Traditional pressure-limited ventilation delivers every breath at a fixed inflating pressure, so the tidal volume ($V_T$) reaching the lung depends passively on respiratory-system compliance. When compliance changes — repeatedly in surfactant deficiency and evolving bronchopulmonary dysplasia (BPD) — a fixed peak inspiratory pressure (PIP) produces a drifting $V_T$: too large as compliance improves, too small as it worsens. Volume-targeted ventilation (VTV) inverts the relationship: the clinician sets the $V_T$, and the ventilator adjusts the inflation pressure to achieve it. Volume Guarantee (VG) is Dräger's implementation of VTV and, with PRVC and AutoFlow, among the most widely used hybrid modes in neonatal intensive care. It attacks two forms of ventilator-induced lung injury at once: capping volume limits volutrauma and biotrauma, while stabilising minute ventilation prevents the hypocarbia associated with cerebral white-matter injury.

Definition, Taxonomy and Platform Implementation

Volume-Targeted Ventilation as a Mode Family

VTV is a family of strategies with one defining feature: a tidal volume is set and pressure is the dependent variable. A VTV breath is a pressure-controlled breath with volume feedback, distinct from volume-controlled ventilation (VCV), in which a set flow delivers a guaranteed volume while pressure varies.

How Volume Guarantee Is Implemented on Dräger Devices

On the Dräger Babylog VN500 and Evita, VG is not a stand-alone mode but a targeting option layered onto a pressure-controlled or assisted mode such as synchronised intermittent mandatory ventilation (SIMV), assist/control (A/C) or pressure support (PSV). The clinician prescribes the target $V_T$, the mode and rate, and the pressure ceiling ($P_{\max}$), and the ventilator updates working PIP breath by breath. Two features are essential: the proximal Y-piece flow sensor, without which VG cannot operate, and a pressure ceiling, since VG may raise PIP only between the set floor and $P_{\max}$.

Equivalents on Other Platforms

PRVC (Siemens/Getinge Servo), AutoFlow (Getinge/Maquet) and volume-assured pressure support (Hamilton) share the architecture of a pressure-controlled breath with a proximal volume feedback loop, differing in measurement site and adjustment aggressiveness. Because VTV measures expired volume at the airway opening, cross-platform comparisons of set $V_T$ are meaningful only when measurement site and leak behaviour match.

The Servo-Control Mechanism

Breath-to-Breath Operation

Each VG breath is a feedback cycle. During inflation the proximal flow sensor integrates flow to measure inspired volume ($V_i$); during exhalation it measures expired volume ($V_{Te}$). The controller subtracts measured volume from the set target and adjusts the next inflation pressure proportionally, making VG a discrete, breath-to-breath servo. The pressure change per breath is deliberately limited — typically to about $\pm 3\text{ cmH}_2\text{O}$ — so the mode cannot chase noise with large swings. If the required pressure would exceed $P_{\max}$, it raises a "volume not reached" alarm.

Auto-Weaning as Compliance Changes

When compliance improves after surfactant or diuresis, the same PIP delivers a larger $V_T$; the controller detects the excess and lowers PIP on subsequent breaths. This is auto-weaning: support withdrawn in proportion to physiological improvement without a clinician turning a dial. When compliance falls — atelectasis, pneumothorax, mucus plugging — the controller raises PIP until the target is achieved or the ceiling is reached. Working PIP thus becomes a bedside read-out of lung mechanics — and its principal hazard: an unrecognised deterioration or large leak can drive PIP to the ceiling and cause the barotrauma the mode was chosen to avoid.

flowchart LR
    A["Set target V_T (4-6 mL/kg) and P_max"] --> B["Inflate at current working PIP"]
    B --> C["Y-piece sensor measures inspired (V_i) and expired (V_Te) volume"]
    C --> D{"Compare V_Te with set V_T"}
    D -->|"Below target"| E["Raise next PIP by up to +3 cmH2O"]
    D -->|"Above target"| F["Lower next PIP by up to -3 cmH2O"]
    E --> G{"Within P_max and PIP floor?"}
    F --> G
    G -->|"Yes"| B
    G -->|"Ceiling reached"| H["Alarm: volume not reached - reassess leak, mechanics, effort"]

    style A fill:#eff6ff,stroke:#3b82f6,stroke-width:2px;
    style D fill:#fefce8,stroke:#eab308,stroke-width:2px;
    style H fill:#fee2e2,stroke:#ef4444,stroke-width:2px;

Physiological Equations and Tidal Volume Targets

Compliance and Driving Pressure

Respiratory-system compliance links the volume delivered to the pressure used to deliver it:

$$C_{rs} = \frac{\Delta V}{\Delta P} = \frac{V_T}{PIP - PEEP}$$

where $C_{rs}$ is compliance ($\text{mL}\cdot\text{cmH}_2\text{O}^{-1}$), $\Delta V$ is delivered volume ($\text{mL}$) and $\Delta P$ is driving pressure ($\text{cmH}_2\text{O}$), approximated as PIP minus PEEP. Rearranged, $V_T = C_{rs}\times(PIP - PEEP)$: at fixed PIP, doubling $C_{rs}$ doubles $V_T$ and risks volutrauma, while halving it risks hypoventilation.

Tidal Volume per Kilogram

The target is scaled to the infant's weight, conventionally as a dose in millilitres per kilogram:

$$V_{T,\text{set}}\ [\text{mL}] = d\ [\text{mL}\cdot\text{kg}^{-1}]\times W\ [\text{kg}]$$

where $d$ is the dose, conventionally $4\text{–}6\ \text{mL}\cdot\text{kg}^{-1}$ of current weight, and $W$ is weight in kilograms. Because preterm infants gain weight daily, the millilitre target must be recalculated with every weight.

Leak Fraction

With an uncuffed endotracheal tube, a fraction of inspired volume escapes unmeasured. The leak fraction is:

$$L = \frac{V_i - V_e}{V_i}\times 100\%$$

where $V_i$ is inspired and $V_e$ is expired tidal volume ($\text{mL}$). A leak up to about $30\%$ is usually tolerable; above $40\text{–}50\%$ the expired signal becomes unreliable and the controller may raise PIP to chase a volume being lost.

Alveolar Ventilation and Carbon Dioxide

Carbon dioxide removal depends on alveolar ventilation:

$$\dot V_A = f\,(V_T - V_D), \qquad PaCO_2 \approx \frac{0.863\times\dot V_{CO_2}}{f\,(V_T - V_D)}$$

where $\dot V_A$ is alveolar ventilation ($\text{mL}\cdot\text{min}^{-1}$), $f$ is respiratory rate ($\text{min}^{-1}$), $V_D$ is anatomical plus instrumental dead space ($\text{mL}$), $PaCO_2$ is arterial carbon dioxide tension ($\text{mmHg}$), $\dot V_{CO_2}$ is carbon dioxide production ($\text{mL}\cdot\text{min}^{-1}$) and $0.863$ is a unit conversion constant. The relationship is inverse, $PaCO_2\propto 1/\dot V_A$, so an unintended rise in delivered $V_T$ produces hypocarbia directly. The Y-piece sensor adds instrumental dead space of the order of $1\text{ mL}$, not negligible in the smallest infants.

The Evidence Base: Volume-Targeted versus Pressure-Limited Ventilation

The Cochrane Review

The reference synthesis is the Cochrane review by Klingenberg and colleagues (2017): twenty randomised trials in infants below 44 weeks' postmenstrual age, sixteen parallel-group (977 infants) and four crossover (88 infants). No trial was blinded, and GRADE certainty was moderate to low. Death or BPD at 36 weeks' postmenstrual age fell to $\text{RR }0.73$ ($95\%\text{ CI }0.59\text{–}0.89$; NNTB $8$, $95\%\text{ CI }5\text{–}20$), and mechanical ventilation shortened by $1.35\text{ days}$ ($95\%\text{ CI }-1.83\text{ to }-0.86$).

Secondary Outcomes

Outcome	Typical RR (95% CI)	NNTB (95% CI)
Death before discharge	0.75 (0.53–1.07)	—
Death or BPD at 36 weeks	0.73 (0.59–0.89)	8 (5–20)
Hypocarbia	0.49 (0.33–0.72)	3 (2–5)
Grade 3–4 IVH	0.53 (0.37–0.77)	11 (7–25)
PVL with or without grade 3–4 IVH	0.47 (0.27–0.80)	11 (7–33)
Pneumothorax	0.52 (0.31–0.87)	20 (11–100)

Caveats

Three caveats matter. Death alone was not significantly reduced ($\text{RR }0.75$, $95\%\text{ CI }0.53\text{–}1.07$), so the composite benefit is driven largely by BPD. The trials were unblinded, so bias cannot be excluded. And the meta-analysis lacked power to show improved neurodevelopment, so volume targeting reduces short-term pulmonary and cranial morbidity rather than being a proven neurodevelopmental therapy.

Target Selection and Initial Settings

Choosing the Target Tidal Volume

For most preterm infants with respiratory distress syndrome (RDS) the target is $4\text{–}6\ \text{mL}\cdot\text{kg}^{-1}$ of current weight, stratified by physiology:

• Early RDS and acute hypoxaemic respiratory failure: $4\text{–}5\ \text{mL}\cdot\text{kg}^{-1}$. The sick lung has low, heterogeneous compliance, so the lower end limits volutrauma.
• Evolving BPD: $5\text{–}6\ \text{mL}\cdot\text{kg}^{-1}$. Regional overdistension raises the dead-space fraction, which a slightly larger target offsets.
• Permissive hypercapnia: $PaCO_2$ $45\text{–}55\ \text{mmHg}$ (to $60\text{ mmHg}$) with $pH > 7.20\text{–}7.25$. Mild hypercapnia permits a lower $V_T$ and PIP, never hypocarbia.

Initial Settings and Titration

Parameter	Typical initial setting	Titration target / safety limit
Target $V_T$	$4\text{–}5\ \text{mL}\cdot\text{kg}^{-1}$ (RDS); $5\text{–}6\ \text{mL}\cdot\text{kg}^{-1}$ (BPD)	Confirm against $PaCO_2$; reserve $>6\text{ mL/kg}$ for hypercapnia
$P_{\max}$ (ceiling)	$5\text{–}10\ \text{cmH}_2\text{O}$ above the anticipated working PIP	Leave servo headroom; avoid very high ceilings
Backup rate	$40\text{–}60\ \text{min}^{-1}$ (higher in ELBW)	Titrate to $PaCO_2$; avoid tachypnoea-induced hypocarbia
Inspiratory time	$0.30\text{–}0.40\ \text{s}$	Long enough to deliver, short enough to avoid air trapping
PEEP	$5\text{–}7\ \text{cmH}_2\text{O}$ in RDS; lower in BPD if hyperinflated	Optimise oxygenation and lung volume
Inspiratory flow	Above $6\text{–}10\ \text{L}\cdot\text{min}^{-1}$	Match demand; avoid flow starvation

Blood-Gas Targets

VG stabilises volume, not blood gases. Typical targets are $pH$ $7.25\text{–}7.35$, $PaCO_2$ $40\text{–}55\ \text{mmHg}$, $PaO_2$ $50\text{–}80\ \text{mmHg}$ and $SpO_2$ $90\text{–}95\%$. The critical constraint is the lower $PaCO_2$ limit: a falling $PaCO_2$ should prompt a reduction in set $V_T$ or rate.

Figure 1: A published titration algorithm for tidal volume-targeted ventilation, comparing measured expired tidal volume with the set target to adjust peak inspiratory pressure (PIP) and PEEP within the pressure ceiling. (Source: Chakkarapani AA, Adappa R, Mohammad Ali SK, et al. International Journal of Pediatrics & Adolescent Medicine 2020. DOI: 10.1016/j.ijpam.2020.11.002).

Pitfalls, Troubleshooting and Alarms

Uncuffed-Tube Leak, False Readings and the Flow Sensor

When expired volume under-reads, the controller sees a deficit and raises PIP; if the leak is flow-dependent, the higher PIP worsens it, producing a positive-feedback spiral of rising pressure with unchanged measured volume. Treat it mechanically — reposition the tube, correct depth, reduce neck extension, suction, and upsize if the leak exceeds roughly $40\text{–}50\%$ — never by raising the target or ceiling. The proximal flow sensor compounds the problem: secretions, condensation and water on its wires distort the flow signal and produce false volume readings, spuriously low or spuriously high. Sensors should be drained, dried and replaced per manufacturer guidance, and an unexplained change in measured $V_T$ without a change in chest excursion should prompt sensor inspection first.

Over-Aggressive Auto-Weaning and Patient Effort

During assisted breaths the infant's own effort contributes to $V_T$; when effort increases, the ventilator reads a satisfactory volume and reduces PIP, potentially leaving an improving infant unsupported and at risk of fatigue. A deep spontaneous effort can likewise be misread and cause an inappropriate pressure reduction on later breaths. Poor synchrony makes both delivered and measured volume unreliable.

Alarms and Their Interpretation

"Volume not reached" means the target could not be achieved despite reaching $P_{\max}$ — a large leak, mechanical deterioration or an unrealistic target. Low tidal or minute volume indicates genuine under-delivery. A high-pressure alarm signals falling compliance or an approaching ceiling. Neither should be silenced without deciding which input — target, leak or mechanics — has changed.

flowchart TD
    A["On VG: review V_Te, leak percent and PIP trend"] --> B{"V_Te on target and leak under 30 percent?"}
    B -->|"Yes"| C["Maintain target; wean FiO2 and PEEP"]
    B -->|"No"| D{"Is V_Te persistently below target?"}
    D -->|"Yes, PIP at ceiling"| E["Volume not reached - check leak, sensor, mechanics"]
    D -->|"Yes, PIP below ceiling"| F["PIP rising as intended; confirm calibration"]
    D -->|"No, V_Te above target"| G{"Is PaCO2 below 35 mmHg?"}
    G -->|"Yes"| H["Reduce set V_T toward 4 mL/kg and rate - avoid hypocarbia"]
    G -->|"No"| I["Reduce set V_T by 0.5 mL/kg and reassess gas"]
    E --> J{"Leak above 40 percent or sensor fault?"}
    J -->|"Yes"| K["Reposition or upsize tube; drain or replace sensor"]
    J -->|"No"| L["Reassess PEEP, tube patency and effort"]

    style A fill:#eff6ff,stroke:#3b82f6,stroke-width:2px;
    style E fill:#fef2f2,stroke:#dc2626,stroke-width:2px;
    style H fill:#fee2e2,stroke:#ef4444,stroke-width:2px;
    style C fill:#f0fdf4,stroke:#16a34a,stroke-width:2px;

Comparative Matrix of Ventilator Control Strategies

Feature	Pressure-Controlled (PC)	Volume-Controlled (VC)	Volume-Guaranteed (VG)
Primary set variable	PIP and inspiratory time	$V_T$ and inspiratory flow	Target $V_T$ with variable PIP
Improving compliance	$V_T$ rises; volutrauma and hypocarbia	Pressure falls; volume constant	PIP falls automatically (auto-weaning)
Worsening compliance	$V_T$ falls; hypoventilation	Pressure rises; volume constant	PIP rises to ceiling; alarm
Leak compensation	None	Partial (sensor dependent)	Active, via expired-volume feedback
Principal hazard	Volutrauma, hypocarbia, unstable $V_T$	Barotrauma from uncontrolled pressure	Ceiling-driven pressure, false sensor readings

Key Takeaways

1. Volume Guarantee is a pressure-controlled breath with volume feedback. The clinician sets the tidal volume and pressure ceiling; the ventilator recomputes PIP each breath from measured expired volume.
2. The servo is discrete and deliberately damped. Volume is measured at the Y-piece and the next PIP is adjusted within about $\pm 3\text{ cmH}_2\text{O}$, bounded by the ceiling and floor.
3. Auto-weaning is the signature behaviour. As compliance improves PIP falls automatically; as compliance falls PIP rises until the ceiling triggers a "volume not reached" alarm.
4. Target $4\text{–}6\ \text{mL}\cdot\text{kg}^{-1}$ of current weight, using $4\text{–}5$ in acute RDS and $5\text{–}6$ in evolving BPD, and recalculate the millilitre setting at every weight change.
5. Hypocarbia is the neurotoxic end of the volume spectrum. Keep $PaCO_2$ above roughly $35\text{ mmHg}$; an over-generous target can cause PVL even while the mode prevents gross volutrauma.
6. The Cochrane evidence is moderate-certainty and clinically meaningful. Volume targeting reduced death or BPD, hypocarbia, severe IVH, PVL and pneumothorax; death alone and neurodevelopment were not significantly improved.
7. Troubleshoot the leak and the sensor before the numbers. A leak above $40\text{–}50\%$ and a contaminated flow sensor are the commonest causes of misleading volume readings and runaway pressure.