Manual ventilation using a Bag-Valve-Mask (BVM) device is a fundamental technique used to deliver rescue breaths to a person who is not breathing or not breathing effectively. The BVM, sometimes referred to as an Ambu bag, is a self-inflating device that provides positive pressure ventilation to the patient’s lungs. Understanding the proper amount of air, or tidal volume, to deliver is paramount in a respiratory or cardiac emergency. The core challenge is estimating the correct volume without the sophisticated monitoring equipment typically found in a hospital setting.
Understanding Tidal Volume and Resuscitation Physiology
Tidal volume is the amount of air that moves in or out of the lungs during a single breath. In resuscitation, the goal is to provide enough air for adequate oxygenation and carbon dioxide removal, not simply to mimic a normal resting breath. While a normal adult tidal volume is roughly 500 milliliters, the amount required during an emergency is calculated to maximize gas exchange while minimizing potential harm.
The physiological balance is delicate because both under-ventilation and over-ventilation carry significant risks. Under-ventilation fails to deliver enough oxygen to the body’s tissues, quickly leading to hypoxia and worsening the patient’s condition. Conversely, hyperventilation can cause barotrauma—physical injury to the lungs from excessive pressure—potentially leading to a pneumothorax or lung collapse.
Over-ventilation dangerously increases intrathoracic pressure, squeezing the great blood vessels near the heart. This elevated pressure reduces the amount of blood returning to the heart, decreasing cardiac output and impairing the effectiveness of chest compressions during CPR. Excessive force can also open the esophageal sphincter, causing air to enter the stomach. This gastric inflation increases the risk of vomiting and subsequent aspiration of stomach contents into the lungs.
Recommended Tidal Volume and Practical Application for Adults
Current guidelines recommend a specific tidal volume for adults that balances the need for oxygenation with the risks of over-inflation. The recommended volume is approximately 500 to 600 milliliters (mL) of air, translating to about 6 to 7 mL per kilogram of ideal body weight. This range provides adequate ventilation while minimizing the risk of gastric insufflation or lung over-distension.
Since emergency responders rarely have a way to measure the exact volume of air being delivered, the practical application relies on a visual cue. Rescuers should deliver just enough air to achieve a gentle, visible rise of the patient’s chest over a period of about one second. This visible chest rise serves as the primary indicator that the lungs have been adequately inflated with the target volume.
The rate at which breaths are delivered is equally important for adults. For a patient in cardiac arrest receiving chest compressions, the recommended rate is one breath every six seconds (ten breaths per minute), delivered without interrupting compressions. For an adult who has a pulse but is not breathing adequately, the rate is slightly faster, at one breath every five to six seconds. Maintaining this slow rate prevents inadvertent hyperventilation and its detrimental effects on circulation.
Adapting Ventilation for Pediatric and Infant Patients
Volume requirements change significantly when ventilating pediatric and infant patients due to their smaller lung capacity and anatomical differences. The guiding principle of delivering “just enough to cause visible chest rise” remains the standard, but the volume needed is substantially less than for an adult. The weight-based recommendation is typically 6 to 8 mL/kg for children and 4 to 6 mL/kg for infants.
Practically, the rescuer must use only a small fraction of the adult BVM bag squeeze, or ideally, use a correctly sized pediatric bag. Pediatric BVM devices are designed with smaller reservoirs (500 to 1000 mL), which inherently limits the maximum volume delivered in a single squeeze. Infants and children have a higher risk of gastric inflation and barotrauma because their lungs are smaller and less compliant.
The correct ventilation rate for smaller patients is faster due to their higher metabolic rates. For children and infants, the recommended rescue breathing rate is one breath every two to three seconds (twenty to thirty breaths per minute). Proper technique often involves a two-person method: one rescuer secures a tight mask seal with both hands, forming an “E-C” clamp, while the second rescuer delivers the controlled, small-volume breaths.
Key Components of the Bag-Valve-Mask System
The Bag-Valve-Mask is a system of interconnected parts designed to facilitate manual positive pressure ventilation. The self-inflating bag, which the rescuer squeezes, typically holds a volume much larger than the required tidal volume; an adult bag may contain 1,000 to 1,500 mL of air. This large reservoir emphasizes that only a partial squeeze is necessary to achieve the target 500 to 600 mL volume for an adult.
The mask component must create a tight seal over the patient’s nose and mouth to ensure the delivered air enters the lungs and does not leak out. Attached to the bag is a one-way, non-rebreathing valve that directs air into the patient during the squeeze and then allows the patient’s exhaled air to exit to the environment, preventing rebreathing of carbon dioxide.
The BVM system also includes an oxygen reservoir bag, which connects to a supplemental oxygen source flowing at a high rate, such as 15 liters per minute. This reservoir allows the device to deliver a high concentration of oxygen, up to nearly 100%, with each rescue breath. Other accessories, such as a Positive End-Expiratory Pressure (PEEP) valve, can be added to help keep small airways open, further improving oxygen delivery.