11.4 Positive Pressure Ventilation, BVM Mechanics, PEEP & CPAP

11.4 Positive Pressure Ventilation, BVM Mechanics, PEEP & CPAP

Physiology of Positive Pressure Ventilation vs Spontaneous Breathing

Understanding the profound physiological distinction between spontaneous ventilation and artificial positive pressure ventilation (PPV) is essential for modern paramedic practice under the Canadian Paramedic Competence Framework (CPCF Appendix A #24, #25).

During normal spontaneous breathing, inspiration is an active, negative-pressure phenomenon. Contraction and flattening of the diaphragm, coupled with elevation of the ribs by external intercostal muscles, expands the thoracic cavity. Intrathoracic and intrapleural pressures become subatmospheric (-5 to -8 cmH2O relative to ambient pressure), expanding lung parenchyma and drawing air into the alveoli. Critically, this negative intrathoracic pressure functions as a biological "thoracic pump," actively pulling venous blood from the inferior and superior vena cava into the right atrium, thereby augmenting cardiac preload, stroke volume, and cardiac output.

In stark contrast, artificial ventilation delivers positive pressure: air is forced into the lungs from the mouth under positive mechanical pressure. During PPV, intrathoracic and alveolar pressures become positive throughout inspiration. This fundamental reversal of thoracic mechanics induces major, potentially lethal adverse effects across multiple organ systems:

1. Hemodynamic Compromise (Decreased Cardiac Output): Positive intrathoracic pressure directly compresses the thin-walled inferior vena cava, superior vena cava, and right atrium. Systemic venous return (cardiac preload) is dramatically impeded. In a hypovolemic patient or a patient in cardiopulmonary arrest, this loss of preload causes a catastrophic collapse in left ventricular stroke volume, mean arterial pressure (MAP), and coronary perfusion pressure (CPP).
2. Pulmonary Barotrauma & Volutrauma: High peak inspiratory pressures can overdistend fragile alveoli, rupturing alveolar-capillary membranes and causing alveolar air leakage into the pleural space (pneumothorax, tension pneumothorax) or mediastinum (pneumomediastinum).
3. Gastric Insufflation & Aspiration: In an unprotected airway, gas flows along the path of least resistance. The lower esophageal sphincter (LES) normally maintains an opening pressure barrier of approximately 20 cmH2O. If positive pressure ventilation generates peak inspiratory pressures exceeding 20 cmH2O (caused by aggressive bag squeezing, high flow rates, or partial upper airway obstruction), gas is forced down the esophagus into the stomach. Gastric distension pushes the diaphragm upward, restricting lung expansion, while precipitating massive vomiting and fatal pulmonary aspiration.

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Manual Resuscitation Mechanics: BVM Ergonomics & Rates

The Bag-Valve-Mask (BVM) resuscitator is the primary manual tool used by paramedics to manage respiratory arrest and cardiac arrest.

Volume Mechanics: The Adult BVM Reservoir Trap

A standard adult self-inflating resuscitator bag contains an internal gas volume of 1200 to 1600 mL. However, human adult physiological tidal volume ($V_t$) in health and disease is only 6 to 7 mL/kg of ideal body weight, translating to roughly 500 to 600 mL. Completely compressing an adult BVM with two hands delivers over 1000 mL—a dangerous, iatrogenic hyperinflation that guarantees gastric distension and severe hemodynamic collapse. Paramedics must compress approximately one-third of the adult bag volume, smoothly delivering 500 to 600 mL over 1 full second, just enough to produce gentle, visible chest rise.

Ergonomics: One-Rescuer vs Two-Rescuer BVM Technique

Numerous clinical trials demonstrate that the traditional one-rescuer BVM technique is plagued by high failure rates due to massive mask leaks, inadequate tidal volumes, and inadvertent neck flexion. The Two-Rescuer Technique is the Gold Standard of prehospital airway care:

• One-Rescuer "E-C" Technique: The rescuer uses their non-dominant hand to form a "C" with thumb and index finger over the mask collar, while the middle, ring, and little fingers form an "E" grasping the bony mandible to pull the jaw into the mask. The dominant hand squeezes the bag. Pitfalls: Difficult to maintain an airtight seal, causes hand fatigue, and frequently pushes the mask down onto the face, inadvertently flexing the neck and obstructing the airway.
• Two-Rescuer "Thenar Eminence" Technique (Gold Standard):
  • Rescuer 1 (at the head): Places the thenar and hypothenar eminences of both hands along the lateral borders of the mask cushion, with thumbs pointing toward the nose and index/middle fingers hooking under the mandibular angles bilaterally. Rescuer 1 lifts the jaw firmly upward into the mask while stabilizing the head. This two-handed seal eliminates mask leaks and maintains a continuous jaw-thrust.
  • Rescuer 2 (at the side): Gently compresses the resuscitator bag with two hands or one hand, delivering 500 to 600 mL smoothly over 1 full second, carefully observing for chest rise and monitoring the capnography waveform.

Prehospital Ventilation Rates Across Clinical Paradigms

The Lethal Danger of Hyperventilation During CPR

During cardiopulmonary resuscitation, emergency responders routinely hyperventilate patients out of anxiety, delivering rates of 20 to 30 breaths/min with excessive volumes. Hyperventilation kills. Sustained positive intrathoracic pressure eliminates the brief periods of negative intrathoracic pressure created by chest recoil between compressions. Without negative recoil pressure, venous return drops to near zero, and Coronary Perfusion Pressure (CPP) falls below the critical 15 mmHg threshold required to achieve ROSC. Concurrently, hyperventilation blows off carbon dioxide ($PaCO_2 < 30 \text{ mmHg}$), triggering profound cerebral vasoconstriction and worsening ischemic brain injury during CPR.

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Positive End-Expiratory Pressure (PEEP) Valve Mechanics

A Positive End-Expiratory Pressure (PEEP) valve is an adjustable spring-loaded valve attached directly to the exhalation port of the bag-valve-mask circuit.

Physiological Mechanism

In normal spontaneous breathing, the vocal cords close momentarily during exhalation to maintain an intrinsic physiological PEEP of 3 to 5 cmH2O, keeping alveoli patent. In an unresponsive patient ventilated via BVM, this intrinsic mechanism is lost, allowing diseased or fluid-filled terminal bronchioles and alveoli to collapse at the end of each exhalation (atelectasis), requiring high opening pressures to re-inflate them during the next breath. A PEEP valve maintains positive mechanical pressure inside the airway throughout exhalation (preventing pressure from dropping to atmospheric zero):

1. Prevents end-expiratory alveolar collapse (atelectrauma).
2. Recruits collapsed, fluid-filled alveoli, expanding Functional Residual Capacity (FRC).
3. Enhances surface area for alveolar-capillary oxygen diffusion.

Clinical Settings & Titration

• Standard Initial Setting: 5 cmH2O.
• Titration Range: 5 to 10 cmH2O (titrate upward in increments of 2.5 to 5 cmH2O for refractory hypoxemia in submersion injury, cardiogenic pulmonary edema, or severe pneumonia).
• Contraindications & Warnings: PEEP increases mean intrathoracic pressure. It is contraindicated in severe hypotension (Systolic BP < 90 mmHg) and suspected untreated tension pneumothorax. Use with extreme caution in traumatic brain injury, as elevated intrathoracic pressure impairs cerebral venous drainage via the internal jugular veins, increasing intracranial pressure (ICP).

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Continuous Positive Airway Pressure (CPAP): Mechanics & Hemodynamics

Continuous Positive Airway Pressure (CPAP) is a form of non-invasive positive pressure ventilation (NIPPV) delivered to a spontaneously breathing patient. Unlike mechanical ventilators that cycle between inspiratory and expiratory pressures (BiPAP), CPAP maintains a constant, preset positive airway pressure (typically 5 to 10 cmH2O) continuously throughout both inspiration and expiration.

CPAP Hemodynamic & Pulmonary Cascades in Cardiogenic Pulmonary Edema:
[Positive Airway Pressure (5-10 cmH2O)]
   ├── [Splints Alveoli Open] ➔ Increases FRC & Gas Diffusion Surface ➔ Resolves Hypoxemia
   ├── [Hydrostatic Force]    ➔ Drives Transudative Fluid from Alveoli back to Capillaries
   ├── [Reduces Preload]      ➔ Lowers Systemic Venous Return ➔ Unloads Failing Right & Left Ventricles
   └── [Reduces Afterload]    ➔ Decreases Left Ventricular Transmural Systolic Pressure ➔ Improves CO

Four Physiological Pillars of CPAP

1. Alveolar Recruitment: Splints fluid-drowned or atelectatic alveoli open, dramatically expanding functional surface area for gas exchange and correcting profound ventilation-perfusion mismatch.
2. Interstitial & Alveolar Fluid Clearance: The positive hydrostatic pressure inside the alveolar airspace counteracts pulmonary capillary wedge pressure, physically pushing transudative fluid out of the alveoli and interstitial spaces back into the pulmonary vascular bed.
3. Decreased Work of Breathing: Offsets intrinsic PEEP in obstructive lung diseases, reducing the diaphragmatic energy expenditure required to initiate each breath and preventing fatal respiratory muscle exhaustion.
4. Hemodynamic Unloading (The Cardiac Benefit): CPAP is the gold-standard therapy for acute cardiogenic pulmonary edema (congestive heart failure). By moderately increasing intrathoracic pressure, CPAP decreases systemic venous return, reducing excessive right and left ventricular preload. Concurrently, it lowers the transmural pressure gradient across the left ventricular myocardium during systole, directly reducing left ventricular afterload. The failing heart pumps against lower resistance, stroke volume improves, pulmonary congestion clears, and invasive intubation is prevented.

Clinical Indications

1. Acute Severe Cardiogenic Pulmonary Edema (CHF): Moderate to severe dyspnea, bilateral crackles, orthopnea, pink frothy sputum, SpO2 < 90% despite oxygen.
2. Severe Exacerbation of COPD / Asthma: Severe dyspnea with accessory muscle use, tachypnea (>24 breaths/min), wheezing, refractory to initial bronchodilator therapy (use with monitoring for dynamic hyperinflation).
3. Near-Drowning / Submersion Injury: Hypoxemia caused by pulmonary surfactant washout and non-cardiogenic pulmonary edema.

Strict Inclusion Criteria

To qualify for prehospital CPAP under Canadian protocols, the patient must meet all of the following:

• Awake, alert, and capable of following verbal commands (GCS ≥ 10).
• Able to maintain and protect their own airway spontaneously.
• Demonstrating signs of severe respiratory distress (tachypnea >24 breaths/min, accessory muscle use, SpO2 < 90% on supplemental oxygen).
• Hemodynamically stable: Systolic Blood Pressure ≥ 90 to 100 mmHg (varies by provincial protocol).

Absolute & Relative Contraindications

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Patient Coaching, Mask Leak Troubleshooting & Complication Management

CPAP is an intense, intimidating intervention. An anxious, hypoxemic patient who feels smothered by a tight, hissing mask will instinctively pull it off, triggering clinical failure.

Pre-Application Coaching & Acclimatization

1. Verbal Coaching: Calmly explain the sensation to the patient: "This mask delivers a firm cushion of air that will help push the fluid out of your lungs and make breathing much easier. It will feel like sticking your head out the window of a moving car."
2. Hand-Held Acclimatization: Connect the CPAP circuit to the oxygen source and set the initial pressure to 5.0 cmH2O. Hold the mask gently over the patient's nose and mouth with your hand—without attaching the head straps. Instruct the patient to breathe slowly through their mouth and nose until they become comfortable with the airflow.
3. Securing the Harness: Once the patient acclimates (typically 30 to 60 seconds), gently fasten the head harness. Tighten the lower straps first, then the upper straps, maintaining balanced tension.
4. Pressure Titration: Begin at 5.0 cmH2O. If respiratory distress, accessory muscle use, and hypoxemia persist after 3 to 5 minutes, titrate the PEEP valve upward in increments of 2.5 cmH2O up to a standard maximum of 10.0 cmH2O (up to 15 cmH2O under specialized Advanced Care Paramedic protocols).

Troubleshooting Mask Leaks

An airtight mask seal is mandatory to maintain positive circuit pressure:

• Facial Hair / Beard Leaks: If a thick beard prevents a seal, apply a thin layer of water-soluble lubricant along the mask cushion to improve contact, or gently snug the harness straps.
• Edentulous Patients (Missing Dentures): If the patient has well-fitting dentures, leave them in place to maintain facial contours and support the mask cushion. If dentures are loose or floating, remove them and adjust the forehead spacer.
• Eye Irritation: Air leaking around the nasal bridge into the eyes indicates that the mask is too large or the forehead cushion requires adjustment. Never overtighten straps to the point of causing soft tissue ischemia.

CPAP Failure & Emergency Transition Criteria

The paramedic must continuously monitor the patient's GCS, respiratory rate, continuous capnography waveform, heart rate, and blood pressure. Immediately abort CPAP and transition to BVM with PEEP if any of the following occur:

1. Deterioration of mental status (GCS drops < 10, patient becomes somnolent or unresponsive).
2. Respiratory arrest, agonal respirations, or severe respiratory muscle fatigue.
3. Onset of nausea, retching, or active vomiting.
4. Systolic blood pressure plummets below 90 mmHg.
5. Refractory hypoxemia or developing signs of tension pneumothorax.

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Clinical Scenario: Acute Cardiogenic Pulmonary Edema Managed with Prehospital CPAP

Prehospital Encounter: The Drowning Heart

Paramedics are dispatched on a winter midnight for a 68-year-old male in extreme respiratory distress. On arrival, the crew finds the patient sitting bolt upright on the edge of his bed, gasping for breath, diaphoresis drenching his nightshirt. He can speak only in single, strangled words. Pink, frothy sputum tinges his lips.

Immediate Assessment:
Primary survey confirms a patent airway, severe tachypnea at 38 breaths/min with profound supraclavicular and intercostal indrawing. SpO2 is 74% on room air. Radial pulse is 122 bpm, irregular (atrial fibrillation); blood pressure is hypertensive at 194/108 mmHg. Auscultation reveals loud, wet inspiratory and expiratory crackles extending bilaterally from the lung bases all the way to the apices, accompanied by cardiac gallop.

Executing CPAP Therapeutics:
The paramedic recognizes acute severe cardiogenic pulmonary edema. Because the patient is alert (GCS 14), spontaneously breathing, and hypertensive (BP > 100 mmHg), he meets all inclusion criteria for prehospital CPAP with zero contraindications.

• The paramedic coaches the patient, explaining the rushing air sensation.
• The circuit is powered to 5.0 cmH2O with 100% FiO2. The mask is held gently over the patient's face for 45 seconds while he synchronizes his breathing, then the harness is secured snugly with no audible leaks.
• The crew co-administers sublingual nitroglycerin (0.4 mg spray) under medical directive, supported by CPAP's preload-reducing mechanics.

Clinical Course & Resolution:
After 4 minutes at 5 cmH2O, the patient's SpO2 rises to 85%, but severe dyspnea persists. The paramedic titrates the CPAP pressure upward to 7.5 cmH2O. Within 8 minutes, the transformation is dramatic: the patient's respiratory rate drops from 38 to 22 breaths/min; accessory muscle indrawing resolves; SpO2 climbs to 96%; and blood pressure moderates to 152/86 mmHg. Crackles recede to the lower third of the lung fields. Upon arrival at the emergency department, the emergency physician confirms that prehospital CPAP successfully averted endotracheal intubation and intensive care unit admission.

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Exam Pitfalls & High-Yield Pearls

• BVM Tidal Volume: Standard adult BVM delivers 500 to 600 mL (approx 6-7 mL/kg) over 1 full second to produce gentle chest rise. Never squeeze the entire 1600 mL adult bag.
• Hyperventilation Consequences: Hyperventilation elevates intrathoracic pressure, eliminates venous return, drops coronary perfusion pressure (CPP), and kills cardiac arrest patients. Controlled rate: 10 breaths/min (1 breath q 6s) with advanced airway; 10-12 breaths/min (1 breath q 5-6s) in respiratory arrest.
• CPAP Contraindications Checklist: Memorize the absolute contraindications to CPAP: Apnea/respiratory arrest, altered mental status (GCS < 10), vomiting/secretions, pneumothorax, hypotension (SBP < 90-100 mmHg), and facial trauma.
• CPAP Hemodynamic Benefit: In CHF, CPAP reduces BOTH preload (decreased venous return) and afterload (decreased LV transmural pressure), directly unloading the failing left ventricle.
• Dentures in CPAP: Leave well-fitting dentures in place during CPAP application to maintain facial seal; remove only if loose or causing an obstruction.