Respiratory - Acid Base Interpretation, Acute Respiratory Failure, COPD

PaCO2 vs. HCO3

• PaCO2

  • ACIDIC!! (when it increases, there is ACIDOSIS)

  • controlled by the lungs

  • can change within MINUTES

• HCO3

  • ALKALOTIC (when it increases, there is ALKALOSIS)

  • controlled by the kidneys

  • changes overs HOURS to DAYS

anion gap - what is it? normal range?

• the difference between positive and negative anions

• in most instances of METABOLIC ACIDOSIS, there is an increase in the anion gap

• normal range - 5-15 mEq/L

anion gap - why is it important?

• it’s helpful in determining the cause of and/or response to treatment for metabolic acidosis

  • ex: if a patient with DKA presents with an anion gap of 25, one would expect the anion gap to decrease gradually as the patient responds positively to treatment (AND since electrolytes are assessed frequently, acidosis can be monitored through the anion gap without getting frequent ABGs)

problems associated with an INCREASE in the anion gap (hint: KUSSMAUL)

• K - ketoacidosis

• U - uremia

• S - salicylate intoxication

• M - methanol toxicity

• A - alcoholic ketosis

• U - unmeasured osmoles: ethylene glycol, paraldehyde

• L - lactic acidosis: shock, hypoxemia

Acute Respiratory Failure - what is it?

• a rapidly occurring inability of the lungs to maintain adequate oxygenation of the blood with OR without impairment of CO2 elimination

  • ABG would show PaO2 of 60 or less with OR without an elevation of PaCO2 to 50 or more with pH less than 7.30

Acute Respiratory Failure - Hypoxemic (conditions that lead to this type)

• CONDITIONS causing LOW blood oxygen levels

  • ARDS

  • asthma

  • atelectasis

  • interstitial fibrosis

  • pneumonia

  • pulmonary edema (HF)

  • pulmonary embolism (MASSIVE)

  • smoke inhalation

Acute Respiratory Failure - Hypercapnic (conditions that lead to this type)

• CONDITIONS causing HIGH blood PaCO2 levels

  • CNS depression from oversedation

  • COPD (acute exacerbation)

  • head trauma

  • musculoskeletal disorders or trauma

  • sleep apnea

  • status asthmaticus

Acute Respiratory Failure - Type 3 Hypoxemic AND Hypercapnic (conditions that lead to this type)

• ARDS (late)

• COPD (late, acute exacerbation)

• status asthmaticus (late)

Acute Respiratory Failure - Hypoxemic (s/s)

• pulmonary - tachypnea, adventitious breath sounds, accessory muscle use

• cardiac - tachyarrhythmias (initial), bradyarrhythmias (late), HTN or hypotension, cyanosis (central (lips, earlobes))

• neurological - anxiety, agitation

Acute Respiratory Failure - Hypercapnic (s/s)

• pulmonary - shallow breathing, bradypnea, lungs may be clear or there may be adventitious breath sounds

• neurological - progressive decreased level of consciousness (lethargic, obtunded, stuporous, unresponsive)

Acute Respiratory Failure - treatments

• maintain airway/improve ventilation

  • positioning upright, suctioning, bronchodilator for wheezing, noninvasive ventilation, intubation if needed, repeat ABGs as needed

• optimize oxygenation

  • adjust FiO2 to keep SaO2 about greater than 90; decrease FiO2 to 0.50 or less as soon as possible; do NOT allow hypoxemia to occur to “prevent O2 toxicity”; use PEEP/CPAP as needed; use pulse oximetry to monitor response to therapy

• optimize circulation, cardiac output

  • manage hypotension, address cardiac arrhythmias

• identify etiology; provide emotional support

Noninvasive Ventilation of ARF (CPAP - what is it?)

• continuous positive airway pressure

  • indicated for patients with hypoxemic respiratory failure who have increased work of breathing (such as cardiogenic pulmonary edema)

  • settings include FiO2 and 1 pressure setting in cm H2O pressure

Noninvasive Ventilation of ARF (BiPAP - what is it?)

• Bilevel positive airway pressure

  • indicated for patients with hypoxemic and/or hypercapnic respiratory failure

  • settings include FiO2 and 2 pressure settings (inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP)

  • IPAP assists ventilation and EPAP assists with oxygenation

Noninvasive Ventilation of ARF - ADVANTAGES

• buys time for medical treatment to take effect

• reduces WOB

• decreases preload/afterload (due to increased intrathoracic pressure drawing blood in and pushing it out easier)

• improves oxygenation

• improves ventilation (BiPAP)

• reduces atelectasis

• prevents intubation and resultant risks

Noninvasive Ventilation of ARF - CONTRAINDICATIONS

• hemodynamic stability or life-threatening arrhythmias

• copious secretions

• high risk of aspiration

• impaired mental status/inability to cooperate (unable to protect airway)

• suspected pneumothorax

• life-threatening refractory hypoxemia (PaO2 less than 60 with FiO2 at 100%

high flow nasal cannula - how much oxygen can it deliver?

• 100% FiO2 and up to 60L/min

high flow nasal cannula - ADVANTAGES

• able to provide high FiO2

• heated and humidified oxygen may imporove secretion clearance and decrease aiway inflammation

• able to meet high inspiratory flow demands of tachypniec patients

• seems to promote alveolar recruitment and increase FRC (functional residual capacity - the amount of air left in the lungs AFTER a normal exhalation)

• decreases dead space ventilation

• more comfortable than CPAP or BiPAP masks, allows access to the mouth without removal of a mask

high flow nasal cannula - DISADVANTAGES

• unable to deliver higher airway pressures (PEEP or CPAP), and the low levels of airway pressure provided are variable when mouth breathing

• provides limited pressure support for a patient with hypercapnic respiratory failure

high flow nasal cannula - INDICATIONS

• community acquired pneumonia (CAP)

• cardiogenic pulmonary edema when NIV is not tolerated

• preoxygenation prior to intubation

• post-extubation (even in low-risk patients)

• for a patient who refuses intubation (DNI) but accepts alternative treatment measures

COPD acute exacerbation - 3 types

• emphysema

• asthma

• chronic bronchitis

COPD acute exacerbation - easier to inspire or expire?

• easier to INSPIRE than EXPIRE (air gets trapped in the lungs)

COPD acute exacerbation - physiological consequences

• dynamic hyperinflation occurs due to too much air in the lungs

• air trapping and auto-PEEP

• LOW expiratory flow rates

• acute exacerbation results in V/Q mismatch due to a problem with ventilation and an increase in PaCO2

• patient may have chronic CO2 retention; if so, the patient will have patial OR complete compensation and high HCO3 on ABG

• can result in RV enlargement (cor pulmonale) and elevated CVP

COPD acute exacerbation - signs and symptoms

• worsening dyspnea

• increase in sputum purulence

• increase in sputum volume

• hypercapnia, hypoxemia

COPD acute exacerbation - MANAGEMENT

• titrate FiO2 to PaO2 GREATER than 60 or SaO2 GREATER than 90% with care NOT to overcorrect hypoxemia and decrease respiratory drive (must address SEVERE hypoxemia; do NOT withhold oxygen only because hypoventilation may occur, cells still need oxygen!!!)

• bronchodilator therapy (inhaled short-acting beta-agonist (SABA) such as albuterol; inhaled anticholinergics such as Ipratropium)

• monitor LOC for decreased responsiveness

• corticosteroid therapy (prednisone, methylprednisolone, fluticasone, budesonide, etc)

• antibiotic therapy (when PNA is the trigger, such as piperacillin-tazobactam, amoxicillin, levofloxacin, etc)

• proceed with mechanical ventilatory support if needed (invasive or non-invasive)