventilatory support in an ICU

Weaning procedures are usually started only after the underlying disease process that necessitated mechanical ventilation has significantly improved or is resolved. The patient should also have an adequate gas exchange , appropriate neurological and muscular status, and stable cardiovascular function.

Weaning indices are objective criteria that are used to predict the readiness of patients to maintain spontaneous ventilation.

The rapid shallow breathing index (f/V_T, where 'f' is the respiratory rate and 'V_T' is the tidal volume measured during the first minute of a T-piece trial) is superior to conventional parameters in predicting the outcome of weaning while arterial blood gases and respiratory rate.

The intact airway reflexes and a cooperative patient are mandatory prior to completion of the weaning process unless the patient has a cuffed tracheostomy tube.

Mechanical Criteria for weaning/extubation

Criteria                                                                                measurement

Inspiratory pressure                                                         <-25cmH2O

Tidal Volume                                                                       >5ml/kg

Vital Capacity                                                                      >10ml/Kg

Minute Ventilation                                                             <10ml

Rapid Shallow Breathing Index                                          <100

With the patient breathing spontaneously

Adequate oxygenation should be maintained that is the arterial haemoglobin saturation of >90% at FiO2 of 0.4-0.5 with less than 5cm of H2O of PEEP

                              RSBI  =  f(breaths/min) /VT(L)

   Patients with RSBI of 100 or less can be successfully extubated

The most common techniques to wean a patient from the ventilator include SIMV, PRESSURE SUPPORT VENTILATION, low levels of CPAP, spontaneous breathing short periods on T-piece.

Hyponatraemia

What are the symptoms and signs of hyponatraemia? Calculate the sodium deficit in a 70 Kg adult male with serum sodium of 122mmol/L. How will you replace the deficit?

Hyponatremia is a plasma sodium concentration less than 135 mEq/L.

S/S of hyponatraemia:

·        Nausea/vomiting

·        Visual disturbances

·        Depressed levels of consciousness

·        Agitation,confusion,coma

·        Muscle cramps,weakness and myoclonus

·        Coma and death

Sodium deficit in a 70kg male with serum sodium of 122mmol/L

Na+ deficit = TBW*(140-122)

TBW is approximately 60% of body weight in males(TBW is total body water)

Na+ deficit = 70*0.6*(135-122)=756 mEq/L

Replacement of the deficit:

The optimal rate of correction seems to be 0.6 to 1 mmol/L/hr until the sodium concentration is 125 mEq/L; correction then proceeds at a slower rate.
 One half the deficit can be administered over the first 8 hours, and the next half can be administered over 1 to 3 days if symptoms remit. Sodium concentration should be monitored every 1 to 2 hours during rapid correction.
Menstruanting female patients are at greater risk for developing significant neurologic sequelae after hyponatremia., Estrogens seem to alter the function of Na⁺/K⁺-ATPase in the rat brain, which could alter the brain's compensatory mechanisms for hyponatremia.
Management of hyponatremia involves elimination of the underlying condition when possible (e.g., stop TURP syndrome as soon as possible).

The use of normal saline (308 mOsm/L) alone may worsen the hyponatremia, depending on the patient's serum and urine osmolality  Severe coma or seizures can be managed with one or more approaches, including 3% hypertonic saline (513 mEq/L), fluid restriction, or furosemide

Scavenging System in anesthesia workstation

What is scavenging in OT? what are five basic parts of a scavenging system? What are the hazards of a scavenging system?

Scavenging is the collection and the subsequent removal of waste anesthetic gases from the operating room 

Scavenging systems generally have five components  (1) the gas-collecting assembly, (2) the transfer means, (3) the scavenging interface, (4) the gas-disposal assembly tubing, and (5) an active or passive gas-disposal assembly An “active system” uses a central evacuation system to eliminate waste gases. The “weight” or pressure of the waste gas itself produces flow through a “passive system.

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Gas-Collecting Assembly

The gas-collecting assembly captures excess anesthetic gas and delivers it to the transfer tubing. Waste anesthetic gases are vented from the anesthesia system either through the APL valve or through the ventilator relief valve which  accumulates in the gas-collecting assembly and is directed to the transfer means. 

Transfer Means

The transfer means carries excess gas from the gas-collecting assembly to the scavenging interface. The tubing must be either 19 or 30 mm, and should be sufficiently rigid to prevent kinking, and as short as possible to minimize the chance of occlusion.and is sometimes  color code with yellow  to distinguish it from 22-mm breathing system tubing.. The two tubes frequently merge into a single hose before they enter the scavenging interface.

Scavenging Interface

The scavenging interface is the most important component of the system because it protects the breathing circuit or ventilator from excessive positive or negative pressure. The interface should limit the pressures immediately downstream from the gas-collecting assembly to between -0.5 and +10 cm water with normal working conditions. Interfaces can be open or closed, depending on the method used to provide positive and negative pressure relief.

Open Interfaces

An open interface contains no valves and is open to the atmosphere, allowing both positive and negative pressure relief. Open interfaces is used  with active disposal systems that use a central evacuation system. Open interfaces require a reservoir because waste gases are intermittently discharged in surges, whereas flow from the evacuation system is continuous.

 An open canister provides reservoir capacity which should be large enough to accommodate a variety of waste gas flow rates. Gas enters the system at the top of the canister and travels through a narrow inner tube to the canister base. Gases are stored in the reservoir between breaths. Positive and negative pressure relief is provided by holes in the top of the canister 

Closed Interfaces

A closed interface communicates with the atmosphere through valves. All closed interfaces must have a positive-pressure relief valve to vent excess system pressure if obstruction occurs downstream from the interface. A negative-pressure relief valve is mandatory to protect the breathing system from subatmospheric pressure if an active disposal system is used. Two types of closed interfaces are commercially available. One has positive pressure relief only; the other has both positive and negative pressure relief.

Gas-Disposal Assembly Conduit

The gas-disposal assembly conduit  conducts waste gas from the scavenging interface to the gas-disposal assembly. It should be collapse-proof and should run overhead, if possible, to minimize the chances of accidental occlusion

Gas-Disposal Assembly

The gas-disposal assembly ultimately eliminates excess waste gas  There are two types of disposal systems: active and passive.

The most common method of gas disposal is the active assembly, which uses a central evacuation system. A vacuum pump serves as the mechanical flow-inducing device that removes the waste gases usually to the outside of the building.

interface with a negative-pressure relief valve is mandatory because the pressure within the system is negative. A reservoir is very desirable, and the larger the reservoir, the lower the suction flow rate needed.

Hazards

Scavenging systems add complexity to the anesthesia system.

 A scavenging system extends the anesthesia circuit all the way from the anesthesia machine to the ultimate disposal site. 

Obstruction of scavenging pathways cause excessive positive pressure in the breathing circuit, resulting in  barotrauma . 

Excessive vacuum  to a scavenging system can cause  undesirable negative pressures within the breathing system. 

Scavenging system catching  fires in engineering equipment rooms that house the vacuum pumps used for waste anesthetic gas evacuation. due to oxygen enrichment 

Trendelenburg position and its anaesthetic considerations

What are the various indication and disadvantages of trendelenburg position and how to prevent them?

Tilting a supine patient head down, the Trendelenburg position (German surgeon, Trendelenburg, who described its use for abdominal surgery)

Indications 

Abdominal and laparoscopic surgery, and to prevent air emboli and facilitate cannulation during central line placement. 

Disadvantages 

The Trendelenburg position has significant cardiovascular and respiratory consequences.

1)  The head-down position increases central venous, intracranial, and intraocular pressures. 

2) Prolonged head-down position  can lead to swelling of the face, conjunctiva, larynx, and tongue with an increased potential for postoperative upper airway obstruction. 

3) The cephalic movement of abdominal viscera against the diaphragm worsens the ventilation–perfusion ratio by gravitational accumulation of blood in the poorly ventilated lung apices and  decreases functional residual capacity and pulmonary compliance. 

4) In spontaneously ventilating patients, the work of breathing increases. 

5) The stomach  lies above the glottis so Endotracheal intubation is often preferred to protect the airway from pulmonary aspiration related to reflux and to reduce atelectasis.

Associated care in trendelenburg position

1) Care is taken to prevent patients  sliding off operating tables with resulting head injuries. With modern surgical tables and procedural techniques, steep head-down tilt is not often warranted. 

2) Retension material or shoulder braces for arm placement is avoided in these position to prevent brachial plexus injury.

3) Ventilation should be assisted or controlled and higher inspiratory pressures are needed to expand the lung.

4) Cranial vascular congestion and increased intracranial pressure can result from head-down tilt and in patients with known or suspected intracranial disease, the position should be used only in those rare instances in which alternate posture cannot be found and  Maintenance of the position should then be as brief as possible.

4)Because of the risk of edema to the trachea and mucosa surrounding the airway during surgeries in the Trendelenburg position for prolonged periods, it may be prudent to verify an air leak around the endotracheal tube or visualize the larynx before extubation.