Respiratory failure Flashcards
Type 1 Respiratory failure : Definition
Hypoxaemia with normocapnia
* caused by ventilation/perfusion mismatch
Type 1 Respiratory failure : Pathophysiology
- Damage to lung tissue which impairs the diffusion of oxygen across the alveolar membrane and into the blood
- Impaired oxygenation of the blood is the main effect - causing hypoxia
- Hypoxia triggers increased ventilation to improve oxygenation
- Increased respiratory rate - expells CO2
- CO2 levels is therefore - either normal or low despite hypoxemia
Type 1 Respiratory failure : ABG values
ABG values;
* PaO2 < 8 kPa
- PCO2 < 6.5 kPa
Type 1 Respiratory failure : Causes
- Pulmonary oedema :
fluid accumulation in the alveoli impairs gas exchange - Pneumonia :
inflammation and infection in the lung tissues can reduce oxygen diffusion - COPD :
narrowing of the bronchioles (Chronic bronchitis) or dilation of alveoli (Emphysema) reduces oxygen entry and surface area for oxygen diffusion. - Asthma - severe bronchoconstriction during exacerbation can lead to poor oxygenation
- Pulmonary fibrosis : Fibrosis reduces surface area and increases the diffusion distance of the alveolar membrane leading to poor oxygen diffusion
Type 1 Respiratory failure : Pulmonary embolism and T1RF
- Blood clot in the pulmonary vessels reduces the perfusion to alveolar capillaries
- Impairing oxygen uptake and leading to hypoxia
- Ventilation remains intact but compromise in perfusion leads to V/Q mismatch
Reduced perfusion with normal ventilation - Pulmonary embolism
Type 2 Respiratory failure : Definition
- Hypoxaemia with hypercapnia, caused by alveolar hypoventilation which prevents oxygenation and elimination of CO2
- Hypercapnia is more significant.
Type 2 Respiratory failure : Pathophysiology
- Primary issue is inability to ventilate the airways by allowing air to move in and out
- There is hypoventilation of both expulsion of CO2 and inhalation of oxygen
- Inadequate gas exchange leads to - Hypoxaemia (Low O2) and Hypercapnia (High CO2)
This can be due to -
* Damage to the lung tissues limiting airflow due to narrowed bronchioles or increased resistance due to lack of lung compliance
* Mechanical impairment of intercostal and respiratory muscles for ventilation
Type 2 Respiratory failure : Causes
Increased airway resistance or lack of lung tissue compliance to expel
1. * COPD - narrowing of airways (Chronic bronchitis) and destruction of alveoli (emphysema) reduces ability of lungs to expel CO2 and causes impaired gas exchange
1. * Asthma - severe constriction of bronchioles prevents ventilation
Issues with mechanical ventilation
1 . Neuromuscular disorders
* Weakness of respiratory muscles prevents effective ventilation leading to impaired CO2 clearance
e.g. Myasthenia Gravis, Guillian Barre syndrome
2 . Chest wall deformities
* Restrict lung expansion thus preventing ventilation of gasses.
e.g. Scoliosis, Rib fracture, Pneumothorax
3 . Drug overdose
* Depress respiratory drive leading to muscle paralysis and inability to ventilate the lungs
CO2 : Physiology in the blood
- CO2 in blood binds with water producing carbonic acid (H+ ions)which decreases PH
- CO2 + H2O -> H2Co3 -> HCO3- + H+
- Body can adjust HCO3 to counterbalance if PH abnormality is caused by respiratory system
HCO3 : Physiology in the blood
- HCO3- produced in proximal tubule of the kidneys
- HCO3 is a base which helps to mop up H+ ions
- HCO3- + H+ -> H2CO3 -> H2O + Co2 : overall reduction in H+ ions, thus PH increases
- Raised HCO3 increases PH due to less H+ ions
- Low HCO3 decreases PH due to more H+ ions
Respiratory acidosis : Definition
- Inadequate alveolar ventilation leading to CO2 retention and reduced PH
PH : < 7.35
Respiratory acidosis : Causes
Causes
* Respiratory depression
* Guillian barre : paralyse leading to inadequate ventilation
* Asthma
* COPD
Respiratory alkalosis : Definition
Excessive alveolar ventilation (hyperventilation) resulting in more CO2 than normal being exhaled
High PH, Low CO2 : often 2nd to response to hypoxia.
Respiratory alkalosis : Causes
- Anxiety
- Pain : causing raised RR
- Hypoxia : raised RR
-
PE : leads to T1RF secondary to V/Q mismatch due to embolism preventing perfusion of lung tissue thus leading to lack of oxygenation
* Hypoxia then triggers increase in ventilation - increasing the respiratory rate
* More CO2 is expelled from the lungs - leading to respiratory alkalosis
5 . Pneumothorax
Respiratory acidosis/alkalosis : Compensatory mechanism
- metabolic compensation
- by varying the HCO3 lost in renal tubules to restore PH to normal range
- Takes a few days for metabolic compensation to occur as it requires the kidneys to adjust HCO3 excretion/retention
Thus if immediate compensation for respiratory disorder is seen - assume respiratory derangement is chronic or ongoing for couple of days e.g. CO2 retention, leads to chronic raised bicarbs
Metabolic acidosis - definition
Definition : Excess acid in the bloodstream secondary to a metabolic cause
* Low PH, Low HCO3, low BE
Metabolic acidosis - Pathophysiology
- Due to accumulation of acids in the body - thus higher concentration of H+ ions (anions) in the blood
Or
2 . Bicarbonate deficiency -
* Deficiency of HCO3- which helps to neutralise the H+ ions from the acids
* thus leading to more free H+ ions in the blood, resulting in acidity.
Metabolic acidosis - Causes - Increased H+ in the blood
Increase in acid concentration of the blood
-
Lactic acidosis
1. Lack of oxygen to tissues due to hypoxia or hypo perfusion for respiration - anaerobic respiration to metabolise glucose
1. This produces lactic acid as a by product
1. Lactic acid dissociates and releases H+ ions in the blood stream which lowers the pH of the blood -
Diabetic ketoacidosis
1. Fatty acids are broken down via lipolysis instead producing Ketone as a byproduct
Ketones release H+ ions in the blood when they dissociate - **Chronic renal failure **
1. Organic acids such as uric acid or sulphur containing amino acids accumulate and are not able to be excreted properly. -
Addison’s disease
1. Aldosterone deficiency leads to impaired N+/K+ regulation in the kidneys
1. Impaired reabsorption of N+ ions into the blood in exchange for K+ secretion in the urine
1. Excess K+ remains in the blood
1. K+/H+ in the cell membrane of cells - moves the extra K+ into the cell and releases H+ ions in the blood stream
1. Extra H+ ions in the blood - reduce its ph
Metabolic acidosis - Causes - Loss of HCO3- ions
-
Severe Diarrhoea
Bicarbonate rich intestinal and pancreatic secretions are excreted via the GI tract before they can be reabsorbed - **Renal tubular acidosis **
Proximal convultured tubule is unable to reabsorb HCO3- ions leading to excess secretion
Metabolic acidosis : Compensatory mechansism
- Respiratory compensation - (IMMEDIATE)
* HCO3 - + H+ -> H2CO3 -> H2O + Co2
* Chemoreceptor detect low pH and trigger increase in RR
* Increase in RR allows for more CO2 to be expelled which is an acidic gas - Renal compensation - DELAYED
Occurs if metabolic acidosis is not 2nd to renal problem
* Kidneys excrete more H+ ions in the urine to restore pH
HCO3 - + H+ -> H2CO3 -> H2O + Co2
* Reabsorb more HCO3 ions in the urine
Metabolic acidosis : 2nd electorlyte abnormalities
-
Hyperchloraemia
* Kidneys counteract excess HCO3- loss by reabsorbing more Cl- ions
* For every HCO3- ion lost, there is a new CL- ion reabsorbed
* Leading to : Normal anion gap also known as Hyperchloremic metabolic acidosis -
Hyperkalaemia
* H+ ions excess in the plasma can be exchanged with K+ ions in the cell
* This lowers the H+ ions free in the blood and balances the PH
* This can lead to excess K + ions in the blood —> Hyperkalaemia
Metabolc acidosis : Anion gap - definition
Difference between the
* measured positively charged ions in the blood
Minus
- measured negatively charged ions in the blood
Anion Gap = [Sodium (Na+)] - ([Chloride (Cl-)] + [Bicarbonate (HCO3-)])
Metabolc acidosis : Anion gap - Which ions are measured?
- Sodium (Na+) is the major cation in the blood.
- Chloride (Cl-) and bicarbonate (HCO3-) are the major anions.
- Unmeasured cation (K+), unmeasured anions (Albumin)
Metabolic acidosis : Normal anion gap
Normal anion gap range as 3 - 11 mmol/L. - referred as overall ‘Neutral’
Metabolic acidosis : Causes of high anion gap
Definition
* Due to increase in positive ions - high H+ conc in the blood
* + ions > - ions
Causes
* Due to endogenous or exogenous increase in H+ ion production
DKA, Lactic acidosis, Aspirin OD, Renal failure (reduced excretion of H+ ions by kidneys)
Metabolic acidosis : causes of normal anion gap
Definition
* Metabolic acidosis with normal anion gap
Causes :
* Loss of bicarbonate ions which is replaced by chloride in the plasma - thus overall stable anion concentration
GI losses - Diarrhoea, Ileostomy, prox colostomy
, Renal tubular acidosis
Metabolic Alkalosis : Definition
Definition :
* metabolic alkalosis is caused by an increase in bicarbonate HCO3 ion concentration in the blood
* Elevates blood PH above 7.45
Metabolic alkalosis - decreased H+ conc, leading to increase HCO3 conc, raised PH, raised CO3, raised BE
Metabolic Alkalosis : Causes
1 . Gastric losses e.g. vomiting
* Gastric fluid is made up of HCL which is has a high concentration of H+ ions
* Stomach acid is lost before they enter the intestines which is where HCO3- secretions occur
* Lack of HCO3- released by the pancreas means it builds up in the blood instead
2 . Hyperaldosteronism
* Excess aldosterone stimulates kidney tubules to increase absorption of water by retaining Na+ ions
* This also makes the distal tubules of the kidneys - excrete more + and reabsorb HCO3- ions
* Thus - excess H+
3 . Hypokalaemia
* Decreased level of potassium in the blood which can be due to excessive loss of GI tract or via the kidneys
* Low K and low blood volume triggers RAAS mechanism - aldosterone is released
* Aldosterone triggers reabsorption of HCO3- ions in the proximal convulated tubule.
4 . Renal loss of H+ ions
e.g. loop/thiazide diuretic, HF, nephrotic syndrome, cirrhosis
