12.1 Non-Invasive Ventilation in Cardiac Patients
Key Takeaways
- CPAP delivers one continuous pressure (typically 5-12 cmH2O) that recruits alveoli and offloads inspiratory work; bilevel adds an inspiratory pressure above the expiratory pressure, and that IPAP-EPAP gap is what augments tidal volume and clears CO2.
- Positive intrathoracic pressure is a hemodynamic drug: it lowers venous return and preload and lowers LV transmural pressure and therefore afterload, which is why NIV is first-line in acute cardiogenic pulmonary edema and reduces intubation rates.
- The same physiology is harmful in the preload-dependent patient: in RV infarction, RV failure or hypovolemia, positive pressure drops cardiac output and raises RV afterload, so hypotension after starting NIV means lower the pressure and give volume, not raise the pressure.
- Typical starting settings are CPAP 5-8 cmH2O titrated to 10-12, or bilevel 10-12/5 cmH2O titrated in 2 cmH2O steps toward 6-8 mL/kg predicted body weight; reassess pH, PaCO2, respiratory rate and oxygenation within 1-2 hours.
- Failure to improve pH, respiratory rate, or oxygenation after a 1-2 hour trial, or any decline in mental status, means intubate rather than continue NIV - delayed intubation is the classic NIV harm.
Why NIV Is a Cardiac Therapy, Not Just a Respiratory One
In acute cardiogenic pulmonary edema (ACPE) the failing left ventricle raises left atrial and pulmonary capillary hydrostatic pressure, alveoli and interstitium flood, intrapulmonary shunt rises, lung compliance falls, and the patient recruits accessory muscles. The work of breathing in florid flash pulmonary edema can consume 20-25% of total cardiac output and generates large negative intrathoracic swings that further increase venous return into an already congested circuit and increase left ventricular (LV) wall stress. Non-invasive ventilation (NIV) interrupts that cycle at several points at once, which is why applying a mask can transform a patient's appearance within 15-20 minutes without a single milligram of new drug reaching them.
CPAP Versus Bilevel: The Distinction CMC Tests
Continuous positive airway pressure (CPAP) delivers one constant pressure across the entire respiratory cycle. It does not deliver a breath and does not augment tidal volume. It splints open flooded and collapsed alveoli, raises functional residual capacity (FRC), reduces intrapulmonary shunt, and counterbalances intrinsic PEEP so the inspiratory muscles do less work. It fixes oxygenation.
Bilevel positive airway pressure (bilevel NIV, commercially BiPAP) delivers an inspiratory positive airway pressure (IPAP) and a lower expiratory positive airway pressure (EPAP). The difference between the two is the delivered pressure support, and that gap is what augments tidal volume and therefore minute ventilation. This yields the single most useful titration rule on the exam: an oxygenation problem is treated by raising EPAP and FiO2; a CO2 or pH problem is treated by widening the IPAP-EPAP gap (raising IPAP). Raising IPAP and EPAP together by the same amount improves oxygenation but does nothing for ventilation, because the supporting pressure difference is unchanged.
The Hemodynamic Effects
| Mechanism | Physiologic result | Consequence in the cardiac patient |
|---|---|---|
| Raised intrathoracic pressure compresses the vena cavae and right atrium | Venous return and RV preload fall | Relieves congestion in the volume-overloaded LV failure patient; causes hypotension in the hypovolemic or RV-dependent patient |
| LV transmural pressure = LV systolic pressure minus intrathoracic pressure | Positive intrathoracic pressure reduces LV transmural pressure | True afterload reduction - stroke volume rises in the failing, afterload-sensitive LV |
| Alveolar recruitment and reduced shunt | PaO2 rises, hypoxic pulmonary vasoconstriction eases | Lower pulmonary vascular resistance, less RV strain, better myocardial oxygen supply |
| Reduced work of breathing | Respiratory muscle oxygen consumption falls | Lower total myocardial oxygen demand, less catecholamine surge, less ischemia |
| Lung overdistension at high pressure | Alveolar vessels compressed, pulmonary vascular resistance rises | Worsens RV afterload in RV failure, pulmonary hypertension, RV infarction |
The flip side matters as much as the benefit. A patient with RV infarction, acute RV failure, pulmonary hypertension, tamponade physiology or frank hypovolemia is preload-dependent. In that patient the same positive pressure that unloads a failing LV removes the venous return the RV needs and simultaneously raises RV afterload. Cardiac output falls, sometimes abruptly. When blood pressure drops after NIV is applied, the correct response is to reduce the expiratory pressure, give volume, and reassess - not to escalate pressure because the saturation is still low.
Indications and Contraindications
Strongest indications: acute cardiogenic pulmonary edema (bilevel or CPAP, both supported by the 2017 ERS/ATS NIV guideline and by meta-analyses showing reduced intubation rates); COPD exacerbation with hypercapnic acidosis (pH below 7.35 with PaCO2 above 45 mmHg - the highest-quality NIV indication of all); selected hypoxemic respiratory failure; post-extubation support and post-extubation respiratory failure in high-risk patients; obesity hypoventilation and obstructive sleep apnea; palliative relief of dyspnea in the do-not-intubate patient.
Contraindications: cardiac or respiratory arrest; inability to protect the airway or clear secretions; depressed or agitated mental status that prevents cooperation; facial trauma, burns, or recent upper airway or upper GI surgery; active vomiting or upper gastrointestinal bleeding; undrained pneumothorax; bowel obstruction; and hemodynamic instability or life-threatening arrhythmia that demands immediate intubation and full support.
A 74-year-old with known HFrEF presents with flash pulmonary edema: BP 186/104, HR 118, RR 34, SpO2 85% on a non-rebreather, diffuse crackles. CPAP is applied at 10 cmH2O with FiO2 0.60 and IV nitroglycerin is started. Twenty minutes later BP is 146/84, RR 22, SpO2 94%, and the patient is speaking in sentences. Which mechanism best explains the contribution of CPAP to this improvement?
Settings, Titration, and the Trial Period
Starting Points
| Parameter | CPAP | Bilevel NIV |
|---|---|---|
| Initial pressure | 5-8 cmH2O | IPAP 10-12 / EPAP 5 cmH2O |
| Usual working range | 8-12 cmH2O | IPAP 12-20 / EPAP 5-10 cmH2O |
| Titration step | 2 cmH2O to target SpO2 and comfort | IPAP in 2 cmH2O steps toward 6-8 mL/kg predicted body weight exhaled tidal volume |
| Pressure ceiling | About 12-15 cmH2O | IPAP about 20-25 cmH2O (above roughly 20-25 cmH2O the lower esophageal sphincter opens and gastric insufflation rises) |
| FiO2 target | SpO2 92-96%, or 88-92% if chronic hypercapnia | Same |
| Backup rate | Not applicable | 10-14 breaths/min in spontaneous-timed mode when respiratory drive is unreliable or sedation is present |
Humidification is standard; unhumidified high flows dry secretions and worsen mucus plugging. Set the inspiratory time or rise time to match the patient's own effort - a rise time that is too slow feels like air hunger, and one that is too fast feels like a slap.
The Trial Period and the Definition of Failure
NIV is a time-limited trial, and knowing when to stop is the highest-yield safety point in the topic. Obtain a baseline arterial blood gas, then reassess at 1-2 hours. Improvement means a rising pH, a falling PaCO2, a respiratory rate trending down from above 30 toward the low 20s, better oxygenation, and a calmer patient.
Signs of NIV failure that should trigger intubation rather than more NIV:
- pH still below 7.25 or falling despite adequate pressure support
- Respiratory rate persistently above 30-35 or unchanged from baseline
- No improvement or worsening in PaO2/FiO2
- New or worsening encephalopathy, agitation, or inability to cooperate
- Hemodynamic deterioration, new arrhythmia, or ongoing ischemic chest pain
- Copious secretions the patient cannot clear, or vomiting
- Persistent large leak and dyssynchrony that cannot be corrected
The trap is the patient who is "about the same" at two hours. Delayed intubation after prolonged failing NIV carries worse outcomes than early intubation, because the patient arrives at laryngoscopy exhausted, acidemic, and catecholamine-depleted.
High-Flow Nasal Cannula as an Alternative
High-flow nasal cannula (HFNC) delivers heated, humidified gas at 30-60 L/min with an independently set FiO2 from 0.21 to 1.0. It generates a modest, flow-dependent positive airway pressure of roughly 3-5 cmH2O at 50-60 L/min with the mouth closed, washes carbon dioxide out of the nasopharyngeal dead space, meets or exceeds the patient's inspiratory flow demand so entrainment of room air is minimal, and is far better tolerated than a mask. It is a reasonable first choice in hypoxemic respiratory failure without significant hypercapnia, in patients who cannot tolerate a mask, and for pre-oxygenation and post-extubation support. It is not a substitute for bilevel NIV when the problem is hypercapnic acidosis, and it delivers far less reliable positive pressure than CPAP in ACPE. The ROX index, calculated as SpO2/FiO2 divided by respiratory rate, is used to track HFNC performance; a value of 4.88 or higher at 2, 6, and 12 hours predicts success, and a falling ROX should prompt escalation.
| Feature | CPAP | Bilevel NIV | HFNC |
|---|---|---|---|
| Pressure delivered | One continuous pressure | Two pressures (IPAP and EPAP) | Flow-dependent, roughly 3-5 cmH2O |
| Augments tidal volume | No | Yes | No |
| Clears CO2 | Indirectly | Yes - primary strength | Dead-space washout only |
| Best first-line use | Acute cardiogenic pulmonary edema | Hypercapnic COPD exacerbation, ACPE with hypercapnia | Hypoxemic failure without hypercapnia, post-extubation |
| Preload reduction | Marked | Marked | Minimal |
| Tolerance / ability to eat and speak | Poor to fair | Poor to fair | Good |
| Aerosol and secretion clearance | Impaired | Impaired | Preserved |
A patient with an inferior STEMI and confirmed right ventricular involvement becomes hypoxemic and is placed on bilevel NIV at 14/6 cmH2O. Within 10 minutes the blood pressure falls from 96/60 to 72/48 mmHg, heart rate rises to 116, and the jugular venous pressure is elevated with clear lung fields. What is the most appropriate nursing action?
Nursing Care During NIV
Interface and Skin
Mask fit determines success. An oronasal (full face) mask is standard for acute respiratory failure; nasal masks leak through the mouth in the dyspneic patient; a total-face mask or helmet may rescue the patient with facial anatomy that will not seal. Fit two fingers under the head straps - overtightening is the single most common error and worsens both leak (by deforming the cushion) and skin injury. Apply a hydrocolloid or thin foam dressing to the nasal bridge prophylactically, inspect the bridge, cheeks, and behind the ears at least every 4 hours, and rotate interfaces when the therapy will run longer than 24-48 hours. Device-related pressure injury on the nasal bridge can appear within hours.
Aspiration and Gastric Distension
Pressures above roughly 20-25 cmH2O overcome the lower esophageal sphincter and insufflate the stomach. Keep the head of bed at 30-45 degrees, assess abdominal distension each shift, and place a gastric tube if distension develops or the patient is receiving prolonged bilevel support - though be aware a tube under the mask cushion creates a leak channel. Do not feed a patient orally under a tight-fitting mask in acute distress; if the patient vomits into an oronasal mask, remove the mask immediately, suction, and reassess airway protection.
Synchrony, Leak, and Comfort
- Large leak causes auto-triggering, failure to cycle off, and alarm fatigue. Fix the mask before touching the settings.
- Ineffective triggering in the air-trapped COPD patient usually responds to raising EPAP toward the level of intrinsic PEEP.
- Prolonged inspiratory time and delayed cycling cause the patient to fight the machine at end-inspiration; shorten inspiratory time or adjust cycle sensitivity.
- Claustrophobia is best handled by coaching, holding the mask in place by hand for the first several minutes before strapping it, and continuous bedside presence during initiation. Sedation to force tolerance is hazardous: a small dose of an anxiolytic in a patient who then hypoventilates or loses airway protection converts a controlled situation into an emergency. Use it only with continuous monitoring and a clear plan for intubation.
- Eye irritation from an upward leak is a marker of poor fit and a source of corneal abrasion.
Monitoring Set
Continuous SpO2, cardiac rhythm, and blood pressure; respiratory rate and pattern including accessory muscle use and paradoxical abdominal motion; exhaled tidal volume and leak on the device display; level of consciousness; arterial blood gas at baseline and 1-2 hours after initiation or any significant change; and hemodynamic response, particularly in patients with RV disease, pulmonary hypertension, or marginal blood pressure. Document the trial start time and the reassessment findings explicitly - that documentation is what drives the timely decision to intubate.
The Do-Not-Intubate Patient
NIV in a patient who has declined intubation is a palliative comfort measure, and the goal changes from avoiding intubation to relieving dyspnea. Settings should prioritize comfort over blood gas normalization, the mask should come off for conversation and sips, and opioids for dyspnea are complementary, not contradictory, to NIV. Clarify and document in advance what constitutes failure in this context, because "NIV failure" in a DNI patient means transitioning to comfort-focused care rather than escalating to the airway.
A patient with a COPD exacerbation and decompensated heart failure has been on bilevel NIV at 12/5 cmH2O with FiO2 0.50 for 60 minutes. The initial arterial blood gas showed pH 7.24 and PaCO2 70 mmHg; the repeat gas shows pH 7.22 and PaCO2 74 mmHg. Respiratory rate remains 32/min and the patient is now drowsy and intermittently pulling at the mask. Which action should the nurse prioritize?