Understanding the Various Modes of Ventilation

Introduction:

Patients under general anesthesia during surgery find it difficult to breathe on their own. Thus machines were developed to perform the function of breathing. Different ventilators were developed that use different parameters like inspiratory and expiratory pressures and volume and tidal volume.

What Is Ventilation in Anesthesia?

Ventilation is a form of life-support that helps a patient breathe in and out during surgery or when the patient cannot breathe on their own due to chronic illness, disability, or other devastating conditions. Ventilation is established in a patient with a device called a ventilator. A ventilator performs the function of the lungs either partially or completely. It helps to deliver oxygen and remove carbon dioxide from the lungs. Ventilators also maintain the pressure inside the alveoli to prevent them from collapsing.

What Are the Different Types of Ventilators in Anesthesia?

There are many types of ventilators, which are classified based on various parameters. Some of the classifications are listed below:

• According to the mechanism of action:

  • Mechanical thumbs.

  • Bag squeezers.

  • Minute volume dividers.

  • Intermittent blowers.

• According to the source of power:

  • Compressed gas.

  • Electricity.

  • Compressed gas and electricity in combination.

• According to the drive mechanism and circuit designation:

  • Single circuit (piston ventilators).

  • Double circuit (bellow ventilators).

• According to the cycling mechanism:

  • Time-cycled control.

  • Electronic control.

  • Contemporary electronic ventilators require either an electrical only or both an electrical and a pneumatic power source.

• According to the type of bellows:

  • Ascending bellows.

  • Descending bellows.

What Are the Different Modes of Ventilation in Anesthesia?

The ventilators used earlier have only a few ventilation modes in the operating room. But with an increase in the number of surgeries being done every day in hospitals, along with advanced technology, newer, improved, and efficient ventilation modes have been developed in anesthesia. As a result, sophisticated modalities for ventilation were introduced to meet the growing population's demand for surgery. The various modes of ventilation are:

• Volume Control Ventilation (VCV):

  • Volume control ventilation is provided by all ventilators.

  • The preset volume in this mode is delivered in a stable and constant flow.

  • The peak pressure during inflation (breathing in) changes according to the patient's airway resistance and compliance.

  • Volume is adjusted to prevent atelectasis (partial or complete collapse of the lung or part of the lungs). While monitoring peak inflation pressure, the respiratory rate is adjusted to get a reasonable volume of carbon dioxide during exhalation (tidal volume).

  • The advanced ventilators can provide tidal volumes ranging from twenty to one thousand and five hundred milliliters.

  • The settings to be followed in volume-controlled ventilation are:

    • Tidal volume as six to ten milliliters per kilogram of bodyweight.

    • Breathing rate as eight to twelve breaths per minute.

    • Positive end-expiratory pressure(PEEP) as zero to five centimeters H2O and titrated.

• Pressure Control Ventilation (PCV):

  • In this ventilation mode, the tidal volume is changed, but inspiratory pressure is maintained at the same value.

  • The volume of air inspired varies according to the changes in the patient's compliance and airway resistance. The flow is maintained in a decelerating flow pattern, which means that the flow is increased at the beginning of inspiration to set the pressure and later reduced to maintain the pressure set throughout the time of inhalation.

  • Then, to avoid the collapsing of the lungs and structural damage to the lungs on overexpansion of the lungs during mechanical ventilation, a reasonable tidal volume is produced by adjusting the target pressure.

  • Also, the rate is manipulated to get a balanced tidal volume of carbon dioxide. Pressure-controlled ventilation is beneficial in providing oxygenation in laparoscopic surgeries for treating obesity when compared to volume-controlled ventilation. It has several other uses:

    • In pregnant patients.

    • In surgeries performed on newborn babies.

    • In patients suffering from acute respiratory distress syndrome (ARDS).

• Pressure Control Ventilation-Volume Guaranteed (PCV-VG):

  • It is a newly developed mode of ventilation that integrates the benefits of both volume-controlled and pressure-controlled ventilation in mechanical ventilation.

  • In this, the ventilators work in pressure control ventilation mode, along with a value of target tidal volume is set.

  • It delivers a constant and fixed tidal volume along with all the advantages of pressure-controlled ventilation mode. It ensures a constant supply of uniform and unobstructed tidal volume irrespective of changes in compliance caused due to surgical exposure or relaxation, retractors, position change, and packs.

  • The preset tidal volume is delivered with reduced pressure in a decelerating flow pattern. The patient receives the first breath as a volume-controlled breath.

  • The patient's lung compliance is measured using the first breath, and then the inspiratory pressure is determined for the upcoming PCV-VG breaths.

  • The characteristics features of PCV-Vg breath are square pressure in a waveform and decelerating flow pattern.

• Synchronized Intermittent Mandatory Ventilation (SIMV):

  • It works in a unique way, enhancing and boosting patients' own efforts, and is designed to deliver definitive tidal volumes and rates.

  • The ventilator interacts with the patient's breathing needs and responds by working in a synchronized manner.

  • It works to sense the patient's breathing capacity and allow them to breathe independently between the mechanical ventilation.

  • It provides assistance and compulsory breaths when the patient's breathing efficiency and rate reach a point below the predetermined value.

  • These are called synchronized ventilators and help the patient breathe more naturally than with controlled ventilation and tackle the challenges faced when patients compete with controlled ventilators.

  • They can overcome the fluctuations that happen during general anesthesia while agents like narcotics, neuromuscular blocking agents, sedatives, and inhalational agents are given.

  • There are two types of SIMV modes: volume control(SIMV-VC) and pressure control(SIMV-PC).

• Pressure-Support Ventilation (PSV):

  • Pressure support ventilation is used for a patient under general anesthesia who can breathe independently.

  • The supraglottic devices that help keep the airway open, like the laryngeal mask airway (LMA), have made spontaneous breathing maintainable.

  • However, to maintain spontaneous ventilation, the anesthesia levels should be less, which will not help in acquiring a sufficient depth of anesthesia to perform surgery. PSV mode is beneficial in this aspect.

Conclusion

Mechanical ventilation has become one of modern anesthesia's most integral and ever-evolving parts. They come with advanced and improved technology to overcome the difficulties faced during surgeries. In addition, they are introduced with more versatile features to meet every type of patient's needs. So, it is recommended that the anesthesiologists and other operating room staff be updated and have a clear knowledge of new ventilators.