Mechanical Ventilation Learning Package, Exams of Nursing

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Mechanical Ventilation Learning

Package

Mechanical Ventilation Learning

Package

Mechanical Ventilation Learning

Package

Sharon-Ann Shunker, CNC, Liverpool ICU

SWSLHD

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1. INTRODUCTION The 3 physiological functions that are responsible for adequate tissue and cellular oxygenation are:  Pulmonary gas exchange  Oxygen delivery  Oxygen Consumption The respiratory system supports gas exchange through the process of ventilation, diffusion and perfusion for the uptake of oxygen and the removal of carbon dioxide from the body. Mechanical ventilation can be provided via non-invasive or invasive means and involves the delivery of positive pressure breaths. Gas flow is delivered via a constant or decelerating pattern and the volume is dependent on inspiratory time, gas flow and pressure applied at the airway. Pressure, flow, time and volume are all interrelated. Lung elasticity, chest wall and abdominal characteristics determine compliance (Oh,

Respiratory Anatomy and Physiology^45 Functionally the respiratory system consists of the conducting and respiratory zones, namely the upper and lower respiratory tract. The conducting zones consist of the nasal cavities, pharynx, larynx and trachea. The purpose of the conducting zones is to filter, humidify, warm and allow the passage of air to the lower respiratory zones. The respiratory zones are the site of gas exchange, containing the respiratory bronchioles, alveolar ducts and alveoli. (Tortora & Grabowski, 2000). (Source: http://webschoolsolutions.com/patts/systems/lungs.htm)

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LH_ICU2016_Learning_Package_Mechanical_Ventilation_Learning_Pac Functions of the respiratory system The major function of the respiratory system is to supply oxygen and eliminate carbon dioxide from the body. In addition to the vital function of gas exchange, the respiratory system fulfils the following functions^13  Acid base regulation – Through the process of ventilation, the lung removes CO 2 and regulates the pH of the body. Regulation of pH is accomplished by removing volatile acid (ie acid converted into the gaseous state; in this case, carbonic acid converted to CO 2 ).  Blood reservoir – The lung receives the venous blood from the right ventricle. Due to the capacity of the pulmonary circulation to receive blood, the lung acts as a reservoir from which the left side of the heart draws blood.  Filtering mechanism – The lung also constantly filters the air we breathe and removes trapped particles through the mucocillary clearance mechanism and the lymphatic system. The lung also acts as a filtering mechanism for blood by removing particles such gas bubbles, small fibrin or blood clots, fat cells, aggregates of platelets or WBC, and other pieces of cellular debris.  Metabolism – The lung produces some very important chemicals that serve physiological regulatory functions such as vascular dilatation, blood clotting, lung structural stability and neurotransmitters. Some chemicals passing through the lungs are converted into their more active form, such as angiotensin I, produced by the kidneys, which is converted to angiotensin II, a potent vasoconstrictor Control of respiration 7, Breathing is usually involuntary but voluntary breathing is necessary when the person is doing other activities such as walking, talking, singing, etc. In these cases homeostatic changes in ventilatory rate and volume are adjusted automatically by the nervous system to maintain normal gas exchange. The lung is innervated by the autonomic nervous system (ANS). Fibres of the sympathetic division in the lung branch from the upper thoracic and cervical ganglia of the spinal cord, while fibres of the parasympathetic division travel in the vagus nerve, which is important in the regulation of ventilation. The respiratory centers in the brain stem control involuntary ventilation by transmitting impulses to the respiratory muscles causing them to contract or relax. The pneumotaxic center in the upper pons functions to maintain rhythmic respirations. It stimulates the expiratory center, which then sends inhibitory signals to the inspiratory center. Inspiration ends and expiration begins. Strong stimuli from the pneumotaxic center result in shorter inspiration, and mild stimuli results in a longer one. The apneustic center sends stimuli to the inspiratory center and prolongs inspiration. The pneumotaxic center usually overrides the apneustic center. Impulses are transmitted from lung receptors, which are receptors that respond to physical changes in the pulmonary system, and chemoreceptors, which are receptors that respond to changes in oxygen or carbon dioxide concentrations, through the sympathetic and parasympathetic divisions of the ANS and the respiratory centers in the brain stem.^25

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3. PULMONARY CIRCULATION Certain problems with ventilation and perfusion can relate to the distribution of blood flow and hemodynamics that are unique to the pulmonary circulation and important for gas exchange in the lungs^13 Anatomy of the Circulation Pulmonary Artery: The pulmonary artery divides into left and right branches which supply blood to the two respective lungs. The walls of the vessels have large diameters and are thin and distensible giving the pulmonary artery system a large compliance of almost 7ml/mmHg. Bronchial vessels: These carry oxygenated blood to the supporting lung tissues, after it has passed through the tissues it empties into the pulmonary veins and left atrium. Lymphatics: They extend from the supportive tissue of the lungs into the hilum and empty into the right lymphatic duct. They remove particulate matter from the alveoli and plasma protein leaking from the lung capillaries, thereby helping to prevent pulmonary edema.^13 Pressures in the Pulmonary System Pressure in the pulmonary artery: During systole the pressure in the pulmonary artery is essentially equal to the pressure in the right ventricle. As the pulmonary valve closes the pressure in the ventricle falls precipitously whereas the pulmonary artery pressure falls more slowly as blood flows though the lung capillaries. Normal systolic PA pressure is 25mmHg, diastolic is 8mmHg and mean is 15mmHg. Pulmonary capillary pressure: Mean pressure is about 7mmHg. Left Atrial and Pulmonary Venous Pressure: The mean pressure in the left atrium and major pulmonary veins averages 2mmHg in the recumbent position.^13 Intrapulmonary Circulation (Percussionaire.com)

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LH_ICU2016_Learning_Package_Mechanical_Ventilation_Learning_Pac (Source: pulmonary circulation, web.cateret.edu)

Middle and lower Zone – best

match of Ventilation & Perfusion

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LH_ICU2016_Learning_Package_Mechanical_Ventilation_Learning_Pac Distribution of ventilation in the normal upright person It can be seen from this that the best match of ventilation to perfusion occurs in the middle lung zones in the upright person. The goal of many respiratory therapies is to optimise both ventilation and perfusion to the alveoli. The relationship between ventilation and perfusion is optimal when there is a match between ventilation and perfusion, that is, where the alveoli are receiving normal ventilation and perfusion. Disease states can alter this relationship, as depicted in the following diagram (Berghuis, et al, 1992, Spacelabs Biophysical Measurement Series; Respiration, page 8). Ventilation / perfusion relationships

1. Normal lung unit, receiving normal ventilation and perfusion
2. Shunt unit, not ventilated but receiving normal perfusion
3. Silent unit, neither ventilated or perfused
4. Deadspace unit, ventilated but not perfused
5. Example of alveoli that is under-ventilated and under-perfused

RV 1200 FRC 2400 ERV 1200 TV 500 IC 3600 IRV 3100 TLC 5800 Closing Capacity Closing Volume Airway ClosureBegins

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5. LUNG VOLUMES AND CAPACITIES Respiratory volumes and capacities Source: Marieb 1992, pages 742– As noted above, interpleural pressures result from the relationship between forces generated by the chest wall and lung. This relationship also determines the resting volume of the lungs (at end of normal expiration). This volume is called the functional residual capacity (FRC). This is the point where chest wall forces and lung forces are in balance. The following concepts are very important to appreciate.

Air participating in gas exchange = 350 mls Air in conducting Airways (anatomical deadspace) = 150mls

Tidal Volume = 500mls

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6. DEAD SPACE VENTILATION Dead space is the amount of gas that is involved in ventilation but does not take part in gas exchange. (Not all the air in each breath is used for the exchange of oxygen and carbon dioxide. About a third of every resting breath is exhaled exactly as it came into the body.) There are four types of dead space^7 :  Anatomic dead space – This refers to the amount of gas that fills the conducting passages of the airway and is not involved in gas exchange. In most adults, this value is estimated at 2 mL/kg of body weight. For the normal sized adult, it is usually about 150 mL. Therefore, if the normal tidal volume is 500 mL, only 350 mL of tidal volume is actually involved in gas exchange as illustrated below.  Alveolar dead space – This is the amount of gas filling the alveoli that does not contribute to gas exchange.  Mechanical dead space – This is the contribution to the patient's dead space through the addition of respiratory circuit attachments, etc.  Physiologic / total dead space – This value is the sum of anatomic and alveolar dead space. It represents the total volume in the airways and alveoli not participating in gas exchange. The relevance of anatomical dead space As the dead space increases, the amount of gas that actually contributes to gas exchange decreases. The volume of gas that takes part in gas exchange is alveolar ventilation. In the mechanically ventilated patient there is mechanical dead space that is contributed by the circuit. This is compressible dead space that can be reduced in patients with conditions such as ARDS and ALI, when it is difficult to achieve optimal tidal volumes.^7

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7. LUNG MECHANICS: COMPLIANCE AND RESISTANCE The mechanical characteristics of the lung greatly influence both normal lung function and pulmonary disability. The two major factors involved in lung mechanics are compliance and resistance, as outlined below.^50 Compliance Normally inspiration is an active process, accomplished through the expansion of the lungs and the thorax. The ease with which the lungs and thorax can be expanded, or distended, is referred to as compliance. Total compliance therefore depends not only on the elasticity of the lung tissue, but also on that of the thoracic cage. Compliance determines the change in volume for a given change in pressure. For example, if a patient is able to sustain a large increase in tidal volume with a small fall in pleural pressure then their lungs are compliant. If a patient requires a large fall in pleural pressure for a relatively small increase in tidal volume then their lung tissue is non-compliant. Compliance is reduced by any factor that: Reduces the natural elasticity of the lungs, eg fibrosis, or interstitial oedema. Reduces the total number of functional alveoli, eg atelectasis or airway obstruction. Increases the stiffness of the chest wall, eg splinting because of pain. Decreases the stiffness of the chest wall, eg post-sternotomy, resulting in decreased FRC. Checks the ability of the thorax to increase in volume, eg abdominal distension. Compliance is therefore a relationship between volume and pressure and can be estimated by dividing the change in volume by the change in pressure as follows: Compliance = change in volume change in pressure For example, your patient is receiving the following ventilator parameters: PEEP – 10 cm H 2 O Tidal volume – 1000 mL End inspiratory hold or plateau pressure – 35 cm H 2 O In this case, the change in volume is 1000 mL and the change in pressure is 25 cm H 2 O. The change in pressure is determined by subtracting the level of PEEP (10 cm) from the end inspiratory hold pressure (35 cm). Remember, we are interested in the change in pressure and in this case the pressure is rising from a baseline of 10 cm to a total pressure of 35 cm of water, the resultant change in pressure is therefore 25 cm. The total lung compliance (lung and chest wall) for this patient is: 1000 35- = 1000 = 40mL /cmH 2 O 25

Dynamic compliance = Tidal volume Peak inspiratory pressure - PEEP

High Resistance

Static compliance = Tidal volume Plateau pressure - PEEP

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Low Compliance

Clinically, there are two types of compliance measurements that can be determined These are dynamic compliance and static compliance. Dynamic compliance is calculated by the following formula: Static compliance is calculated by the following formula: To obtain the static compliance, an inspiratory pause must be initiated. This pause will result in a period of no gas flow and allow the pressure in the alveoli to equilibrate with the ventilator circuit pressure. The measurement of static compliance may be useful in eliminating the following variables that may influence compliance readings: resistance to flow, distribution of gases and recruitment time of closed lung units. Compliance alters during phases of a maximal inspiration. At lung volumes near RV and TLC, the lung tissue is less compliant (ie not as distensible). This results in an ‘S’ shaped curve (see following figure). Conceptually, this is similar to blowing up a balloon. It is more difficult to inflate a balloon at the beginning of inflation. Once the balloon starts to inflate, less pressure (or work) is required to inflate the balloon. As the balloon reaches its total capacity and draws near to bursting, a greater pressure is required to achieve a unit volume increase. Thus the lung, like a balloon, requires greater pressures at the beginning (near functional residual capacity) and end of inspiration (near total lung capacity) for relatively small increments in tidal volume; this is a state of decreased lung compliance. In the middle of inspiration, little pressure is required for increases in volume – ie the lungs are more compliant.

Compliance45, Compliance refers to the distensibility of the lung tissue. A patient with a low compliance or non-compliant lungs is said to have ‘stiff’ lungs. Signs of non-compliant lungs may include high airway pressures for a given tidal volume. Lungs that have decreased in compliance will require higher airway pressures to deliver a given tidal volume. Potential complications of increased airway pressures include: barotrauma, mediastinal emphysema, pneumothorax, and tension pneumothorax. Compliance is calculated by dividing the change in volume by the change in pressure. The normal value (full size adult) for compliance (total lung) is approximately 70– mL/cm H 2 O. Compliance for a patient who is intubated and ventilated is approximately 40–60 mL/cm H 2 O; this will vary depending on whether you are measuring static or dynamic compliance. Compliance is related to lung size; larger lungs have higher compliance. Elasticity is often mistaken to mean compliance. Elastance is the reciprocal of V O L U M E Pressure

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LH_ICU2016_Learning_Package_Mechanical_Ventilation_Learning_Pac Compliance ‘S’ curve Pulmonary Surfactant increases compliance by decreasing the surface tension of water. The internal surface of the alveolus is covered with a thin coat of fluid. The water in this fluid has a high surface tension, and provides a force that could collapse the alveolus. The presence of surfactant in this fluid breaks up the surface tension of water, making it less likely that the alveolus can collapse inward. If the alveolus were to collapse, a great force would be required to open it, meaning that compliance would decrease drastically45,

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8. OXYGENATION Tissue oxygenation depends on oxygen delivery and consumption. Three processes involved in oxygenation are:^17  Intake of Oxygen (Pulmonary Gas Exchange or Pulmonary Ventilation).  Delivery of oxygen to the cells (Arterial Oxygenation) – DO 2  Oxygen consumption -use of oxygen by the cells for metabolism (Cellular Oxygenation) – VO 2. The components of oxygenation (Source: Kidd & Wagner, 1997, pg:159) Pulmonary gas exchange:17, Pulmonary gas exchange involves breathing O 2 in so it can be exchanged at an alveolar level with CO2. The O 2 is then attached to haemoglobin in the pulmonary capillaries. Pulmonary gas exchange is dependent on three processes:  Ventilation  Diffusion – determined by:  Surface Area  Thickness  Length of exposure  Perfusion- determined by:  Haemoglobin  Affinity of O 2 to haemoglobin  Blood flow /Cardiac output

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LH_ICU2016_Learning_Package_Mechanical_Ventilation_Learning_Pac Process of Respiration (Source: Thelan et al, 1998, pg:619) Delivery of Oxygen (DO 2 )^17 Arterial oxygenation is the process of delivering oxygen to the cells (Kidd & Wagner, 1997). Oxygen delivery to the cells depends on:  Cardiac output  The amount of Haemoglobin (Hgb).  Oxygen saturation  Oxygen binding capacity (normally 1.34 mls of oxygen binds to every 1g of haemoglobin ) Each of these factors need to be optimised to improve oxygen delivery. The relationship between the oxygen carrying capacity of blood, cardiac output, concentration of haemoglobin and saturation of haemoglobin is expressed in the following equation. DO 2 = Cardiac output x Hgb concentration x Oxygen Saturation x 1. There is also a relationship between partial pressure of oxygen dissolved in the plasma (PaO 2 ) and saturation of Hgb with oxygen (SaO 2 ) on the oxyhaemoglobin curve. Oxyhaemoglobin – dissociation curve .13, Oxygen is transported in the blood in two ways. The majority of the blood (97%) is bound to haemoglobin, whereas the other 3% is dissolved in the plasma. Each haemoglobin molecule can combine with four oxygen molecules. When four oxygen molecules are bound to a haemoglobin it is said to be fully saturated. The extent to which haemoglobin is bound to haemoglobin depends largely on the PO 2 of blood. However, the relationship is not linear. When oxygen saturation is plotted against the partial pressure of oxygen an