Skills - Rationale Flashcards
Respiration Process
Respiration
- Pulmonary ventilation (air movement in and out of the lungs)
- External respiration (gas exchange - O2 loading and CO2 unloading)
- Transport of respiratory gases (blood transports gases between lungs and tissue cells)
- Internal respiration (gas exchange between blood and tissue cells at capillary levels)
Tidal volume (Vt)
Tidal volume is the amount of gas expired per breath
Typically 500mL at rest
Dead space Volume (VD)
Dead space volume is the sum of the anatomical dead space (due to volume of airways - ~150mL) and physiological dead space (due to alveoli being ventilated but not perfused - usually insignificant)
Minute Volume (VE)
Minute volume is the amount of gas expired per minute
Alveolar ventilation (VA) - Formula
Alveolar ventilation is the amount of gas that reaches alveoli per minute
Formula VA = (Tidal volume - dead space) x Respiratory rate
Lung volumes
- Measurement
- Average size
- Description
Tidal volume (TV) ~500mL - amount of air inhaled or exhaled with each breath under resting conditions
Inspiratory reserve volume (IRV) ~3100mL - amount of air that can be forcefully inhaled after normal tidal volume inhalation
Expiratory reserve volume (ERV) ~1200mL - amount of air that can be forcefully exhaled after a normal tidal volume exhalation
Residual volume (RV) ~1200mL - amount of air remaining in lungs after a forced exhalation
Total lung capacity (TLC) ~600mL - max amount of air contained in lungs after a maximum inspiratory effort (TLC = TV+IRV+ERV+RV)
Vital capacity (VC) ~4800mL - max amount of air that can be expired after a maximum inspiratory effort (VC+TV+IRV+ERV = should be 80% TLC)
Inspiratory capacity (IC) 3600mL - max amount of air that can be inspired after normal expiration (IC=TV+IRV)
Functional residual capacity (FRC) ~2400mL - volume of air remaining in the lungs after a normal tidal volume expiration (FRC = ERV+RV)
Mechanics of respiration
- Pressures
- Components and functional abilities of lungs
Pressure
- Atmospheric pressure (at sea level) is 760mmHg
- Intrapulmonary pressure - rises and falls with phases of breathing and is the pressure within the alveoli of lungs
- Intrapleural pressure - pressure within the pleural cavity and is negative to intrapulmonary pressure and atmospheric pressure by ~4mmHg
Components and functional abilities of lungs
- Elastic ability of lungs - lungs assume smallest size possible at any given time
- Alveoli - surface tension caused by fluid film in alveoli acts to cause alveoli to fall to smallest size
- Chest wall - thoracic cage is elastic and acts to pull the thorax outwards and enlarge lungs
Atelectasis (lung collapse)
- Natural prevention
- Causes
Body naturally prevents atelectasis due to the difference between the intrapulmonary and inrapleural pressures (negative pressure)
Atelectasis occurs when the pressure equalises
Causes include
- Air entering pleural cavity
- Rupture of visceral pleura
- Air in intrapleural space (pneumothorax)
Boyle’s Law
Boyle’s law states that volume changes lead to pressure changes which lead to the flow of gases to equalize the pressure
The pressure of gases varies inversely with its volume
Inspiration
Active process requiring muscular effort
- High to low pressure
- Thoracic cavity volume increases
- Fall in alveolar pressure
- Gases always flow down their pressure gradient
- Flow ceases when intrapulmonary pressure is 0 and equal to atmospheric pressure
Expiration
Passive process due to lung recoil
- Thoracic cavity volume decreases
- Elastic lungs recoil passively and intrapulmonary volume decreases
- Intrapulmonary pressure rises
- Air gases flow out of lungs
Dalton’s law of partial pressure
Dalton’s law states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in mixture
Partial Pressure of air at atmospheric level and alveoli
Atmospheric:
- Nitrogen = 78.6% with PP of 597mmHg
- Oxygen = 20.9% with PP of 159mmHg
- Carbon dioxide = 0.04% with PP of 0.3mmHg
- Water vapour = 0.46% with PP of 3.7mmHg
- Total = partial pressure of 760mmHg
Alveoli:
- Nitrogen = 74.9% with PP of 569mmHg
- Oxygen = 13.7% with PP of 104mmHg
- Carbon dioxide = 5.2% with PP of 40mmHg
- Water vapour = 6.2% with PP of 47mmHg
- Total = partial pressure of 760mmHg
Henry’s Law
Henry’s law states that when a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure.
The greater the concentration of a particular gas in the gas phase the more that gas will dissolve in the liquid
Transport of CO2 in the body
CO2 is transported in the blood from the tissue cells to lungs in 3 ways
- Dissolved in plasma
- Chemically bound to hemoglobin in RBC
- As bicarbonate ion in plasma
Haldane effect
Haldane effects states that
- Deoxygenated hemoglobin combines more readily with CO2 than oxygenated hemoglobin
- It is the greater ability of reduced hemoglobin to form carbaminohaemoglobin and buffer H+ by combining with it
- As CO2 enters systemic bloodstream it allows more CO2 to combine with hemoglobin and more bicarbonates ions to be formed
- Haldane effect encourages CO2 exchange in both tissues and lungs
Bohr Effect
Bohr effects state that with more CO2 entering bloodstream it causes more oxygen to dissociate from hemoglobin
Capnography
- What it is
- How it works
- What it measures
- Normal limits
Capnography is the measurement of end tidal carbon dioxide levels
How it works
- Uses infrared waves to measure CO2 levels as infrared is absorbed by gases that have 2 or more different atoms
- 5 characteristics of waveform - Height, frequency, rhythm, baseline and shape
What it measures
- Proportion of CO2 in expired air
- End-tidal CO2 represents the gold standard for confirming advanced airway placement
- If return of spontaneous circulation occurs, a spike in end tidal CO2 often appears before pulse detected
- See slides for image of normal wavelength
Normal limits for end tidal CO2 are 35-45 mmHg
Causes of abnormal end tidal carbon dioxide
Elevated ETCO2
- Decreased ventilation secondary to head trauma, overdose, respiratory failure, sedation, stroke
- Increased carbon dioxide production due to fever or shivering
Decreased ETCO2
- Ventilation problem due to esophageal intubation, airway obstruction
- Inadequate blood flow due to cardiac arrest, tension pneumothorax, pericardial tamponade, reduced CO
- Ventilation-perfusion mismatch due to pulmonary embolism
- Decreased production of CO2 due to hypothermia
- Sampling error due to inadequate tidal volume delivery or CO2 sampling tubing blocked
Definition of respiratory failure
- Acute respiratory failure
- Chronic respiratory failure
Respiratory failure is defined as a partial pressure of arterial oxygen that falls below 60mmHg (hypoxia) or pressure of arterial CO2 above 50mmHg (hypercapnia)
Acute respiratory failure
- Characterized by life threatening derangement in PaO2, PaCO2 and acid base balance
Chronic respiratory failure
- Alterations in arterial blood gas valves and can cause very different normal vital signs
Common respiratory conditions
- Chronic obstructive pulmonary disease (COPD)
- Asthma
- Bronchiectasis
- Sarcoidosis
- Lung cancer
- Influenza
- Pneumonia
Gas Exchange as ventilation : perfusion ratio
- Hypercapnia
- Hypoxaemia
Gas exchange is the balance between alveolar ventilation and pulmonary capillary blood flow
Changes expressed as ventilation: perfusion ratio (V/Q)
- High ratio indicates greater than normal ventilation and lower than normal perfusion
Hypercapnia
- Increase in PaCO2 due to increased tidal volume and/or respiration rate which decreases alveolar ventilation and CO2 removal can alter acid base balance
Hypoxaemia
- Oxygenation of arterial blood decreases
- Contributing mechanisms include decreased alveolar oxygenation, decreased diffusion of oxygen from alveoli to pulmonary capillaries and decreased pulmonary capillary perfusion
Respiratory Assessment
- Primary Survey
- Clinical history
- Physical assessment
Primary Survey
- Gross signs of respiratory compromise including airway obstruction, stridor,, increased respiratory effort and marked accessory muscle use, tachypnoea, decreased speech tolerance, pallor or cyanosis, hypoxia, paradoxical chest wall movement, decreased air entry and altered level of consciousness
Clinical History
- Current event
- Underlying illness/disease
- Current symptoms
Physical assessment
- Inspection, palpation, percussion and auscultations
- See textbook for details
Support for respiratory function
- Oxygen therapy (low flow, high flow)
Oxygen therapy
- Low flow - fraction of inspired oxygen varies from breath to breath and depends on patient minute ventilation
- High flow - accommodates patient inspiration demands and maintains fixed FiO2 irrespective of patient respiratory rate and tidal volume
- Note that oxygen administration can elevate the PaCO2 due to changes in ventilation: perfusion ratio in lung therefore causing hypercapnia
Non invasive ventilation
- Benefits
- Evidence needed for respiratory support
- Contraindications
Benefits
- Improved survival, fewer complications, increased comfort and decreased cost
- Can be used in acute, chronic, hypoxaemic or hypercapnic conditions
Evidence needed for respiratory support
- Moderate to severe respiratory distress
- Increased respiratory rate
- Use of accessory muscles
- Arterial blood gas show respiratory acidosis or PaCO2/FiO2 under 200
Contraindications
- Impaired consciousness
- Hemodynamic instability
- Myocardial ischemia, unstable angina
- Unable to protect airway
- Copious respiratory secretions
- Uncooperative, agitated or depressed level of consciousness
- Difficulty fitting mask
- Head trauma with unstable respiratory drive
- Recent upper airway or GI surgery
Invasive mechanical ventilation
- Full ventilation support
- Partial ventilation support
Endotracheal tube or mechanical ventilation
Full ventilation support - maintained effective alveolar ventilation irrespective of patient comfort
Partial ventilation support - patient contributes to breathing and maintaining alveolar ventilation